# The CatWISE Preliminary Catalog: Motions from WISE and NEOWISE Data

Peter R. M. Eisenhardt  Jet Propulsion Laboratory, California Institute of
Technology, 4800 Oak Grove Drive, M/S 169-327, Pasadena, CA 91109, USA
Federico Marocco NASA Postdoctoral Program Fellow  Jet Propulsion Laboratory,
California Institute of Technology, 4800 Oak Grove Drive, M/S 169-327,
Pasadena, CA 91109, USA  IPAC, Mail Code 100-22, California Institute of
Technology, 1200 E. California Blvd., Pasadena, CA 91125, USA  John W. Fowler
230 Pacific St., Apt. 205, Santa Monica, CA 90405, USA  Aaron M. Meisner
Hubble Fellow  National Optical Astronomy Observatory, 950 N. Cherry Ave.,
Tucson, AZ 85719, USA  J. Davy Kirkpatrick IPAC, Mail Code 100-22, California
Institute of Technology, 1200 E. California Blvd., Pasadena, CA 91125, USA
Nelson Garcia  IPAC, Mail Code 100-22, California Institute of Technology,
1200 E. California Blvd., Pasadena, CA 91125, USA  Thomas H. Jarrett
Department of Astronomy, University of Cape Town, Private Bag X3, Rondebosch,
7701, South Africa  Renata Koontz  University of California, Riverside, 900
University Ave, Riverside, CA 92521, USA  Elijah J. Marchese  University of
California, Riverside, 900 University Ave, Riverside, CA 92521, USA  S. Adam
Stanford  Department of Physics, University of California Davis, One Shields
Avenue, Davis, CA 95616, USA  Dan Caselden  Gigamon Applied Threat Research,
619 Western Avenue, Suite 200, Seattle, WA 98104, USA  Michael C. Cushing
Department of Physics and Astronomy, University of Toledo, 2801 West Bancroft
St., Toledo, OH 43606, USA  Roc M. Cutri  IPAC, Mail Code 100-22, California
Institute of Technology, 1200 E. California Blvd., Pasadena, CA 91125, USA
Jacqueline K. Faherty Department of Astrophysics, American Museum of Natural
History, Central Park West at 79th Street, NY 10024, USA  Christopher R.
Gelino  IPAC, Mail Code 100-22, California Institute of Technology, 1200 E.
California Blvd., Pasadena, CA 91125, USA  Anthony H. Gonzalez  Department of
Astronomy, University of Florida, 211 Bryant Space Center, Gainesville, FL
32611, USA  Amanda Mainzer  Jet Propulsion Laboratory, California Institute of
Technology, 4800 Oak Grove Drive, M/S 169-327, Pasadena, CA 91109, USA  Bahram
Mobasher  University of California, Riverside, 900 University Ave, Riverside,
CA 92521  David J. Schlegel Lawrence Berkeley National Laboratory, Berkeley,
CA, 94720, USA  Daniel Stern  Jet Propulsion Laboratory, California Institute
of Technology, 4800 Oak Grove Drive, M/S 169-327, Pasadena, CA 91109, USA
Harry I. Teplitz  IPAC, Mail Code 100-22, California Institute of Technology,
1200 E. California Blvd., Pasadena, CA 91125, USA  Edward L. Wright Department
of Physics and Astronomy, UCLA, 430 Portola Plaza, Box 951547, Los Angeles, CA
90095-1547, USA

###### Abstract

CatWISE is a program to catalog sources selected from combined WISE and
NEOWISE all-sky survey data at 3.4 and 4.6 µm (W1 and W2). The CatWISE
Preliminary Catalog consists of 900,849,014 sources measured in data collected
from 2010 to 2016. This dataset represents four times as many exposures and
spans over ten times as large a time baseline as that used for the AllWISE
Catalog. CatWISE adapts AllWISE software to measure the sources in coadded
images created from six-month subsets of these data, each representing one
coverage of the inertial sky, or epoch. The catalog includes the measured
motion of sources in 8 epochs over the 6.5 year span of the data. From
comparison to Spitzer, the SNR=5 limits in magnitudes in the Vega system are
W1=17.67 and W2=16.47, compared to W1=16.96 and W2=16.02 for AllWISE. From
comparison to Gaia, CatWISE positions have typical accuracies of 50 mas for
stars at W1=10 mag and 275 mas for stars at W1=15.5 mag. Proper motions have
typical accuracies of 10 mas yr-1 and 30 mas yr-1 for stars with these
brightnesses, an order of magnitude better than from AllWISE. The catalog is
available in the WISE/NEOWISE Enhanced and Contributed Products area of the
NASA/IPAC Infrared Science Archive.

catalogs, infrared:stars, proper motions, brown dwarfs

††journal: Astrophysical Journal Supplement

##  1 Introduction

NASA’s Wide-field Infrared Survey Explorer mission (WISE; Wright et al., 2010)
revealed iconic objects, including the first Earth Trojan asteroid (Connors et
al., 2011), the closest and coolest brown dwarfs (Luhman, 2013, 2014), and the
most luminous galaxy yet found in the Universe (Tsai et al., 2015). These
discoveries were made using two or more infrared coverages of the sky (or
epochs) obtained from January 2010 to February 2011. Each epoch typically
consists of a dozen exposures per band taken within two days at a given
position. The satellite was reactivated as NEOWISE and resumed searching for
near-Earth objects in December 2013 (Mainzer et al., 2014), and has continued
to cover the sky every six months since then.

In November 2013, the AllWISE release (Cutri et al., 2013) made available to
the community an atlas from coadding the two dozen exposures per position from
the initial year of WISE surveying, and a catalog of source fluxes and
positions measured from those exposures. With at least two epochs per inertial
position, AllWISE also provided motion estimates, and became the definitive
catalog in the WISE bandpasses. Each year, NEOWISE releases the individual
exposures from the reactivated survey (Cutri et al., 2015), corresponding to
two additional epochs. With the 2019 April 10 NEOWISE release, exposures from
12 epochs are now available. Meisner et al. (2018a) used unWISE processing
(Lang, 2014) to produce an image atlas which combines the 2010 and 2011 data
used for AllWISE with the 2013 through 2016 NEOWISE data. An obvious next step
is to catalog the sources revealed in these combined exposures.

The unWISE Catalog (Schlafly et al., 2019) uses a crowded-field point-source
photometry code called “crowdsource” to do this, measuring source fluxes and
positions in the coadded image, with the measurements at 3.4 µm (W1) and 4.6
µm (W2) carried out independently. In contrast, CatWISE has adapted AllWISE
software to produce a full-sky catalog of sources selected simultaneously in
both W1 and W2, and also provides motion estimates. For the CatWISE
Preliminary Catalog described in this paper, sources were selected from the
ensemble of 8 epochs of WISE and NEOWISE data from Meisner et al. (2018a, the
“full coadd”), and the least-squares best-fit solution for point source flux,
position and motion were determined from measurements on the individual “epoch
coadd” images (Meisner et al., 2018c) rather than individual exposures, to
reduce computational cost. Because the total time spanned by CatWISE epochs at
a given inertial position is over 6 years, compared to a typical value of 6
months for AllWISE, the CatWISE motion estimates are far more accurate. This
in addition to the greater depth of the CatWISE catalog relative to AllWISE
are being used to extend the census of the coldest brown dwarfs in the solar
neighborhood and enable definitive measurement of the form of the low-mass end
of the star formation process (Kirkpatrick et al., 2019). Figure 1 illustrates
an example of the potential of CatWISE to progress in this area.

In §2 of this paper we summarize relevant aspects of the WISE and NEOWISE
mission phases. §3 describes the CatWISE processing steps. §4 assesses the
astrometric and photometric performance of CatWISE using comparisons to Gaia
and Spitzer data. §5 provides some initial examples of science results enabled
by CatWISE, and §6 provides information on accessing CatWISE data products.
Appendices provide additional information on CatWISE column entries, cautions
on known issues in the CatWISE Preliminary Catalog, and details on how
positions were combined from alternating survey scan directions (§2). The
CatWISE website is https://catwise.github.io.

Figure 1: (top) Proper motions that are insignificant in AllWISE become
obvious with the increased time baseline of CatWISE. Discovered via its color
in WISE data (Gelino et al. (2011); Kirkpatrick et al. (2011)), WISEA
J161441.47+173935.4 is a T9 brown dwarf at 10 pc with a proper motion of
$\mu_{\alpha}=554.5\pm 1.2$ mas yr${}^{-1},\mu_{\delta}=-476.7\pm 1.2$ mas
yr-1 as determined by Spitzer (Kirkpatrick et al., 2019). With a 6-month
baseline, AllWISE (top left and center) measures a motion consistent with zero
($\mu_{\alpha}=352\pm 412$ mas yr${}^{-1},\mu_{\delta}=-69\pm 452$ mas yr-1;
bottom center), while with the inclusion of NEOWISE data from 2016 (top
right), the corresponding source in the CatWISE Preliminary Catalog (CWISEP
J161441.63+173933.7) has a highly significant motion ($\mu_{\alpha}=583.6\pm
23.5$ mas yr${}^{-1},\mu_{\delta}=-442.0\pm 25.0$ mas yr-1; bottom right)
consistent with the Spitzer values. The top panels show 2${}^{\prime}\times
2^{\prime}$ cutouts in W2 centered on the WISE position of the source at the
first epoch, marked in all 3 epochs by a red circle.

##  2 Observations

WISE was launched on 2009 December 14, with its 40-cm telescope cooled to 12 K
by an outer cryostat tank, and the W1 and W2 detectors operated at 32 K. The
12 and 22 µm (W3 and W4) detectors were cooled to 7.8 K by an inner cryostat
tank. Both tanks were filled with frozen hydrogen. The cryostat cover was
ejected and the first images obtained on 2009 December 31, and science survey
data were taken starting on 2010 January 7.

WISE uses dichroics to image the same $47\times 47$ arcmin region of sky
simultaneously in all bands using $1024\times 1024$ pixel arrays with
2$\farcs$75 pixels, and obtaining exposures every 11 seconds. The duration of
each W1 and W2 exposure is 7.7 seconds due to readout time and scan mirror
settling, and discarding the initial readout.

The Sun-synchronous polar WISE orbit was designed to precess so that the
satellite stays over the Earth’s terminator. The basic WISE survey strategy is
to point near the zenith, scanning at the orbital rate along lines of ecliptic
longitude, with the image motion compensated by a scan mirror that flies back
for the start of each exposure. The precession rate sweeps across the imaging
field of view in approximately 12 orbits (less than one day) near the
ecliptic, although the detailed survey strategy extends this time to more than
a day. Because images are obtained on both sides of the orbit, the same region
of inertial sky is covered every six months, with the direction of the survey
scans alternating between ascending and descending in ecliptic latitude.
CatWISE processes ascending and descending scans separately for source
measurement.

Figure 2: Temperature of the WISE beamsplitter assembly vs. date. This
temperature is close to that of the telescope and W1 and W2 detectors. Dashed
lines indicate the transitions from full cryogenic to 3-band cryogenic phases,
to the post-cryogenic phase, and to the start of hibernation and reactivation.
Grey shading indicates the date ranges of data used for the CatWISE
Preliminary Catalog. AllWISE included only the left portion of the shaded
range.

The fully cryogenic survey continued until the hydrogen in the outer tank
evaporated on 2010 August 6, exceeding the 7-month cryogen lifetime
requirement and covering the sky 1.2 times (Figure 2). WISE then surveyed an
additional 30% of the sky during its 3-Band (W1, W2, and W3) cryogenic phase,
with the W3 detector operating at reduced sensitivity. After the cryogen in
both tanks was exhausted on 2010 September 29, the post-cryogenic NEOWISE
survey (Mainzer et al., 2011) in W1 and W2 began, as the telescope and
detectors warmed to $\sim 73$ K. Surveying continued until 2011 February 1,
completing a second pass over the sky in W1 and W2, after which the satellite
was placed into hibernation. In September 2013, the spacecraft was brought out
of hibernation, where it had equilibrated at $\sim 200$ K due to thermal
radiation from the Earth, and renamed NEOWISE. On 2013 December 13, with the
telescope and detectors passively cooled below 76 K, NEOWISE resumed surveying
the sky every 6 months in W1 and W2 (Mainzer et al., 2014).

The CatWISE Preliminary Catalog is based on the combination of W1 and W2
exposures in the two sky coverages used for the AllWISE data release (Cutri et
al., 2013) and in the six additional sky coverages from the 2017 NEOWISE data
release. For PSF purposes (§3.2), we use MJD 55480 (2010 Oct. 11) as the
dividing point between cryogenic and post-cryogenic data. The average
observation date is closer to MJD 56700 (2014 Feb. 12), which we adopt as the
epoch for reporting positions when solving for source motion in the
Preliminary Catalog. Source positions, whether incorporating source motion or
not, are given in the equinox J2000 coordinate frame. Magnitudes in the
catalog and throughout this paper are on the Vega system.

##  3 CatWISE Processing

CatWISE adapts the AllWISE pipeline to detect and measure source fluxes in the
combined WISE and NEOWISE images provided by unWISE. A full description of the
AllWISE pipeline is provided in Cutri et al. (2013).

CatWISE processing works in the atlas tile footprint established by the WISE
All-Sky Release, dividing the inertial sky into 18,240 overlapping square
images (tiles), each approximately 1.56° on a side, aligned with the local
right ascension and declination. Except for the “primary flagging” step
(§3.6), processing steps were carried out independently on each tile. For the
Preliminary Catalog, CatWISE uses the full depth unWISE coaddition of 8 epochs
(Meisner et al., 2018a), and the 8 individual unWISE epoch coadds (Meisner et
al., 2018c, after astrometric modifications described in §3.1). The full depth
coadds and the PSF (§3.2) were used to create detection images. Sources were
detected in these images (§3.3), while source properties were determined from
measurements on the epoch coadds, treating ascending and descending epochs
(§2) separately for most tiles and then merging the results (§3.4). Potential
artifacts affecting the sources were identified (§3.5), and sources with
multiple measurements because they were in the tile overlap region were
flagged to indicate which set of measurements should be used (§3.6). Finally,
sources were selected for inclusion in the catalog or reject files (§3.7). We
describe these steps in more detail below.

###  3.1 unWISE Coadds

The coadded images in the atlas released with the AllWISE (and All-Sky)
Catalogs were primarily intended to facilitate source detection, and for this
reason they are convolved with the WISE PSF. The unWISE coadds retain the
resolution of individual WISE exposures. To reduce differences from the
AllWISE processing approach, for source detection ($\S$3.3) in the CatWISE
Preliminary Catalog, we convolved the masked version (Lang, 2014) of the full-
depth unWISE coadds Meisner2018a with the PSF ($\S$3.2). For source
measurement ($\S$3.4) we used the masked version of the unWISE epoch coadds
(Meisner et al., 2018c) without convolution.

An adjustment was made to the world coordinate system (WCS) for the pre-
hibernation unWISE epoch coadds. Although all of the released individual
exposures are tied to 2MASS (Skrutskie et al., 2006), the individual pre-
hibernation exposures released for AllWISE are not on the same astrometric
system as the individual post-hibernation exposures released for NEOWISE. The
NEOWISE images include corrections for the motions of the 2MASS reference
stars, as does the AllWISE Catalog (see §V.2.b of the AllWISE Explanatory
Supplement; Cutri et al. (2013)), but the released pre-hibernation images do
not include those corrections. A table of corrections for these images exists,
however111http://wise2.ipac.caltech.edu/docs/doc_tree/sis/rex19, and these
corrections were applied to the unWISE input, making the astrometric system of
the pre-hibernation epoch coadds consistent with those from the post-
hibernation epoch coadds.

###  3.2 Point Spread Function (PSF)

The WISE and NEOWISE PSFs have been well characterized at the individual
exposure level, but CatWISE works with coadded images rather than individual
exposures, so PSFs appropriate for these coadded images are needed. The W1 and
W2 PSFs vary with position in the focal plane, and changed somewhat between
the cryogenic and post-cryogenic phases of the mission, particularly for W1.
The PSFs are also asymmetric, with an orientation that is fixed with respect
to the focal plane, but the focal plane orientation with respect to equatorial
coordinates varies between exposures. The focal plane orientation also flips
every six months because at a given inertial location the survey scan
direction alternates between ascending and descending in ecliptic latitude
(§2). CatWISE addressed these issues as follows.

A $9\times 9$ grid over the focal plane of cryogenic phase model PSFs and
their uncertainties, $8\times$ oversampled relative to WISE pixels and
covering 220” on a side, is provided in §IV.4.c.iii.1 of the WISE All-Sky
Explanatory Supplement (Cutri et al., 2012). The post-cryogenic PSFs used for
NEOWISE are given in §IV.2.b.i of the NEOWISE Explanatory Supplement (Cutri et
al., 2015). Since many focal plane positions contribute to each source in the
coadded images, CatWISE averaged these model PSFs, weighted only by their
partition sizes in the $9\times 9$ grid. The uncertainty in the averaged PSF
was taken to be the root sum square of two terms: the root mean square of the
individual grid PSF uncertainties divided by $\sqrt{N-1}$ where N is the 81
PSF’s, and the standard deviation of the grid PSF’s about the averaged PSF.
Figure 3 (left) shows the resulting focal plane average post-cryogenic PSF
model for W1. An analogous focal plane average cryogenic PSF was created for
W1, as well as average cryogenic and post-cryogenic PSFs for W2. These are
referred to below as the “basic” PSFs.

Figure 3: Left: The “basic” (averaged over focal plane position) post-
cryogenic PSF model for W1. The grey scale uses a log stretch from 1% to 99%
of the peak value. The model extends to $\pm 110^{\prime\prime}$. Center: The
ascending scan W1 PSF for tile 1497p015 after averaging over cryogenic and
post-cryogenic epochs, and a PA range from 199.93°to 200.56°. This is
illustrative of the type of PSF used by CatWISE for source measurement, with
the exception of 50 tiles near the ecliptic poles. Right: The W1 PSF averaged
over all scans for tile 0890m667, which includes the south ecliptic pole and
all PAs. This is illustrative of the type of PSF used by CatWISE for source
measurement for 50 tiles near the ecliptic poles. Figure 4: Difference image
of W1 post-cryogenic PSFs in ascending vs. descending scans. The image extends
to $\pm 110^{\prime\prime}$.

As can also be seen in the AllWISE PSF’s plotted in Figure 1 of Lang, Hogg, &
Schlegel (2016), the PSFs contain fine structure in the core that is
qualitatively consistent with the expected diffraction pattern for WISE. This
structure is not rotationally symmetric. Figure 4 shows the difference between
ascending and descending orientations of the W1 post-cryogenic PSF. The bright
and dark stripes in and around the core correspond to variations of roughly
25% of the peak of the PSF. The center of the PSF is the most crucial region
for position estimation, so inaccuracies in this part of the PSF have serious
consequences for astrometry, as well as photometry.

These structures are not due to misalignment of the ascending and descending
PSF’s, which would need to be $\sim 1\farcs 0$ to account for the features –
far greater than the difference in astrometry from the ascending vs.
descending PSF’s. For example, the median difference in source astrometry for
tile 0895p227 (where ecliptic longitude and right ascension align) was 26.5
mas in ecliptic longitude, and 0.0 mas in ecliptic latitude. With the PSF’s
intentionally swapped, these values became -1216.0 mas in longitude and 633.6
mas in latitude. Over the sky, the tile median differences using the correct
PSF’s are distributed with a median value of 78.1 mas and $\sigma=33$ mas in
ecliptic longitude, and -14.4 mas and $\sigma=16$ mas in ecliptic latitude.

The structure revealed here has no impact on the measurements carried out by
the WISE and NEOWISE projects, because those measurements were made (and
continue to be made) on individual exposures. The PSF structure is captured in
the models, and the PSF orientation is always the same in the focal plane.
This is not the case for CatWISE, which uses coadds in place of individual
WISE exposures. The coadded image tile orientation is aligned with the local
equatorial coordinates. Therefore CatWISE rotated the basic PSFs by an
appropriate set of position angles (PAs) between focal plane and equatorial
coordinates for each image tile.

The coadded images were assembled from individual exposures with a range of PA
values, so histograms of exposure PA values were constructed for each tile,
using a bin width of 0.1 degrees. Separate histograms were constructed for
ascending vs. descending and cryogenic vs. post-cryogenic exposures. The basic
PSF was rotated by each PA bin center value and these rotated PSFs averaged
together with weights equal to the histogram bin count. Those averages were
then combined for each tile with weights given by the number of corresponding
cryogenic and post-cryogenic exposures to make ascending and descending PSFs
for each tile.

Tiles in the vicinity of the ecliptic poles have large PA ranges, and so the
tile PSF becomes very smeared (see Figure 3, right). For the 50 tiles nearest
the ecliptic poles (listed in Table 1), the ascending and descending PSFs were
averaged together, weighted by the number of ascending and descending
exposures.

The description above applies to the PSF’s used by CatWISE for source
measurement. For the detection step in the Preliminary Catalog (§3.3), PSF’s
analogous to the measurement PSF’s of the 50 tiles nearest the ecliptic poles
were created for every tile, but with $4\times$ oversampling relative to the
unWISE images, rather than the $8\times$ oversampling of the measurement
PSF’s. These detection PSF’s were made by resampling the central portion of
the weighted average of ascending and descending PSF’s for that tile. The
detection PSF’s have $27\times 27$ pixels with each pixel spanning
0$\farcs$6875, i.e. covering an area of 18$\farcs$5625 on a side (6.75 native
WISE pixels).

Table 1: Tiles Processed Using a Single PSF per Band  RA range in decimal
degrees of tiles in column
---
79.1:87.9 | 89.0:95.7 | 96.2:264.2 | 265.0:272.1 | 272.6:280.8
0791m682 | 0890m667 | 0962m697 | 2650p681 | 2726p651
0803m652 | 0891m637 | 0964m667 | 2656p651 | 2729p681
0830m682 | 0908m652 | 0978m652 | 2672p666 | 2741p636
0837m697 | 0909m682 | 0988m682 | 2675p636 | 2746p666
0838m652 | 0920m697 | 1002m667 | 2679p696 | 2761p651
0853m667 | 0924m637 | 2610p681 | 2690p681 | 2762p696
0858m637 | 0927m667 | 2621p651 | 2691p651 | 2769p681
0870m682 | 0943m652 | 2635p666 | 2708p636 | 2783p666
0873m652 | 0949m682 | 2637p696 | 2709p666 | 2796p651
0879m697 | 0957m637 | 2642p636 | 2721p696 | 2808p681

###  3.3 Detection

Source detection for the CatWISE Preliminary Catalog was done simultaneously
in W1 and W2 with the Multiband Detection (MDET; Marsh & Jarrett, 2012) and
Image Co-addition with Optional Resolution Enhancement (ICORE) software
(Masci, 2013) used in the WISE pipeline, following the process described in
§IV.4.b.iii.1 of the WISE All-Sky Explanatory Supplement (Cutri et al., 2012).
The detection PSF’s in each band were used as matched filters to generate
images which are optimal for detection of isolated point sources in that band.
ICORE generated the convolution (or more accurately, the cross-covariance) of
the PSF with the full-depth unWISE coadd, resampling the $2048\times 2048$
2$\farcs$75 pixel unWISE coadds to the $4095\times 4095$ 1$\farcs$375 pixel
detection image format employed by MDET for WISE. The resampling was done
using nearest-neighbor interpolation into a temporary finer grid matching the
PSF grid, before being down-sampled to the output pixel size. Similarly, ICORE
created uncertainty images from the detection PSF’s and the unWISE “std”
images, which are the standard deviation of the individual exposures that
comprise the unWISE full-depth coadds. MDET then subtracted a local background
from each ICORE image using a $21\times 21$ pixel median filter, normalizing
the result by the ICORE uncertainty image. To account for confusion noise, the
uncertainties were augmented by the root-sum-square of the estimated variance
of the ICORE image from the local background. Finally, MDET generated a
detection image from these normalized and background-subtracted ICORE images
by summing the two bands in quadrature, identifying as sources local maxima
(pixels with larger values than their neighbors) that exceeded a specified
detection threshold.

As was done for WISE, the detection threshold was set at an estimated source
reliability of $\sim 50$%. AllWISE, with two sky coverages, selected a
threshold of 2.4, compared to 3.5 used for the single coverage WISE All-Sky
Release. For CatWISE we estimated the threshold based on comparison to deeper
Spitzer data from the SERVS (Mauduit et al., 2012) and S-COSMOS (Sanders et
al., 2007) programs. These programs have a source density of $\sim 100,000\
\rm{deg}^{-2}$, so with the matching radius of 2$\farcs$5 the chance of a
spurious match is not trivial (15%), but still well below the 50% criterion
for the detection threshold. The estimated threshold varied from 1.7 in the
COSMOS field to 2.6 in the Lockman Hole, and a value of 1.8 was selected for
the Preliminary Catalog.

At the high source densities typical for CatWISE in coadds of $\sim 100$ or
more individual WISE exposures, this matched filter detection methodology
yields an asymptotic number of $\sim 60,000$ detected sources per tile for the
Preliminary Catalog (or $\sim 25,000\ {\rm deg}^{-2}$, Figure 5) and hence
becomes incomplete, particularly in W1. The detected source density declines
slightly in the Galactic plane, for reasons not well understood at present.
The limiting source density corresponds to an exclusion radius of 13”. This is
similar to the decline in sources separated by less than 10” shown in Figure
27 of §VI.2.c.iv in the WISE All-Sky Explanatory Supplement (Cutri et al.,
2012), so is likely a consequence of the source detection methodology used in
the WISE pipeline. Schlafly et al. (2019) present an alternative approach
(“crowdsource”) which results in higher detected source densities, but does
not provide motion estimates. We are using the Schlafly et al. (2019) catalog
as the detection list for an updated version of the CatWISE catalog that is
expected to be available in 2020. The completeness and reliability of the
CatWISE Preliminary Catalog are discussed further in §4.1.

Figure 5: Map of CatWISE Preliminary Catalog source density in Galactic
coordinates.

###  3.4 Source Measurement

Source photometry, astrometry, and motion estimation for CatWISE use an
adapted version of the WPHOT software developed for the AllWISE pipeline.
WPHOT carries out point-source extraction, solving for both photometry and
astrometry simultaneously. Source positions detected in each coadded tile
image were propagated to individual exposures, and the flux and $\chi^{2}$
evaluated from fitting the PSF. The source position and flux were refined by
searching for a local minimum in $\chi^{2}$. For AllWISE, a linear motion was
also solved for, based on the observation time of the images.

CatWISE did not have the computational resources to perform the source fitting
on all of the many hundreds of individual exposures that contribute to a
coadded tile image. Because the 12 exposures for a given inertial position in
each sky coverage are obtained within less than two days, the motions of
sources beyond the solar system can be assumed to be fixed for each sky
coverage (or epoch). Therefore CatWISE ran WPHOT treating unWISE epoch coadd
images as the individual images.

The other significant modification to the AllWISE WPHOT process implemented by
CatWISE addressed the alternating scan direction of each epoch and hence
varying PSF orientation. WPHOT was not designed to use a time-dependent PSF,
but with 8 or more epoch coadd images per position, CatWISE elected to measure
source properties with WPHOT separately for the groups of four or more epoch
coadds in each of the two scan directions, and then merge the two results.
This methodology was used for all but the 50 tiles nearest the ecliptic poles,
listed in Table 1. For those tiles, a tile-specific average PSF over all
epochs was used for both scan directions, and all epochs were processed
together with WPHOT.

WPHOT performs PSF-fit photometry and astrometry assuming sources are
inertially fixed (the “stationary fit”), and also searches for a solution
assuming a linear motion with time (the “motion fit”). The $\chi^{2}$
minimization fit can be performed at any location in the flux image. The goal
of astrometry is to find the location at which $\chi^{2}$ has a local minimum.

The stationary fit uses the gradient descent method to minimize the total
$\chi^{2}$ (i.e., the combined W1 and W2 $\chi^{2}$) in the two-dimensional
space of right ascension ($\alpha$) and declination ($\delta$). As one moves
around in the space, $\chi^{2}$ decreases and increases as the local flux
distribution appears more or less like a point source. The $(\alpha,\delta)$
where $\chi^{2}$ is estimated to be minimal is the position assigned to the
source, and the scale factor on the PSF that minimizes $\chi^{2}$ there is the
flux estimate. The value of $\chi^{2}$ is recorded, as are the individual
$\chi^{2}$ for the W1 and W2 flux fits. Under certain conditions such as low
coverage or negative fluxes, the gradient descent algorithm may be unable to
compute estimates, in which case nulls are reported. Standard Gaussian error
analysis provides uncertainties for $(\alpha,\delta)$ and flux. The error
model includes both PSF error and image flux error, and is discussed further
in §3.4.1. More details on WPHOT are given in §IV.4.c.iii of the WISE All-Sky
Explanatory Supplement (Cutri et al., 2012).

The model used for the motion solution replaces the single location of the
stationary solution with locations along a linear function of time. The slopes
of this line in the $(\alpha,\delta)$ directions are the angular motion rates
($\mu_{\alpha},\mu_{\delta}$) in $(\alpha,\delta)$. The motion fit minimizes
the total $\chi^{2}$ over the four-dimensional space of
$(\alpha,\delta,\mu_{\alpha},\mu_{\delta})$. When the motion fit is based on
both ascending and descending epochs, the motions will include parallax
effects (see Kirkpatrick et al., 2014), and so they are not true proper
motions. However, except for the 50 tiles near the ecliptic poles, CatWISE
processed ascending and descending scans separately, so that the measured
motions are close to true proper motions.

####  3.4.1 Uncertainties

WPHOT estimates uncertainties in photometry, position, and motion based on
error propagation from uncertainty images corresponding to the PSF and
exposure images. The PSF uncertainty images from WISE and NEOWISE appropriate
to each PSF were combined using the methodology described in §3.2, and these
dominate the uncertainties estimated for brighter stars ($\lesssim 12$ mag).
For fainter sources ($\gtrsim 15$ mag) the exposure image (or for CatWISE,
epoch image) uncertainties dominate. For these, the “std” unWISE images were
used, which are the sample standard deviation at each pixel of the individual
WISE exposures divided by $\sqrt{N-1}$, where N is the number of exposures.

Reduced $\chi^{2}$ values from the CatWISE PSF fitting measurements showed
that the uncertainties needed adjustment. The PSF uncertainties were scaled
down by 0.9 in W1 and 0.64 in W2 based on bright star reduced $\chi^{2}$
values. Cryogenic PSF uncertainties for W1 were scaled down by an additional
factor of 0.58. Based on faint star reduced $\chi^{2}$ values, the image
uncertainties were scaled up by 1.4 in W1 and 1.15 in W2, and those for pre-
hibernation epochs were scaled up by additional factors of 1.1 in W1 and 1.05
in W2. The units of the unWISE images are Vega nanomaggies (nMgy), i.e.
$0.306681\ \mu$Jy for W1 and $0.170663\ \mu$Jy for W2 (Wright et al., 2010),
and a minimum value of 1 nMgy in W1 and 10 nMgy in W2 was also imposed on the
image uncertainties.

The uncertainties for the 50 tiles near the ecliptic poles, which used
averages of ascending and descending PSFs (§3.2) had somewhat different
adjustments. For these tiles the PSF uncertainties were scaled up by 1.3 in
W1, with no further adjustment for cryogenic values, and were left unchanged
for W2. The image uncertainties were scaled up by 1.75 in W1 and 1.25 in W2,
and those for pre-hibernation epochs were scaled up further by 1.1 in W1 and
1.05 in W2. A minimum value of 1 nMgy in W1 and 5 nMgy in W2 was imposed on
the image uncertainties.

####  3.4.2 Merging Measurements from Ascending and Descending Scans

As noted earlier, for nearly all tiles two independent measurements were made,
one extracted from epoch coadds constructed from ascending scans and one from
descending-scan epochs. These were merged into a single source list as
follows.

Sources were matched based on their identifier in the detected source list
(§3.3). A small number of mismatches occurred because of active deblending in
WPHOT. Active deblending inserts a new source (not from the detection program)
into the fitting region if the $\chi^{2}$ value exceeds a threshold and is
reduced by the insertion by a minimum required amount. Full details may be
found in §IV.4.c of the WISE All-Sky Explanatory Supplement (Cutri et al.,
2012). Actively deblended sources have the same identifier as the parent
source in the detection list. Because active deblending may proceed
differently in the ascending and descending data, when the identifier was not
unique, a nearest-match criterion was also applied. If the number of ascending
sources did not equal the number of descending sources, the left over sources
were discarded, leaving a one-to-one association list.

Corresponding parameters for each matched ascending-descending source pair
were combined when the ascending and descending apparitions have non-null
values (§3.4), otherwise the single non-null values (if any) were retained.
Aperture magnitudes for the ascending and descending scans were measured from
the same full-depth coadd, differing only slightly in the position of the
aperture centers. Hence, the fluxes and flux uncertainties corresponding to
the aperture magnitudes and their uncertainties were simply averaged without
weighting. Positions were averaged using inverse-covariance weighting, with
the averaging done in a local Cartesian projection consistent with the
uncertainty representation. Additional details are provided in Appendix C.
PSF-fit photometry was combined by averaging the flux values using inverse-
variance weighting. This yields refined flux values and reduced uncertainties
that are used to recompute the magnitudes, magnitude uncertainties, and
signal-to-noise ratios.

###  3.5 Artifact Flagging

Bright stars create a variety of scattered light effects and electronic charge
issues that require special handling by the software. These effects include
scattered light halos, diffraction spikes, glints from off-frame bright stars,
optical ghosting from internal reflections within the optical system, and
charge persistence on the arrays. These can create false detections, hereafter
called artifacts, or can contaminate detections of real, astrophysical
sources. The goal of artifact flagging is to label spurious or affected
sources so that the user can easily create source lists for which most of
these problems are eliminated.

The CatWISE Preliminary Catalog employs two types of artifact flagging. One is
the $cc\\_flags$ values copied directly from AllWISE processing. If a source
was found in either the AllWISE Catalog or Reject Table within 2$\farcs$75 of
a CatWISE source, its $cc\\_flag$ was included in the entry for the CatWISE
source; if no AllWISE source was found within this radius, then $cc\\_flags$
contains a null value. For a CatWISE source having multiple AllWISE sources
within the 2$\farcs$75 radius, the most pessimistic value per band was
retained, as described in §3.5.1. These $cc\\_flags$ values indicate whether a
source is likely to be spurious because it is dominated by an artifact
(encoded as an upper-case letter) or is a real source contaminated by an
artifact (encoded as a lower-case letter). Each of the four characters in
$cc\\_flags$ corresponds to artifacts in one of the four WISE bands, so the
first two characters are most relevant for CatWISE, which does not include W3
or W4 data. The possibilities are “0” for no artifact or contamination, “D” or
“d” for a diffraction spike, “H” or “h” for a scattered light halo, “O” or “o”
for an optical ghost, or “P” or “p” for charge persistence. The $cc\\_flags$
field conveys only the main features of the full artifact flag information
contained in the AllWISE $w1cc\\_map$, $w1cc\\_map\\_str$, $w2cc\\_map$, and
$w2cc\\_map\\_str$ fields,222Note that the column descriptions for these
fields in §II.1.a of the AllWISE Explanatory Supplement (Cutri et al., 2013)
are incorrect. Corrected descriptions are provided in Appendix A. which are
also provided in the CatWISE source entry. A more detailed description can be
found in §IV.4.g of the WISE All-Sky Explanatory Supplement (Cutri et al.,
2012).

The second type of artifact flag is called $ab\\_flags$, which was determined
for every CatWISE source and is based on artifact flagging images provided by
unWISE (unWISE bit masks; Meisner et al., 2019a). The $ab\\_flags$ do not
attempt to distinguish between an outright artifact and a real astrophysical
source that suffers from some level of artifact contamination, but are set so
that only egregious artifacts more likely to be spurious detections are
flagged. By analogy with $cc\\_flags$, the $ab\\_flags$ thus contain only
upper-case letters “D”, “H”,“O”, or “P” or the value “0”. Furthermore, because
the CatWISE data deal only with W1 and W2 data, the $ab\\_flags$ values are
only two characters long. Additional details are given in §3.5.2.

Figure 6 shows examples of both types of artifact flags. These $cc\\_flags$
and $ab\\_flags$ values should be regarded as two different yet complementary
methods for tagging sources of special concern to the user. As described in §
IV.4.g of the WISE All-Sky Explanatory Supplement (Cutri et al., 2012), the
$cc\\_flags$ are known to overflag; their purpose is to produce a reliable
catalog of source extractions free of contamination by artifacts, but they do
so at the expense of completeness. AllWISE $cc\\_flags$ are not available for
sources detected only in CatWISE, so requiring a source to have “0” in its
$cc\\_flags$ entry again emphasizes reliability over completeness. In contrast
the $ab\\_flags$, which are available for every CatWISE source, are used to
tag only the more egregious artifacts, thereby emphasizing completeness,
albeit at the expense of reliability. Users of the CatWISE data products can
thus query against these two sets of artifact flags to best fit their needs.

Figure 6: Example artifact flags for $16.5^{\prime}\times 13.0^{\prime}$
cutouts in W1 (top) and W2 (bottom) from the tile 1497p015. Detected CatWISE
sources are shown by the small cyan circles. Sources with upper-case
$cc\\_flags$ are marked with large red circles, and sources labeled as
artifacts by $ab\\_flags$ are marked in medium-sized blue circles.

####  3.5.1 Setting the $cc\\_flags$ Values When There are Multiple Matches

The values of $cc\\_flags$ were taken directly from a join of the AllWISE
Point Source Catalog and Reject Table. When more than one of these AllWISE
sources matched to within 2$\farcs$75 of a CatWISE source, the band-by-band
$cc\\_flags$ value of the CatWISE source was determined as follows. If there
was an upper-case letter for any matching source, it took precedence over any
lower-case letter. The letter “D” is the highest priority followed, in order,
by “P”, “H”, and “O”. If there were no upper-case letters for any matching
source, lower-case letters had the next highest priority, in the same order
(“d”, “p”, “h”, and “o”). If all matching sources had a value of “0” in that
band, then “0” was used.

For the binary bit encoding of AllWISE artifacts given by $w1cc\\_map$ and
$w2cc\\_map$, when there was more than one AllWISE match to a CatWISE source,
a logical ‘OR’ was performed over all the matches for each bit. The
$w1cc\\_map\\_str$ and $w2cc\\_map\\_str$ values were then constructed using
the priority rules given in the previous paragraph.

####  3.5.2 Translating the unWISE Bit Mask Values to $ab\\_flags$

The unWISE bit mask values at the (x,y) pixel location of the source were used
to set the value of $ab\\_flags$, as shown in Table 2, where we use the prefix
“b” to indicate specific unWISE mask bits in column 3. Bits 0–3 and 7–8 are
related to a nearby bright star; bits 11, 12, 25, and 26 indicate optical
ghosts; bits 13–20 flag charge persistence; bits 23 and 24 mark halos; and
bits 27–30 are for diffraction spikes. The checks against bits 21 and 22 are
to ensure that the bright star is not flagged by its own diffraction spikes or
scattered light halo. A complete description of the unWISE mask bits is given
in Meisner et al. (2019a).

Table 2: Logic Used in Converting unWISE Bitmask Values to ab_flags Values Value | Band | Logic
---|---|---
“D” | W1 | b0 or b1 or b7 or b27 or b29 but only if not b21
$\cdots$ | W2 | b2 or b3 or b8 or b28 or b30 but only if not b22
“H” | W1 | b23 but only if not b21
$\cdots$ | W2 | b24 but only if not b22
“O” | W1 | b25 or b26
$\cdots$ | W2 | b11 or b12
“P” | W1 | b13 or b14 or b17 or b18
$\cdots$ | W2 | b15 or b16 or b19 or b20

In addition to the $ab\\_flags$ field, the CatWISE catalog includes fields
with detailed artifact flag information ($w1ab\\_map$, $w1ab\\_map\\_str$,
$w2ab\\_map$, and $w2ab\\_map\\_str$) for each source, analogous to the
similar fields providing detailed artifact information related to
$cc\\_flags$.

###  3.6 Duplicate Source Measurements

WISE tiles, and, therefore, unWISE coadds, overlap each other by $\sim
3^{\prime}$ on the equator. As a result, sources appearing near the edge of a
tile will also be measured in neighboring tiles. To remove these duplicate
measurements we adopted the approach used by Schlafly et al. (2019). For each
source in a tile, its coordinates were used to calculate the minimum distance
to the edge of the tile. The same coordinates were used to calculate the
minimum distance to the edge for all neighboring tiles, and the source was
flagged as “primary” if these other minimum distances were all smaller.

###  3.7 The CatWISE Preliminary Catalog

With the necessary flagging in place, catalog generation could proceed.
CatWISE Preliminary Catalog sources are required to:

1) be from the tile where that source is furthest from the tile edge (i.e.
flagged as “primary,” §3.6)

and

2a) have W1 SNR $\geq 5$ with no identified artifacts (a value of 0 in the
left character of ab_flags)

or

2b) have W2 SNR $\geq 5$ with no identified artifacts (a value of 0 in the
right character of ab_flags).

There are 900,849,014 sources that meet these criteria. The 167,831,546
sources that fail to meet these criteria go into the reject file for their
tile. Individual tile reject files typically contain 8,000 sources, although
near the celestial poles they can contain over 30,000 sources due to large
tile overlap. There are 186 formatted columns of information about each source
in the tile catalog files. Reject files have one additional column, indicating
if the source is primary in its tile. Descriptions of most of the columns are
available in §II.1.a of the AllWISE Explanatory Supplement (Cutri et al.,
2013), and Appendix A provides additional information about CatWISE columns.

The individual Preliminary catalog and reject files for the 18,240 tiles were
transferred to the NASA Infrared Science Archive (IRSA), where they were
merged into the IRSA database. Four columns (with names in italics in Table 4)
were removed from view by IRSA, for reasons explained in that Table.
Information regarding access to the catalog is provided in §6.

CatWISE source designations should have the prefix CWISEP for objects in the
CatWISE Preliminary Catalog, and CWISEPR for objects in the CatWISE
Preliminary Reject Table. The designation for each source, based on its
coordinates for the J2000 equinox following the IAU truncation convention and
without the leading CWISEP or CWISEPR prefix, is given by the field
source_name which is the first column in the files. For example, the quasar 3C
273 is CWISEP J122906.70+020308.6333The CatWISE Preliminary Catalog
coordinates for 3C 273 are within 0$\farcs$011 of the Gaia DR2 coordinates.

##  4 Performance Characterization

The greatest potential for improvement of the CatWISE Preliminary Catalog over
existing WISE catalogs is in providing more accurate motion measurements, due
to the much longer time baseline compared to AllWISE. Hence, characterization
has focused on astrometric properties of the Preliminary Catalog. Photometric
depth is also improved due to the four times larger number of exposures than
AllWISE. Characterization has emphasized comparison to external truth sets,
using Spitzer for photometric comparisons including completeness and
reliability, and Gaia for astrometric comparisons. We begin with the
photometric assessments.

###  4.1 Completeness and Reliability

####  4.1.1 Bright Sources

CatWISE completeness and reliability for sources with W1 or W2 $<$8 mag were
assessed using an updated version of the WISE Bright Star List (BSL) as a
truth set. The list was generated by the WISE team for artifact flagging (see
§4.4.g.vi in the WISE All-Sky Release Explanatory Supplement; Cutri et al.,
2012). To avoid degradation of completeness or reliability due to missed
matches for fast-moving sources, astrometric information for sources moving
faster than 0$\farcs$275 yr-1 was added to the BSL to propagate the positions
of such stars to the CatWISE epoch. Astrometric information was taken from
Gaia DR2 (Gaia Collaboration et al., 2018; Lindegren et al., 2018) or, when
not available (given Gaia’s known incompleteness for bright stars) from the
Hipparcos main catalog (van Leeuwen, 2007), the Luyten Half-Second catalog
(Bakos et al., 2002), or the Gliese-Jahreiß catalog (Stauffer et al., 2010).

CatWISE completeness was determined as the percentage of sources that have
astrometric matches in CatWISE as a function of BSL magnitude. Differential
CatWISE reliability was determined as the percentage of sources that have
astrometric matches in the BSL as a function of CatWISE magnitude. We used a
relatively large matching radius of 5$\farcs$5 (corresponding to two WISE
pixels) to account for the poorer centroiding accuracy expected for highly
saturated sources.

Figure 7 shows the results for completeness, compared to those from AllWISE.
CatWISE achieves $\sim 99\%$ completeness in the BSL W1$\sim 5.5-8$ mag and
BSL W2$\sim 5-8$ mag ranges, slightly lower than AllWISE. For reasons that we
have not investigated, CatWISE completeness drops for brighter stars, falling
to $\sim 50\%$ by W1$\sim 4.3$ mag and W2$\sim 3.6$ mag. AllWISE completeness
remains above 90% even for stars as bright as 0.25 mag, making it preferable
to CatWISE for investigations of the brightest stars.

CatWISE reliability is $\sim 99\%$ for W1$\sim 4.8-8$ mag and W2$\sim 4.5-8$
mag, somewhat better than AllWISE, as can be seen in Figure 8. (For sources
without artifacts in either CatWISE or AllWISE, the median (CatWISE - AllWISE)
value over the 2.5 to 8 mag range is 0.17 mag in W1 and 0.02 mag in W2, and
the median absolute deviation is 0.08 mag in W1 and 0.02 mag in W2). Even
though completeness falls rapidly for brighter stars, CatWISE reliability is
$\sim 90\%$ or better down to 2.5 mag. For the brightest stars, CatWISE
completeness is poor, and CatWISE reliability shows large fluctuations due in
part to the resulting small number statistics.

Figure 7: Differential completeness of the CatWISE Preliminary Catalog as a
function of the Bright Star List’s W1 (left) and W2 (right), compared to
AllWISE.

Figure 8: Differential reliability of the CatWISE Preliminary Catalog as a
function of the Bright Star List’s W1 (left) and W2 (right), compared to
AllWISE. Not shown are points at W2=-0.75 mag and +0.75 mag with reliabilities
of 0 and 0.5 respectively.

####  4.1.2 Faint Sources

Completeness and reliability were assessed for faint sources using the Spitzer
South Pole Telescope Deep Field (SSDF) survey (Ashby et al., 2013) as a truth
set. The SSDF covers $\sim 94$ deg2 with 2 minutes of integration per position
with Spitzer. WISE survey depth increases towards the ecliptic poles, and for
the SSDF, with $\beta\sim-46\degree$, the typical number of CatWISE exposures
is 125, or 16 minutes. Nevertheless the larger Spitzer mirror diameter (85 cm
vs. 40 cm for WISE) and better image quality (2” vs. 6”) makes the SSDF data
more sensitive than CatWISE.

The radius used to match Spitzer SSDF sources with CatWISE sources was
2$\farcs$5, and otherwise the methodology to determine completeness and
reliability was the same as in §4.1.1. The results are shown in Figures 9 and
10.

At this coverage depth, the CatWISE Preliminary Catalog completeness is $\sim
98\%$ for sources brighter than 13th mag, consistent with the performance seen
from comparisons to the Bright Star List (§4.1.1), and remains above 90% for
sources brighter than [3.6] = 16 mag or [4.5] = 15.5 mag, dropping to 50% at
[3.6] = 17.8 mag and [4.5] = 17.4 mag. The SSDF achieves 55% completeness at
[3.6] = 18.5 mag and [4.5] = 18.0 mag (Ashby et al., 2013).

CatWISE reliability at this coverage depth is better than 99% for sources
brighter than 15th mag in both W1 and W2, again consistent with the Bright
Star List results. CatWISE reliability remains above 90% to W1 = 18.2 and W2 =
17.6 for this ecliptic latitude and coverage depth. Reliability at fainter
magnitudes is not well determined because the SSDF data are not deep enough.

Figure 9: Completeness of the CatWISE Preliminary Catalog vs. Spitzer
$3.6\mu$m (left) and $4.5\mu$m (right) magnitude for sources in the SSDF.

Figure 10: Reliability of the CatWISE Preliminary Catalog as a function of W1
(left) and W2 (right), for sources in the SSDF.

###  4.2 Photometric properties

We assessed CatWISE photometric depth using both the SSDF (§4.1.2) and the
COSMOS field. The COSMOS field is an important benchmark for assessing CatWISE
performance, because it has been intensively observed, is near the ecliptic
($\beta=-11\degree$), and is at fairly high Galactic latitude ($b=41\degree$).
Although confusion effects are reduced by the high Galactic latitude, the
ecliptic has the highest zodiacal emission and lowest survey coverage from
WISE, making COSMOS a representative base for performance estimates.

Figure 11 compares CatWISE Preliminary Catalog PSF-fitting photometry to
2$\farcs$9 radius aperture photometry from the Spitzer S-COSMOS program
(Sanders et al., 2007). These observations were obtained using longer
integration times (20 minutes) than for the SSDF, while the CatWISE
integration is lower (12 minutes) than in the SSDF, so the S-COSMOS data are
much deeper than CatWISE. The closest CatWISE source within 2$\farcs$75 was
taken as the match to the S-COSMOS source. Because the CatWISE photometry is
point source fitting, S-COSMOS sources were required to have $<10\%$ flux
increase between the 1$\farcs$9 and 2$\farcs$9 radius apertures. In addition,
because the W1 band is significantly bluer than the [3.6] band, S-COSMOS
sources at [3.6] were required to have $-0.1\leq[3.6]-[4.5]\leq 0$ (see
Figures 2 and 3 in §VI.3.a of the All-Sky Explanatory Supplement; Cutri et
al., 2012). While necessary for comparing W1 and [3.6] photometry, the color
cut significantly reduces the numbers of sources for comparison. Figure 12
gives the analogous comparison to photometry from the SSDF survey.

The comparison between the CatWISE and Spitzer photometry is consistent for
both fields, in both bands. CatWISE photometry becomes $\sim 0.1$ mag fainter
than Spitzer beyond 16th mag, possibly due to the increasing incidence of
extragalactic sources at faint magnitudes. The measured scatter in the SSDF
reaches 0.217 mag, equivalent to an SNR of 5, at [3.6] = 17.64 mag and [4.5] =
16.49 mag. Adjusting for the mean offsets in W1–[3.6] and W2–[4.5] at these
magnitudes, and subtracting 0.14 mag to adjust the SSDF 125 exposure depth to
the 96 exposure depth that we take as the baseline for the CatWISE Preliminary
Catalog, we find that the SNR=5 limits for CatWISE are W1=17.67 mag and
W2=16.47 mag.

Figure 11: Comparison of CatWISE photometry to Spitzer photometry for COSMOS.
Left: Difference between CatWISE W1 PSF and Spitzer S-COSMOS 2$\farcs$9 radius
aperture photometry at [3.6], for sources with $-0.1<[3.6]-[4.5]<0$ and $<10$%
flux increase from the 1$\farcs$9 to 2$\farcs$9 aperture. Median differences
and standard deviations in 0.5 mag bins are shown by the red points and error
bars. Right: The analogous comparison for CatWISE W2 and Spitzer [4.5]
photometry, but without the restriction on Spitzer source color.

Figure 12: Comparison of CatWISE photometry to Spitzer photometry for the
SSDF, using the same methodology as in Figure 11. The outer contour represents
a source density of 10 sources per $0.05\times 0.05$ mag bin, with each
additional contour showing a factor of two increase in source density. Note
the three additional contour levels on the right, due to the lack of the color
cut used on the left.

###  4.3 Astrometric properties

####  4.3.1 Full-sky Astrometric Assessments

The astrometric performance of CatWISE was assessed by comparing to Gaia DR2
(Gaia Collaboration et al., 2018; Lindegren et al., 2018). Within each tile,
the 10 brightest sources in bins of 0.5 mag over the $10<$W1$<17.5$ mag range
were selected, providing a sample of 150 sources per tile, uniformly
distributed on the sky. At low Galactic latitudes, CatWISE does not reach to
W1=17.5 mag, and therefore the faintest bins are empty. The sample for
astrometric comparison consists of 2,699,315 sources. These sources were
cross-matched with Gaia DR2 using a 5$\farcs$5 radius (corresponding to two
WISE pixels), requiring the Gaia counterpart to have measured proper motions.
This returned 2,148,274 unique matches. The completeness of the match
approaches $100\%$ for sources brighter than W1=14 mag, and drops to $\sim
25\%$ at the faintest magnitudes. Gaia astrometry was used to propagate the
Gaia counterparts to the CatWISE epoch, and the standard deviation444The IDL
“robust_sigma” function was used to calculate the standard deviation. between
the CatWISE motion-fit and Gaia position and motion values was computed.

Figure 13 summarizes the results of the full sky comparison. The positional
accuracy floor for bright sources approaches $\sim 50$ mas and remains
approximately constant until W1$\sim 12.5$ mag. At fainter magnitudes, the
dispersion increases to 275 mas (1/10 of a pixel) at W1$\sim 15.5$ mag, while
in the faintest magnitude bin the dispersion is $\sim 700$ mas, with the
dispersion in $\alpha$ being slightly better than in $\delta$. This is
expected since the scan direction is closer to $\delta$ and the PSF is more
elongated in the scan direction. At the bright end, however, the accuracy in
$\delta$ is better than in $\alpha$, an effect not fully understood.

The motion accuracy floor for bright stars is just under 10 mas yr-1,
consistent with the positional accuracy floor – given CatWISE’s 6 year
baseline, and since the motion accuracy scales linearly with time, one would
expect $\sigma_{\mu}\sim\sigma_{\rm pos}/6$. At W1$\sim$15.5 mag, the motion
accuracy is $\sim$30 mas yr-1, a factor of 10 better than AllWISE (Kirkpatrick
et al., 2014). At the faint end, CatWISE is sensitive to proper motion of 100
mas yr-1 until W1$\sim$17 mag.

The two panels on the right of Figure 13 show the $\chi^{2}$ computed using
the CatWISE catalog uncertainties, the Gaia catalog uncertainties, and the
uncertainty introduced by the translation of the Gaia position to the CatWISE
epoch. Since the total uncertainty is dominated by CatWISE, these $\chi^{2}$
values are essentially a measurement of how accurate the CatWISE Preliminary
Catalog uncertainties are. The expected value of the median $\chi^{2}$ with
one degree of freedom is $\sim 0.45$, so the top right panel of Figure 13
indicates that the catalog position uncertainties underestimate the actual
errors by a factor of 2.5–3. In contrast, the motion uncertainties are
consistent with the expected value. We have not investigated the reason for
this, although it suggests an additional error in position that is stable for
a given source. Confusion is one possible explanation. Although the
uncertainties are underestimated, the actual CatWISE position errors are
comparable to those for AllWISE (see §II.5.c of the AllWISE Explanatory
Supplement (Cutri et al., 2013).

Figure 13: CatWISE astrometric performace with respect to Gaia DR2. Left: the
1-$\sigma$ dispersion between CatWISE and Gaia R.A. (specifically,
$\Delta\alpha\ \textrm{cos}(\delta)$) and Dec. (top) and proper motion
(bottom), for a subsample of $\sim$2.1 million sources in the $10<$W1$<17.5$
mag range, uniformly distributed over the entire sky. Right: the median
$\chi^{2}$ computed taking into account CatWISE catalog uncertainties, Gaia
catalog uncertainties, and the uncertainty introduced by the translation of
Gaia’s positions to the CatWISE epoch.

The astrometric performance is however not uniform over the sky. Figures 14–17
show the 1-$\sigma$ dispersion in each tile with respect to Gaia positions and
motion components for the full magnitude range considered (Figures 14 and 16),
and in three smaller magnitude intervals (Figures 15 and 17). The maps for the
full magnitude range are smooth overall, indicating a fairly constant
astrometric performance for CatWISE over the majority of the sky. The main
features can be easily identified – the Galactic plane (and in particular the
bulge), and the Small and Large Magellanic Clouds (SMC and LMC). In those
denser regions, the astrometric accuracy for the bright stars deteriorates to
$\sim 500$ mas for positions and $\sim 30$ mas yr-1 for motions, and to $\sim
1,000$ mas and $\sim 200$ mas yr-1 (or worse) for the faint stars.

Figure 14: 1-$\sigma$ dispersion of the CatWISE $\alpha$ (left) and $\delta$
(right) with respect to Gaia DR2, for sources in the $10<$W1$<17.5$ mag range.

Figure 15: Same as Figure 14, but for three W1 magnitude ranges. Gray tiles
are those where there were no sources in CatWISE in the given magnitude bin.

Figure 16: Same as Figure 14, but for the proper motion components.

Figure 17: Same as Figure 16, but for three W1 magnitude ranges. Gray tiles
are those where there were no sources in CatWISE in the given magnitude bin.

The motion accuracy maps show additional features that appear to be related to
the WISE survey strategy, and to the transition between the cryogenic and
post-cryogenic phases of the mission. In particular, one can see better motion
performance approaching the ecliptic poles (at $l=96^{\circ};b=30^{\circ}$ and
$l=276^{\circ};b=-30^{\circ}$, respectively) compared to regions of similar
Galactic latitude. This is a result of the increased coverage of the sky by
WISE at higher ecliptic latitude. Conversely, there are two strips of slightly
poorer motion performance, which are coincident with the transition between
cryogenic and post-cryogenic phases, where the PSF was changing rapidly as the
telescope was warming up.

The maps for the faintest magnitude interval appear noisy. This is partly
intrinsic, and partly an effect of statistical fluctuations due to the low
number of sources with a Gaia counterpart in each tile (as discussed above,
the completeness for the Gaia match drops to $\sim 25\%$ at the faint end).
Moreover, at lower Galactic latitudes, CatWISE detects only brighter sources
because of confusion noise.

####  4.3.2 Astrometric Assessments in Selected Tiles

We next consider the astrometric performance in more detail by focusing on
four tiles:

  * •

the North Ecliptic Pole tile (NEP, tile 2709p666), a field with maximal WISE
coverage and average source density;

  * •

the South Ecliptic Pole tile (SEP, tile 0890m667) a field with with maximal
WISE coverage and high source density (the SEP tile contains part of the LMC);

  * •

the Galactic Center tile (GC, tile 2657m288) a field with average WISE
coverage and maximal source density; and

  * •

the COSMOS tile (tile ID 1497p015), representative of most of the sky, i.e. a
field with average WISE coverage and source density.

The analysis of §4.3.1 was repeated for these four tiles, but using all of the
sources in each tile. The results are shown in Figure 18 and 19. We then
defined ten metrics to characterize the astrometric performance of CatWISE:

  * •

$\sigma_{\rm min}$ and $\sigma_{\mu,\,\rm min}$ are the accuracy floor for
positions and motions, respectively. These accuracy floors are determined as
the median dispersion with respect to Gaia in the $8<$W1,W2$<10$ mag interval,
except in the GC where we restrict to $8<$W1,W2$<9$ mag since the astrometric
accuracy starts deteriorating significantly beyond W1,W2$\sim$9 mag (see
Figure 18–19).

  * •

W1min, W2min, W1${}_{\mu,\,\rm min}$, and W2${}_{\mu,\,\rm min}$ are the W1
and W2 mag at which $\sigma_{\rm min}$ and $\sigma_{\mu,\,\rm min}$ are
exceeded by no more than 20 mas and 5 mas yr-1, respectively.

  * •

W1500 and W2500 are the W1 and W2 mag at which the accuracy on positions
reaches 500 mas.

  * •

W1μ, 100 and W2μ, 100 are the W1 and W2 mag at which the accuracy on motion
reaches 100 mas yr-1.

The results for the four representative tiles are summarized in Table 3.

For brighter sources, $\sigma_{\rm min}$ and $\sigma_{\mu\,\rm min}$ are
encouragingly small, on the order of 40 mas and 8 mas yr-1, respectively.
W1min and W2min are $\sim 12$ mag while W1${}_{\mu,\,\rm min}$ and
W2${}_{\mu,\,\rm min}$ are $\sim 14.5$ mag. All the above metrics show little
dependence on coverage and source density, except for the GC tile, where they
are degraded to $\sigma_{\rm min}\sim 530$ mas, $\sigma_{\mu,\rm min}\sim 20$
mas, at a limiting depth of only $\sim$8 mag and $\sim$9 mag respectively.
This is consistent with the uniformity of the maps in Figures 14 to 17.

For fainter sources, W1500, W2500, W1μ, 100 and W2μ, 100 show a clearer
dependence on source density and coverage. For COSMOS (i.e. a typical CatWISE
tile) these metrics are $\sim$17 mag. W1500 and W2500 become $\sim$9 mag
shallower in the GC, while W1μ, 100 and W2μ, 100 measurements are upper limits
only. This is again a consequence of the fact that at lower Galactic latitudes
CatWISE only detects brighter sources because of confusion noise, and for
sources this bright the motion accuracy never degrades to the 100 mas yr-1
level.

W1μ, 100 and W2μ, 100 become $\sim$2 mag deeper ($\sim$19 mag) at the ecliptic
poles, thanks to the higher coverage. W1500 and W2500 are much deeper at the
NEP than the SEP, most likely an effect of the much higher source density at
the SEP, and therefore the higher confusion noise. However, this does not seem
to affect the motion metrics.

Table 3: CatWISE Astrometric Performance Evaluation Fields

| 0890m667 | 1497p015 | 2657m288 | 2709p666
---|---|---|---|---
| SEP, LMC | COSMOS | GC | NEP
$l$ (deg) | 276.5 | 237.3 | 359.8 | 96.4
$b$ (deg) | $-30.2$ | 41.4 | 0.6 | 29.5
$\beta$ (deg) | $-89.6$ | $-10.2$ | $-5.4$ | 89.6
Exp. | 7154 | 90 | 86 | 7839
$\\#$ | 71462 | 58961 | 63368 | 61702
$\sigma_{\rm min}$ (mas) | 52.9 | 27.3 | 526.4 | 37.7
W1min (mas) | 11.0 | 12.5 | 8.0 | 12.0
W1500 (mag) | 15.1 | 17.0 | 8.4 | 18.5
W2min (mag) | 11.0 | 12.5 | 8.0 | 12.0
W2500 (mag) | 15.0 | 16.8 | 8.3 | 19.0
$\sigma_{\mu,\rm min}$ (mas yr-1) | 7.4 | 8.5 | 22.2 | 7.3
W1μ,min (mag) | 14.5 | 13.5 | 9.0 | 15.5
W1μ,100 (mag) | 18.2 | 16.8 | $>11.0$ | $>19.0$
W2μ,min (mag) | 14.5 | 13.5 | 9.0 | 15.5
W2μ,100 (mag) |  $>$20.5 | 16.7 |  $>$11.5 |  $>$20.0

Note. — $l$, $b$, and $\beta$ are the Galactic longitude, Galactic latitude,
and ecliptic latitude for the center of the tile, in degrees. Exp. indicates
the number of exposures for the tile, $\\#$ the number of sources (combining
catalog and reject entries). The subsequent metrics are described in detail in
§4.3.

Figure 18: Same as Figure 13, but for four tiles: the SEP (0890m667), COSMOS
(1497p015), the GC (2657m288), and the NEP (2709p666). The standard deviation
between CatWISE position measurements and Gaia DR2 position measurements, and
the corresponding $\chi^{2}$ in 0.5 mag bins, is shown in red (R.A.) and blue
(Dec.).

Figure 19: Same as Figure 18, but for motion measurements.

####  4.3.3 Fast Movers

We compared the astrometric performance using merged results from ascending
and descending scans processed with separate PSF’s to that using a single PSF
per band for all scans processed together (see §3.4) on a set of 19 known
ultracool dwarfs, chosen from the literature. Our test set included extremely
cold, very fast moving objects (e.g. WISE J085510.83–071442.5, $\mu_{\rm
tot}\sim 8$ arcsec yr-1; Luhman, 2014), as well as warmer, slower M dwarfs
(e.g. WISE J072003.20–084651.2, $\mu_{\rm tot}\sim 0.12$ arcsec yr-1; Scholz,
2014). Results using separate PSF’s proved superior, delivering good motion
measurements for 17 out of the 19 test objects, while the single PSF option
only recovered 13 out of the 19 test objects, with a clear drop in performance
for objects with $\mu_{\rm tot}\gtrsim 2.5$ arcsec yr-1. Only one out of the
five fastest moving objects in our test sample is correctly measured using the
single PSF method, while using separate ascending and descending scan PSF’s
and merging the results measures four out of these five.

The two objects that our pipeline is unable to correctly measure – WISE
J163940.83–684738.6 and WISEPC J205628.90+145953.3 – are missed at the
detection stage (§3.3) because they are partly blended with brighter nearby
sources. These two sources were recovered when the detection step was run
without the PSF convolution (§3.1 and 3.3), yielding reasonable motions, but
they are not present in the CatWISE Preliminary Catalog.

The fastest movers in our test set have larger parallaxes (up to 502 mas for
WISE J104915.57-531906.1, Lazorenko & Sahlmann, 2018), which will affect
motion measurements when measuring ascending and descending scans together. To
test this, we also processed the fastest movers using a single PSF but
measuring ascending and descending scans separately and merging them. This
gave better performance than processing scans together, recovering motions
correctly for three of the five fastest movers.

Comparing astrometry to Gaia for all sources in these tiles showed that even
when measuring ascending and descending scans separately and merging them,
using separate PSFs further improves performance. The position sigmas were 10%
to 20% better at all magnitudes when using separate PSFs than when using a
single PSF. For brighter sources ($<12$ mag), motion sigmas improved by $\sim
20\%$ using separate PSFs, while at fainter magnitudes the single and separate
PSFs yielded similar motion sigmas. Processing ascending and descending scans
separately was beneficial both because it allowed the PSF asymmetry to be
addressed, and because it removed the effect of parallax on measured motions.

##  5 Example Science Applications

An initial application of the CatWISE catalog was the first secure W2
detection for the Y dwarf WD0806-661 B (Meisner et al., 2018b), leading to a
[4.5] - W2 color consistent with the population of known Y dwarfs. CatWISE
represents a major hunting ground for cold brown dwarfs. The improved depth
(§4.2) and motion sensitivity (§4.3) allows for a deeper, more complete search
for the coldest constituents of the solar neighborhood, a crucial population
if we wish to constrain the low-mass end of the mass function (see e.g.
Kirkpatrick et al., 2019).

Towards this goal we are mining CatWISE using both a “classical” and a
“machine learning based” approach. Here “classical” means a search based on
color and motion cuts applied to the catalog data to select cold brown dwarf
candidates, while machine learning uses the previously known population of
cold brown dwarfs as a training set to develop a classifier that is then
applied to the CatWISE catalog.

Early results include the discoveries of CWISEP J193518.59–154620.3 (Marocco
et al., 2019) and WISEP J144606.62–231717.8 (Marocco et al., 2020), two of the
coldest brown dwarfs identified to date, with infrared colors comparable with
those of the coldest brown dwarf known, WISE J085510.83–071442.5 (Luhman,
2014). Spitzer follow-up of additional cold brown dwarf candidates is
presented in Meisner et al. (2019b).

Prospects are also good for discovering distant galaxy clusters using CatWISE.
Gonzalez et al. (2019) used the AllWISE Catalog together with Pan-STARRS to
carry out the Massive and Distant Clusters of WISE Survey (MaDCoWS),
identifying over 2000 galaxy cluster candidates with photometric redshifts in
the $0.7-1.5$ range. The additional depth of CatWISE compared to AllWISE
increases the W1 detection limit for L* galaxies from $z\sim 0.8$ to $z\sim
2$. Figure 20 illustrates the increase in the number of distant galaxies with
substantial photometric redshifts in the COSMOS field (Laigle et al., 2016)
detected by CatWISE compared to AllWISE. Between $z=1.5$ and 1 clusters
transition from being more actively star-forming than the field to being
mainly quiescent in terms of star formation (e.g., Brodwin et al., 2013), so
CatWISE offers the potential of reaching the era where major formation and
assembly of clusters is underway.

Finally, the AllWISE Catalog played an important role in the discovery of ULAS
J1342+0928, at $z=7.54$ the most distant quasar known (Bañados et al., 2018).
The additional depth of CatWISE may enable extending this to redshifts as
large as 9, when the seeds of reionization were emerging in the overwhelmingly
neutral Universe. With over 3000 refereed papers making use of the existing
WISE catalogs, many other uses for CatWISE can be anticipated.

Figure 20: CatWISE detects sources $\sim 0.5$ mag fainter than AllWISE,
providing many times more sources at $z>1.5$. The vertical axis is the number
of COSMOS sources that were detected by CatWISE and AllWISE in the 1497p015
tile and had matches within 2$\farcs$5 of sources with the indicated
photometric redshifts from Laigle et al. (2016).

##  6 Data Access

Current information about CatWISE data products is provided at
https://catwise.github.io.

The merged Preliminary catalog and reject files are available from IRSA
(https://irsa.ipac.caltech.edu) in the WISE/NEOWISE Enhanced and Contributed
Products area. IRSA’s catalog search tools allow for complex search queries.
IRSA also hosts the AllWISE Explanatory Supplement (Cutri et al., 2013), which
provides full details on the AllWISE processing algorithms, and includes
descriptions of the AllWISE Catalog columns, many of which are applicable to
CatWISE. Appendix A provides additional information about CatWISE columns.

The individual tile files have also been transferred to a data repository at
the National Energy Research Scientific Computing Center (NERSC), and are
available at https://portal.nersc.gov/project/cosmo/data/CatWISE/prelim in
18,240 pairs of gzipped ASCII files (one catalog and one reject file per tile)
in IPAC table format, organized into 359 directories, one for each decimal
degree of right ascension from 0° to 358° (there are no tiles beginning with
359). Text files providing the format and a brief description of the columns
in the catalog and reject files are also provided there. The catalog and
reject files for the 50 tiles near the ecliptic poles (Table 1), where a
single PSF per band was used for processing, include the string “opt0” in
their names. Files for tiles where different PSFs were used for ascending and
descending scans (§3.2) include the string “opt1” in their names.

CatWISE was only possible because it uses and adapts software developed at
IPAC for WISE to the maximum extent practical. We thank Tim Conrow, Frank
Masci, and Ken Marsh for many helpful discussions and assistance in developing
CatWISE. We also thank the anonymous referee for a careful reading and
numerous suggestions which improved this paper. CatWISE uses data products
from WISE, which is a joint project of the University of California, Los
Angeles, and the Jet Propulsion Laboratory (JPL)/California Institute of
Technology (Caltech), funded by the National Aeronautics and Space
Administration (NASA), and from NEOWISE, which is a JPL/Caltech project funded
by NASA. Characterization of CatWISE performance uses data from Gaia and from
Spitzer. CatWISE is led by JPL/Caltech, with funding from NASA’s Astrophysics
Data Analysis Program (ADAP), and is also supported in part by ADAP grant
NNH17AE75I at Lawrence Berkeley Laboratory. CatWISE is also supported by the
Fellowships and Internships in Extremely Large Data Sets (FIELDS) program
funded by NASA at UC Riverside. FM acknowledges support from the NASA
Postdoctoral Program at the Jet Propulsion Laboratory, administered by
Universities Space Research Association under a contract with NASA.

##  Appendix A CatWISE Preliminary Catalog Column Descriptions

We adopt MJD 56700 (2014 Feb. 12) as the epoch for reporting positions when
solving for source motion in the Preliminary Catalog. Source positions,
whether incorporating source motion or not, are given in the equinox J2000
coordinate frame. Column entries involving right ascension that are in units
of arcseconds have the cos(declination) term applied. The convention for
designating sources from the CatWISE Preliminary Catalog and Reject Table is
specified in §3.7.

There are 186 formatted columns of information about each source in the
CatWISE Preliminary Catalog. The CatWISE Preliminary Reject Table adds a
column to indicate whether the source is “primary” (see §3.6). Descriptions of
nearly all the columns can be obtained from IRSA at
https://irsa.ipac.caltech.edu/data/WISE/CatWISE/gator_docs/catwise_colDescriptions.html.
Most of the columns have the same names as in the AllWISE Catalog, and are
described in §II.1.a of the AllWISE Explanatory Supplement (Cutri et al.,
2013). Table 4 provides information about selected columns in the CatWISE
Preliminary Catalog that augments or supersedes the information provided by
IRSA. Four columns (w1fitr, w2fitr, glon, glat) in the NERSC release are
hidden in the IRSA release, and their names are shown in parentheses in Table
4.

Table 4: Additional Information on CatWISE Preliminary Catalog Columns Name | Units | Type | Format | Description
---|---|---|---|---
(w1fitr) | arcsec | R*4 | f7.2 |  Fitting radius for W1. In the CatWISE Preliminary Catalog, w1fitr and w2fitr are always -99.99, 7.5, or 13.2. Both are hidden in the IRSA release, because they do not appear to follow the prescription given in http://wise2.ipac.caltech.edu/docs/release/allsky/expsup/sec4_4c.html#wpro where rfit=1.25 FWHM (7$\farcs$5 for W1 and W2) or $2r_{sat}$ (saturation radius), whichever is larger.
(w2fitr) | arcsec | R*4 | f7.2 |  Fitting radius for W2. See w1fitr.
dist_ad | arcsec | R*4 | f9.3 |  Radial distance between ascending and descending apparitions. The field name in the IRSA release is dist_ad, but in the NERSC release, this field is called dist.
elon_avg | deg | R*8 | f11.6 |  Averaged ecliptic longitude between ascending and descending apparitions
elat_avg | deg | R*8 | f11.6 |  Averaged ecliptic latitude between ascending and descending apparitions
chi2pmra | $\cdots$ | R*4 | 1pE10.3 |  $\chi^{2}$ for difference between PMRA for ascending and descending apparitions (1 degree of freedom)
chi2pmdec | $\cdots$ | R*4 | 1pE10.3 |  $\chi^{2}$ for difference between PMDec for ascending and descending apparitions (1 degree of freedom)
par_pm | arcsec | R*4 | f11.3 |  Parallax estimate from motion solution. Computed using PMRA and PMDec to propagate ascending and descending motion solution positions to MJD 56700, then dividing ecliptic longitude difference by 2. Because scan longitudes on opposite sides of orbit had a $5\degree$ bias from a great circle until the end of 2014, gradually increasing to $18\degree$ by the end of 2017, the actual baseline is $<2$au, declining from 1.996 to 1.951 au over this period, although the difference is negligible given the uncertainty in parallax. Will be null unless km = 3.
par_stat | arcsec | R*4 | f11.3 |  Parallax estimate from stationary solution. Computed using PMRA and PMDec to propagate ascending stationary-solution position from ascending observation epoch to descending observation epoch, then dividing ecliptic longitude difference by 2 (see entry for par_pm regarding dividing by 2). Will be null unless ka = 3, km $>0$, and all w?mjdmin/max/mean values are non-null in both ascending and descending WPHOT output.
dist_cc | arcsec | R*4 | f13.3 |  Maximum distance between CatWISE and AllWISE sources providing cc_flags. See §3.5.1. In the NERSC release, this field is called dist_x.
cc_flags | $\cdots$ | char | a16 |  Worst case character cc_flag from AllWISE. See §3.5.1.
w1cc_map | $\cdots$ | int | i13 |  The description in the AllWISE Explanatory Supplement is incorrect, so an updated description is provided here. Worst case contamination and confusion map in W1 for matching AllWISE sources. Contains integer equivalent of 12-bit binary number specifying if W1 measurement in AllWISE is believed to be contaminated by or a spurious detection of an image artifact. Elements of binary array (with bit number indicated below each code) are given in Table 5. Bits 0, 1, 3 and 4 indicate whether band-detection is contaminated by an artifact. If brightness of detection is less than expected for artifact, source is regarded as spurious and bits 7, 8, 10, and 11 are also set accordingly. Letters denote contamination by different types of artifacts (see §3.5). For example, a measurement believed to be a spurious detection of a scattered light halo and contaminated by a diffraction spike has a binary bit map value of “010000001001” and w1cc_map=1033. When there is more than one AllWISE match to a CatWISE source, a logical ‘OR’ is performed over all the matches for each bit.
w1cc_map_str | $\cdots$ | char | a20 |  The w1cc_map_str description in the AllWISE Explanatory Supplement is incorrect, so an updated description is provided here. Worst case contamination and confusion string in W1 for matching AllWISE sources. This character string denotes all artifacts that may contaminate the AllWISE W1 measurement of this source, in the priority order D,d,P,p,H,h,O,o. For example, a real detection contaminated by a diffraction spike and a latent (persistent) image has a w1cc_map_str=“dp”. A spurious detection of a diffraction spike also contaminated by a latent image has w1cc_map_str=“Dp”. This string is “null” if there are no artifacts that affect the measurement in this band. The value that appears in first element of the cc_flags string is the left-most character in w1cc_map_str. If w1cc_map_str is “null”, then the corresponding cc_flags entry is “0”.
w2cc_map | $\cdots$ | int | i13 |  Worst case w2cc_map from AllWISE. See w1cc_map.

w2cc_map_str | $\cdots$ | char | a20 |  Worst case w2cc_map_str from AllWISE. See w1cc_map_str.
ab_flags | $\cdots$ | char | a9 |  Two character (W1 W2) artifact flag. See §3.5.2.
w1ab_map | $\cdots$ | int | i9 |  W1 artifact code value. See §3.5.2.
w1ab_map_str | $\cdots$ | char | a13 |  W1 artifact string. See §3.5.2.
w2ab_map | $\cdots$ | int | i9 |  W2 artifact code value. See §3.5.2.
w2ab_map_str | $\cdots$ | char | a13 |  W2 artifact string. See §3.5.2.
(glon) | deg | R*8 | f12.6 |  Galactic longitude, calculated using a pole of $\alpha=192.85\degree\ \delta=27.13\degree$, and a zero of longitude of $\alpha=266.4\degree\ \delta=-28.94\degree$ (both in J2000). Hidden in IRSA. See Appendix B for details.
(glat) | deg | R*8 | f12.6 |  Galactic latitude. Hidden in IRSA. See glon.
p | $\cdots$ | int | i6 |  Flag to indicate if source is “primary” in tile. See §3.6.

Table 5: cc_map Bit Definitions O | H | 0 | P | D | 0 | 0 | o | h | 0 | p | d
---|---|---|---|---|---|---|---|---|---|---|---
11 | 10 | 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0

##  Appendix B Caveats

The CatWISE Preliminary Catalog contains a number of features that users
should be aware of. Among these are:

  * •

The number of sources per square degree has relatively small variation over
the sky (Figure 5).

  * •

Catalog performance is less good in high source density regions (the Galactic
plane and the ecliptic poles). Figures 14 through 19 and Table 3 illustrate
this.

These issues appears to arise in the detection step. Relatively few deblended
sources are added in the measurement step. We are using the unWISE Catalog
(which contains many more sources in high density regions; Schlafly et al.,
2019) as a detection list for an updated version of the CatWISE catalog that
is expected to be available in 2020.

  * •

The completeness and reliability for bright sources is low (Figure 7).

As an example, the brightest two catalog sources in W2 (CWISEP
J005153.01–235140.3 and CWISEP J223327.80+060246.2) are spurious, and it is
likely that other spurious bright sources are present in the catalog. Users
should visually inspect interesting sources selected from the catalog using
AllWISE or unWISE images before devoting significant resources to follow up
observations.

Additional features present in the CatWISE Preliminary Catalog include:

  * •

Tabulated position uncertainties are significantly smaller than measured
position scatter with respect to Gaia, as illustrated in Figure 13 (upper
right) and Figure 18 (right).

  * •

A floor of 10 mas yr-1 was imposed on the tabulated motion uncertainties,
making them significantly larger than the measured scatter with respect to
Gaia motion, as illustrated in Figure 13 (lower right) and Figure 19 (right).

  * •

The Galactic coordinates in the CatWISE Preliminary Catalog were calculated
using a pole of $\alpha=192.85\degree\ \delta=27.13\degree$, and a zero of
longitude of $\alpha=266.4\degree\ \delta=-28.94\degree$ (both in J2000).
These directions are not perpendicular by 22$\farcs$663, and consequently
yielded “nan” entries for the Galactic coordinates of the 31 catalog sources
and 6 reject sources within 88$\farcs$3 of the north Galactic pole, and the 39
catalog sources and 5 reject sources within 87$\farcs$8 of the south Galactic
pole. The discrepancy in the glon and glat Galactic coordinates of sources
compared to coordinates calculated using the IAU-defined system (Blaauw et
al., 1960) increases with absolute Galactic latitude from $<1"$ in the
Galactic plane to $88"$ near the Galactic poles. These glon and glat values
(including the 81 with glat = “nan”) appear in the NERSC release of the
CatWISE Preliminary catalog and reject table files. In the IRSA release, these
columns are hidden, and in addition the 81 glat “nan” values have been
replaced with $+90\degree$ or $-90\degree$ as appropriate.

##  Appendix C Combining Ascending and Descending Scan Positions

Here we provide additional details on how ascending and descending scan
positions are combined for the CatWISE Preliminary Catalog. Positions are
averaged using inverse-covariance weighting. The averaging is done in a local
Cartesian projection consistent with the uncertainty representation.

A transformation matrix T is defined as follows: starting with a Cartesian
$(x,y,z)$ system whose z axis points to the celestial north pole and whose x
axis points to the vernal equinox, we perform two Euler rotations that place
the z’ axis of the rotated system on the ascending celestial $(\alpha,\delta)$
position with the y’ axis aligned with the local north-south direction, and
the x’ axis aligned with the local east-west direction. First, rotate about
the z axis by $\phi_{1}=\alpha-90^{\circ}$, then rotate about the x’ axis by
$\phi_{2}=\delta-90^{\circ}$. Then T is given by:

$T\equiv\begin{bmatrix}T_{11}&T_{12}&T_{13}\\\ T_{21}&T_{22}&T_{23}\\\
T_{31}&T_{32}&T_{33}\end{bmatrix}$ (C1)

with elements:

$\displaystyle T_{11}=\cos\phi_{1}$ (C2) $\displaystyle T_{12}=\sin\phi_{1}$
$\displaystyle T_{13}=0$ $\displaystyle T_{21}=-\cos\phi_{2}\sin\phi_{1}$
$\displaystyle T_{22}=\cos\phi_{2}\cos\phi_{1}$ $\displaystyle
T_{23}=\sin\phi_{2}$ $\displaystyle T_{31}=\sin\phi_{2}\sin\phi_{1}$
$\displaystyle T_{32}=-\sin\phi_{2}\cos\phi_{1}$ $\displaystyle
T_{33}=\cos\phi_{2}$

This corresponds to a z’ axis that looks outward from the origin, so any
nearby $\alpha^{\prime},\delta^{\prime}$ position will have an
$(x^{\prime},y^{\prime},z^{\prime})$ vector in the rotated system whose
$z^{\prime}$ component will be positive and close to 1. By “nearby” we mean
within a few arcseconds of the origin, since it is very rare for an extracted
source’s position to vary between the ascending and descending solutions by
more than that. This justifies our Cartesian approximation.

The coordinate system is computed for the ascending position, so that the
ascending position of the source has coordinates $(0,0,1)$. The descending
position $(\alpha^{\prime},\delta^{\prime})$ is mapped into that system as
follows. We construct the vector v to the descending position in the original
celestial coordinate system:

$\displaystyle v_{1}=\cos\alpha^{\prime}\cos\delta^{\prime}$ (C3)
$\displaystyle v_{2}=\sin\alpha^{\prime}\cos\delta^{\prime}$ $\displaystyle
v_{3}=\sin\delta^{\prime}$

and transform it into the new system,

$(x^{\prime},y^{\prime},z^{\prime})\equiv Tv$ (C4)

wherein its $x^{\prime}$ and $y^{\prime}$ coordinates are the offsets of the
descending position from the ascending position.

The $(x^{\prime},y^{\prime})$ coordinates are averaged with (0,0) using
inverse covariance weighting. We construct $2\times 2$ error covariance
matrices $\Omega_{a}$ and $\Omega_{d}$ for the ascending and descending
vectors using the corresponding $\sigma_{\alpha}$, $\sigma_{\delta}$, and
$\sigma_{\alpha\delta}$:

$\begin{split}\Omega_{a}=\begin{bmatrix}\sigma^{2}_{\alpha}&\sigma_{\alpha\delta}|\sigma_{\alpha\delta}|\\\
\sigma_{\alpha\delta}|\sigma_{\alpha\delta}|&\sigma^{2}_{\delta}\end{bmatrix}_{a}\\\
\Omega_{d}=\begin{bmatrix}\sigma^{2}_{\alpha}&\sigma_{\alpha\delta}|\sigma_{\alpha\delta}|\\\
\sigma_{\alpha\delta}|\sigma_{\alpha\delta}|&\sigma^{2}_{\delta}\end{bmatrix}_{d}\end{split}$
(C5)

A minimum value of $10^{-8}$ is enforced for any zeroes on the diagonal. The
merged (i.e., inverse-covariance-weighted average) vector
($x_{m},y_{m},z_{m}$) and associated covariance matrix are computed as
follows:

$W_{a}=\Omega_{a}^{-1},\;\;W_{d}=\Omega_{d}^{-1}$ (C6)
$\Omega=(W_{a}+W_{d})^{-1}\equiv\begin{bmatrix}\Omega_{11}&\Omega_{12}\\\
\Omega_{21}&\Omega_{22}\end{bmatrix}$ (C7)
$\displaystyle\begin{bmatrix}x_{m}\\\
y_{m}\end{bmatrix}=\Omega\left(W_{a}\begin{bmatrix}0\\\
0\end{bmatrix}+W_{d}\begin{bmatrix}x^{\prime}\\\
y^{\prime}\end{bmatrix}\right)=\Omega\,W_{d}\begin{bmatrix}x^{\prime}\\\
y^{\prime}\end{bmatrix}$ (C8) $\displaystyle
z_{m}=\sqrt{1-x^{2}_{m}-y^{2}_{m}}$
$\sigma_{\alpha}=\sqrt{\Omega_{11}},\;\;\sigma_{\delta}=\sqrt{\Omega_{22}},\;\;\sigma_{\alpha\delta}={\rm
sign}(\Omega_{12})\sqrt{|\Omega_{12}|}$ (C9)

The celestial coordinates corresponding to the $(x_{m},y_{m},z_{m})$ vector
are obtained using the inverse of the transformation matrix T described above,
which is the transpose because T is orthonormal:

$\begin{bmatrix}v^{\prime}_{1}\\\ v^{\prime}_{2}\\\
v^{\prime}_{3}\end{bmatrix}\equiv\begin{bmatrix}T_{11}&T_{21}&T_{31}\\\
T_{12}&T_{22}&T_{32}\\\
T_{13}&T_{23}&T_{33}\end{bmatrix}\begin{bmatrix}x_{m}\\\ y_{m}\\\
z_{m}\end{bmatrix}$ (C10)

$\alpha_{m}=\tan^{-1}{\left(\frac{v^{\prime}_{2}}{v^{\prime}_{1}}\right)},\;\;\delta_{m}=\sin^{-1}{v^{\prime}_{3}}$
(C11)

The image pixel coordinates are then computed for the merged position using
the coadd WCS information.

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