# NIRCam Performance on JWST In Flight

Marcia J. Rieke Steward Observatory, University of Arizona, 933 N. Cherry Ave,
Tucson, AZ 85721, USA Marcia J. Rieke<EMAIL_ADDRESS>Douglas M. Kelly
Steward Observatory, University of Arizona, 933 N. Cherry Ave, Tucson, AZ
85721, USA  Karl Misselt  Steward Observatory, University of Arizona, 933 N.
Cherry Ave, Tucson, AZ 85721, USA  John Stansberry  Space Telescope Science
Institute, 3700 San Martin Drive, Baltimore, MD 21218  Martha Boyer  Space
Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218
Thomas Beatty  Department of Astronomy, University of Wisconsin, Madison,
Madison, WI 53706  Eiichi Egami  Steward Observatory, University of Arizona,
933 N. Cherry Ave, Tucson, AZ 85721, USA  Michael Florian  Steward
Observatory, University of Arizona, 933 N. Cherry Ave, Tucson, AZ 85721, USA
Thomas P. Greene NASA Ames Research Center, Space Science and Astrobiology
Division, MS 245-6, Moffett Field, CA, 94035, USA  Kevin Hainline Steward
Observatory, University of Arizona, 933 N. Cherry Ave, Tucson, AZ 85721, USA
Jarron Leisenring Steward Observatory, University of Arizona, 933 N. Cherry
Ave, Tucson, AZ 85721, USA  Thomas Roellig  NASA Ames Research Center, Space
Science and Astrobiology Division, MS 245-6, Moffett Field, CA, 94035, USA
Everett Schlawin Steward Observatory, University of Arizona, 933 N. Cherry
Ave, Tucson, AZ 85721, USA  Fengwu Sun Steward Observatory, University of
Arizona, 933 N. Cherry Ave, Tucson, AZ 85721, USA  Lee Tinnin  Steward
Observatory, University of Arizona, 933 N. Cherry Ave, Tucson, AZ 85721, USA
Christina C. Williams  National Optical-Infrared Research Laboratory , 950 N
Cherry Ave, Tucson, AZ 85719  Christopher N. A. Willmer Steward Observatory,
University of Arizona, 933 N. Cherry Ave, Tucson, AZ 85721, USA  Debra Wilson
Steward Observatory, University of Arizona, 933 N. Cherry Ave, Tucson, AZ
85721, USA  Charles R. Clark  Goddard Space Flight Center, Greenbelt, MD
Scott Rohrbach  Goddard Space Flight Center, Greenbelt, MD  Brian Brooks
Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218
Alicia Canipe  Space Telescope Science Institute, 3700 San Martin Drive,
Baltimore, MD 21218  Matteo Correnti  INAF, Osservatorio Astronomico di Roma,
via Frascati 33, 00078, Monteporzio Catone, Rome, Italy  ASI-Space Science
Data Center, Via del Politecnico, I-00133, Rome, Italy  Audrey DiFelice  Space
Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218  Mario
Gennaro  Space Telescope Science Institute, 3700 San Martin Drive, Baltimore,
MD 21218  Julian Girard  Space Telescope Science Institute, 3700 San Martin
Drive, Baltimore, MD 21218  George Hartig  Space Telescope Science Institute,
3700 San Martin Drive, Baltimore, MD 21218  Bryan Hilbert  Space Telescope
Science Institute, 3700 San Martin Drive, Baltimore, MD 21218  Anton M.
Koekemoer Space Telescope Science Institute, 3700 San Martin Drive, Baltimore,
MD 21218  Nikolay K. Nikolov  Space Telescope Science Institute, 3700 San
Martin Drive, Baltimore, MD 21218  Norbert Pirzkal  Space Telescope Science
Institute, 3700 San Martin Drive, Baltimore, MD 21218  Armin Rest Space
Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218
Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD
21218  Massimo Robberto  Space Telescope Science Institute, 3700 San Martin
Drive, Baltimore, MD 21218  Johns Hopkins University, 3400 N. Charles Street,
Baltimore, MD 21218, USA  Ben Sunnquist  Space Telescope Science Institute,
3700 San Martin Drive, Baltimore, MD 21218  Randal Telfer  Space Telescope
Science Institute, 3700 San Martin Drive, Baltimore, MD 21218  Chi Rai Wu
Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218
Malcolm Ferry  Lockheed Martin Advanced Technology Center, 3251 Hanover St.,
Palo Alto, CA 94304  Dan Lewis  Lockheed Martin Advanced Technology Center,
3251 Hanover St., Palo Alto, CA 94304  Stefi Baum  Faculty of Science, 230
Machray Hall, 186 Dysart Road, University of Manitoba, Winnipeg, MB Canada R3T
2N2  Charles Beichman NASA Exoplanet Science Institute/IPAC, Jet Propulsion
Laboratory, California Institute of Technology, 1200 E California Blvd,
Pasadena, CA 91125  René Doyon  Département de physique, Université de
Montréal, C.P. 6128, Succursale Centre-Ville, Montréal, QC Canada H3C 3J7
Alan Dressler  The Observatories, The Carnegie Institution for Science, 813
Santa Barbara St., Pasadena, CA 91101, USA  Daniel J. Eisenstein Center for
Astrophysics $|$ Harvard & Smithsonian, 60 Garden St., Cambridge MA 02138 USA
Laura Ferrarese  National Research Council Canada, Herzberg Astronomy and
Astrophysics, 5071 West Saanich Rd., Victoria, BC, Canada V9E 2E7  Klaus
Hodapp Institute for Astronomy, 640 N Aohoku Pl Hilo, HI 96720, USA  Scott
Horner  881 Spinosa Dr., Sunnyvale, CA 94087  Daniel T. Jaffe  The University
of Texas at Austin, Department of Astronomy RLM 16.342, Austin, TX 78712  Doug
Johnstone  National Research Council Canada, Herzberg Astronomy and
Astrophysics, 5071 West Saanich Rd., Victoria, BC, Canada V9E 2E7  Department
of Physics and Astronomy, University of Victoria, Victoria, BC, V8P 5C2,
Canada  John Krist  Jet Propulsion Laboratory, 4800 Oak Grove Drive M/S
183-900, Pasadena, CA 91109  Peter Martin  Canadian Institute for Theoretical
Astrophysics, University of Toronto, McLennan Physical Laboratories, 60 St.
George Street, Toronto, Ontario, Canada M5S 3H8  Donald W. McCarthy  Steward
Observatory, University of Arizona, 933 N. Cherry Ave, Tucson, AZ 85721, USA
Michael Meyer  Department of Astronomy, University of Michigan, 1085 S.
University, Ann Arbor, MI 48109  George H. Rieke Steward Observatory,
University of Arizona, 933 N. Cherry Ave, Tucson, AZ 85721, USA  John Trauger
Jet Propulsion Laboratory, 4800 Oak Grove Drive M/S 183-900, Pasadena, CA
91109  Erick T. Young  USRA, Mountain View, CA, USA

###### Abstract

The Near Infrared Camera for the James Webb Space Telescope is delivering the
imagery that astronomers have hoped for ever since JWST was proposed back in
the 1990s. In the Commissioning Period that extended from right after launch
to early July 2022 NIRCam has been subjected to a number of performance tests
and operational checks. The camera is exceeding pre-launch expectations in
virtually all areas with very few surprises discovered in flight. NIRCam also
delivered the imagery needed by the Wavefront Sensing Team for use in aligning
the telescope mirror segments (Acton et al 2022, McElwain et al 2022).

infrared camera, HgCdTe detector

##  1 Introduction

All of the original concepts for what we now call the James Webb Space
Telescope (JWST) included a relatively large field-of-view camera spanning at
least the $1$ to $5\mu m$ range. NIRCam’s science role was initially focused
on deep surveys to find the most distant galaxies (Stockman 1997), and of
course, an instrument capable of finding these galaxies is also capable of
many other observations. The deep survey use drove the need for as large a
field of view as possible since no one knew how common galaxies at $z>10$
would be at the time NIRCam was being designed. What was known is that
photometric redshifts would be a powerful tool and so NIRCam would need a
filter set that spanned its entire wavelength range, with filter widths that
needed to be optimized between sensitivity and wavelength discrimination.

Over time the role of NIRCam expanded to include providing the imaging used
for wavefront sensing and telescope alignment, which rendered NIRCam a
mission-critical element. The final NIRCam design incorporates two copies
(modules) of the camera to provide full block redundancy to maximize
availability of the wavefront sensing capability. Use of NIRCam as the
wavefront sensor also meant that NIRCam must have very small intrinsic
wavefront error so it does not imprint errors on the other instruments (Acton
et al (2004), Acton et al (2018)).

In the final NIRCam design, two modules provide 2.2 arc min x 2.2 arc min
fields of view each as shown in Figure 1. Each field of view can be observed
using two filters simultaneously through the use of dichroics that split the
wavelength range into $0.6\mu m$ to $2.3\mu m$ and $2.4\mu m$ to $5\mu m$ with
a deadband from $2.3\mu m$ to $2.4\mu m$, effectively doubling the field for
programs needing full filter coverage. NIRCam also includes components to
enable coronagraphy; these do not affect the area available for direct
imaging. Grisms in the long wavelength arms, originally included to expand the
coarse phasing capture range, can be used to provide a time-series
spectroscopic capability useful for exoplanet observations and to provide wide
field spectroscopy, a powerful took for finding emission-line galaxies.

Figure 1: NIRCam’s location in the JWST field of view. Designations for the
short wavelength arrays are shown with the long wavelength arrays called
NRCALONG and NRCBLONG with the fields of view outlined in blue. The long
wavelength arrays cover the same area as the four short wavelength arrays in a
module. The small green squares show the locations of the coronagraphic
fields. V2 and V3 refer to the telescope coordinate system where V3 points
away from the sunshield and V2 is perpendicular to V3 to form a right-handed
coordinate system.

Lockheed Martin’s Advanced Technology Center led the design and construction
of NIRCam. The University of Arizona Team developed the focal plane assemblies
in collaboration with Teledyne Imaging Systems. Arizona also developed the
initial pipeline and reference files which were used by Space Telescope
Science Institute in producing the first flight pipeline. The Space Telescope
Science Institute maintains documentation about the telescope and instruments
including NIRCam at https://jwst-docs.stsci.edu/.

##  2 Design and Construction

The NIRCam design was driven by wanting a large field of view for efficient
surveys and by the wavefront error requirement imposed by its use as the
mission wavefront sensor. NIRCam is required to have less than 90 nm of
absolute wavefront error at $2.12\mu m$. The ground-to-flight change in
wavefront error is required to be less than 40 nm (Huff 2005). The redundancy
imposed by being the wavefront sensor meshes well with the efficient survey
need as having two complete copies of NIRCam doubles the field of view.
Assembly and test of the two modules was facilitated by including a tip-tilt
focus mechanism in each module. This mechanism can be used to align the NIRCam
pupil with the telescope as required for wavefront sensing and the coarse
phasing step of aligning the telescope. The ability to adjust the focus of
each module separately relaxed the need for each module to have precisely the
same focus. The fields of view of the two modules are separated by $\sim 44$
arc sec and cover a total of 9.7 square arc minutes for one wavelength.

The survey use drove the choice of using dichroics to split the light into
short and long wavelength optical trains in each of the redundant NIRCam
modules. Figure 2 presents the light path and layout of one module. Note that
this figure shows the pupil imaging lens in the deployed position rather than
the usual retracted position. The wavelength split also facilitates the
matching of pixel scales to diffraction-limited sampling so NIRCam provides
Nyquist sampling of the point spread function at 2 $\mu m$ and at 4 $\mu m$.
The overlap between the short wavelength and long wavelength fields of view is
$\sim 96\%$ in both modules. Each arm in a module has two wheel mechanisms
mounted together with one wheel carrying mainly filters and the other wheel
carrying optical elements that need to be used at a pupil as well some
additional filters. The wheels are referred to as the filter wheel and the
pupil wheel. They are moved independently.

Another benefit of the short wavelength / long wavelength split is that
detector arrays (referred to as SCAs for Sensor Chip Assemblies) with
different cut-off wavelengths and different anti-reflection coatings could be
used. Teledyne Imaging Systems fabricated the arrays for NIRCam. NIRCam uses
eight 2.5 $\mu m$ cut-off SCAs and two 5 $\mu m$ cut-off SCAs. It is somewhat
easier to make the 2.5-$\mu m$ devices than the 5-$\mu m$ devices so the
wavelength split eased the production process for this relatively large number
of arrays. Last, the wavelength split also eased the blocking requirements on
NIRCam’s bandpass filters. The short wavelength filters do not need blocking
beyond the response band of the 2.5-$\mu m$ detectors. The dichroics have
silicon substrates so the long wavelength filters do not need blocking below
$1.08\mu m$.

A key design choice for NIRCam was the decision to use a refractive design.
This choice was motivated by such a design yielding a more compact and less
massive instrument. Some of the very early designs for NIRCam using reflective
optics yielded an instrument so large that there may not have been room and
mass for a fourth instrument on JWST. The refractive design choice also helped
enable the fully redundant design.

Figure 2: One half of NIRCam showing its optical train and mechanisms.
Starlight from the telescope enters the instrument from the upper left, enters
the module after reflection from the pick-off mirror (1) and then reflects
into the main body of the camera where the dichroic beamsplitter (5) separates
the long and short wavelength light. The other module is a mirror image of the
one shown here, mounted back-to-back behind what is shown.

##  3 Wavefront Sensing

The JWST primary mirror has eighteen fully adjustable hexagonal segments that
must be aligned to perform like a single, monolithic mirror. NIRCam was used
to locate all of the segment images after launch and provided the imagery for
the initial mirror alignment. The first step in phasing the eighteen segments
after segment-level adjustments were completed using dispersed Hartmann
sensors (DHSes) mounted in the short wavelength pupil wheel to provide spectra
at $R\sim 300$ resulting from the interference of two mirror segments. The
F150W2 filter includes two notches in its spectral response shown in Figure 3
which provides the wavelength calibration needed to convert the interference
pattern into a piston difference between the segments. Each DHS spans ten
segment pairs and produces ten spectra. The two DHSes are mounted at 60
degrees with respect to each other to provide 20 separate measurements to
adjust the segments in piston. Relatively large focus steps between segments
can be measured using these data, and were a key step in the initial mirror
alignment but not needed for routine wavefront sensing.

Figure 3: Throughputs of NIRCam’s filters. All terms affecting throughpout
including detector quantum efficiency and telescope reflectivity are included.
The gray bar denotes the dichroic deadband. The letter P indicates filters
that are mounted in the pupil wheels.

Every two days NIRCam takes images for focus diverse phase retrieval to
monitor the alignment of the primary mirror. Mirror adjustments are then
executed when the alignment has drifted to where the overall observatory
wavefront error has increased to $\sim 80$ nm. These adjustments have been
needed on average about every 17 days. Focus diversity is achieved by using
weak lenses in the short wavelength arm with one mounted in the filter wheel
and two in the pupil wheel. These lenses defocus the star light by either +/-
4 waves, +/- 8 waves, and as much as +12 waves using +4 and +8 in series. The
wavefront sensing is executed at a wavelength of $2.1\mu m$. Two of these weak
lenses, +4 and +8, can also be used to defocus stars observed in time series
measurements (see 4.4). Focus diverse phase retrieval and the overall
wavefront sensing process are described in detail in Acton et al (2012),
Perrin et al (2017), and Acton et al (2022).

##  4 Observing Modes

NIRCam has five observing modes. Two of these modes, grism time series and
photometric time series, are very similar. Table 1 presents the basic
characteristics of the modes. The imaging mode serves as the basis for all
five modes with its collection of data from both short wavelength and long
wavelength arms simultaneously and with detectors read out using sampling up
the ramp. The wide field slitless spectroscopic (WFSS) mode includes
acquisition of direct images in addition to the grism images to enable source
detection and wavelength calibration in the spectroscopic images. In this mode
the short wavelength arm is used for direct imaging. For grism time series,
the long wavelength arm uses a grism in series with a bandpass filter for out
of band blocking. The short wavelength arm is used with a choice of weak lens
and the appropriate $\sim 2\mu m$ filter. The photometric time series mode
uses bandpass filters in Module B. In coronagraphic mode, a target star is
placed behind an occulter and images are acquired.

Table 1: NIRCam Observing Modes Mode | Number of |  |
---|---|---|---
SCAs Used | Typical Use | Special Requirements |
Imaging | 2,5,or 10 | General 0.7 to 5 $\mu m$ imaging | Subarrays available, RAPID
mode using 10 SCAs only for |  |  |
single exposures |  |  |
WFSS | 10 |  |
(2 for spectra, 8 for imaging) | Wide field emission line survey |  |
2.4 to 5 $\mu m$ with |  |  |
short wavelength imaging | Careful planning needed for |  |
full spatial coverage |  |  |
Grism Time Series | 2 (Mod A only) | Transit spectroscopy 2.4 to 5 $\mu m$ | Stripe mode can extend
bright limit, target |  |  |
acquisition may be desirable |  |  |
Photometric Time |  |  |
Series | 2 or 5 (Mod B only) | High time resolution photometry |
Coronagraphy | 2 (Mod A only) | High contrast imaging | Target acquisition required

###  4.1 Imaging Mode

As described earlier, NIRCam’s basic imaging mode is used for both science
imaging and for wavefront sensing imaging. Both short wavelength and long
wavelength arms are read out simultaneously, but if a point source or limited
area is to be observed, only one module of NIRCam can be used to save on data
volume. For surveys, both modules can be used. There is no hardware
requirement that the two modules be configured with the same filter choices
although the Astronomer’s Proposal Tool does not support different filters
currently. The modules are constrained to use the same readout patterns. If a
bright source is observed, subarrays can be used to extend the camera’s
dynamic range but all arrays must be read out using the same size subarray so
typically only one short wavelength array and the corresponding long
wavelength array will be read out. In the case of extended bright sources, one
long wavelength and a set of four short wavelength subarrays at the center of
the B module, with approximately overlapping areas are used. For point
sources, one short wavelength and one long wavelength subarray can be
configured, located at the upper left corner of the NRCB1, a short wavelength
SCA. Figure 1 shows the location of NIRCam and its SCAs in the JWST focal
plane.

The throughput of the telescope plus NIRCam system is higher than pre-launch
predictions. This improvement is partly the result of arriving at L2 with
fewer particulates and other contaminants such as ice and partly the result of
conservatism in the pre-launch estimates. Figure 3 shows the throughputs which
are remarkable for a camera employing six lenses, a tribute to the quality of
the lens anti-reflection coatings. The throughput has been measured by
comparing the known flux of calibration stars to the detected flux in a 2.5
arc second radius aperture. The detected flux was corrected to the total flux
using an encircled energy curve based on a PSF computed using the optical path
difference map for the date of the observation available from the data archive
MAST. No correction was made for gaps or strut obscuration in the telescope
throughput.

Figure 4 displays a pixel ramp from data taken during a Cycle 1 calibration
program, illustrating how charge builds up over time from a source and is very
linear. The start of the ramp lies below the linear fit, which is indicative
of charge being lost to traps in the detector material. This loss of charge to
traps could affect low signal observations but in many cases there is
sufficient charge collection from the zodiacal background that this is not an
issue. Figure 5 displays a ramp that reaches saturation, and also shows how a
pixel that reaches saturation affects the slope of an adjacent pixel as charge
migrates from the saturated pixel to adjacent pixels. The migration of
saturated charge is mainly a bright object problem but can complicate
extraction of faint sources on the fringes of a bright PSF. The pipeline
includes a saturation check and a linearity correction step to compensate for
the change in gain of the readout circuit as the charge on the integrating
node increases (see Plazas et al 2017).

Figure 4: On the left, a ramp for a pixel illuminated by a star showing the
linear behavior of the charge collection. On the right, the low signal portion
of the ramp illustrates the non-linearity present at low signal levels.
Figure 5: Ramps from adjacent pixels illustrating how the slope changes when
the adjacent pixel saturates and charge migration begins.

NIRCam is background-limited through wide filters as predicted before launch.
Table 2 gives the current sensitivity for long exposures. The performance of
NIRCam’s detectors varies from SCA to SCA somewhat. Here we will use
instrument-wide averages unless denoted otherwise. The flight read noise and
dark current of the NIRCam short wavelength SCAs are slightly higher than
measured in ground testing which is undoubtedly the result of cosmic-ray hits.
The eight short wavelength SCAs had an average read noise of 6.2 electrons and
the two long wavelength SCAs had an average of 9.1 electrons in 1000 seconds
in ground testing. In flight, these two values changed to 6.4 and 7.5
electrons respectively. The long wavelength SCAs benefit from being surrounded
by a colder environment in flight so that the effect of small light leaks in
the NIRCam baffles is mitigated.

Table 2: NIRCam sensitivity 10 $\sigma$ in 10,000 second in 2.5-pixel radius aperture with a background 1.2x the minimum zodaical light level Filter | $\lambda_{pivot}$ |  |  |  |  |
---|---|---|---|---|---|---
$\mu m$ | nJy |  | Filter | $\lambda_{pivot}$ |  |
$\mu m$ | nJy |  |  |  |  |
F070W | 0.704 | 11.85 |  | F277W | 2.786 | 10.59
F090W | 0.901 | 9.74 |  | F356W | 3.563 | 8.88
F115W | 1.154 | 8.57 |  | F444W | 4.421 | 17.31
F150W | 1.501 | 6.99 |  | F250M | 2.503 | 24.63
F200W | 1.990 | 6.22 |  | F300M | 2.996 | 16.75
F140M | 1.404 | 11.85 |  | F335M | 3.365 | 14.91
F162M | 1.626 | 11.35 |  | F360M | 3.621 | 15.14
F182M | 1.845 | 9.28 |  | F410M | 4.092 | 18.74
F210M | 2.093 | 11.56 |  | F430M | 4.280 | 35.14
F164N | 1.644 | 69.08 |  | F460M | 4.624 | 56.81
F187N | 1.874 | 65.00 |  | F480M | 4.834 | 63.75
F212N | 2.120 | 70.31 |  | F323N | 3.237 | 113.73
F405N | 4.055 | 103.44 |  | F466N | 4.654 | 173.14
F470N | 4.707 | 199.80 |  |  |  |

Because NIRCam generates a large volume of data when all ten SCAs are read
out, a variant of sampling up the ramp is used where two, four, or eight
samples are co-added and then a number of reads are clocked but not included
in the co-adds. Figure 6 illustrates the DEEP8 readout pattern which can be
used for long exposures with all ten SCAs and still have data volume available
for data acquisition in parallel such as imaging with MIRI or NIRISS or
spectra with NIRSpec. Other readout patterns with more samples up the ramp can
be used when data volume is not a limitation.

Figure 6: The DEEP8 readout pattern illustrating the co-addition of eight
samples and skipping of reads to minimize data volume.

Any of the readout patterns with co-addition also include saving of the first
read separately to recover any pixels that reach saturation in subsequent
reads. Currently the Space Telescope Science Institute pipeline is not using
this “Zero read frame” but will do so in the future.

###  4.2 Wide Field Slitless Spectroscopy (WFSS)

This NIRCam mode uses grisms in NIRCam’s long wavelength arm and is
complementary to the NIRISS wide field slitless mode that covers 0.8 to 2.25
$\mu m$ (Dixon et al 2015). The NIRCam grisms have R$\sim 1200$, higher than
NIRISS grisms that have R$\sim 150$. NIRCam WFSS works from 2.44 to 4.98 $\mu
m$. The short wavelength arms can be used simultaneously with the grisms to
acquire a short wavelength direct image of the field. The zero deflection
wavelength for NIRCam’s grisms is 3.94 $\mu m$, and sources not in the direct
imaging field of view can appear in the grism field of view. This is described
in more detail in Greene et al (2017). Commissioning data updated the spatial
locations for best grism coverage as shown in Figure 7. Each module has a pair
of grisms with one grism aligned with detector rows and one aligned with
detector columns. Taking exposures with each grism can enable better
separation of potentially confused sources.

Figure 7:  Location of areas for complete grism spectral coverage using
F322W2. The upper row shows FOVs for the column grisms, and the lower row
displays the row grisms. Diagonal lines indicate the sky area where sources
can appear on the detector in grism mode. The red square outlines the detector
area. Black rectangles outline the original regions for complete spectral
coverage while blue and orange indicate the updated regions for complete
spectral coverage. Star symbols illustrate the fraction of the spectral range
that is captured for the indicated star locations.

Because the WFSS mode mixes background photons from across the wavelength
range with source photons that are dispersed by the grism, this mode is not as
sensitive as NIRSpec’s slit modes. The NIRCam WFSS mode is $\sim 8$ times less
sensitive than NIRSpec, but has the advantage of not requiring pre-knowledge
of source positions. The utility of this mode for observing moderate to high
redshift galaxies is illustrated in Sun et al (2022a) and Sun et al (2022b)
where WFSS flux calibration data taken during commissioning revealed several
$z\sim 6$ galaxies with bright emission lines and little continuum. Figures 8
and 9 show that the grism photon conversion efficiency is higher than pre-
launch predictions because there is little or no water or other organic ices
on JWST’s optical surfaces. The grisms were used to check for ice
contamination during commissioning by observing A stars with no ice absorption
at $3.1\mu m$ detected. Module A has higher throughput than Module B because
the Module A grisms are anti-reflection coated on both sides of the grisms
while Module B grisms are coated only on the flat side. Only one set of grisms
were anti-reflection coated on the groove-side as there was no prior
experience demonstrating that coating grooves would not lead to problems.
Figures 10 presents the $10-\sigma$ detection limits in 10,000 seconds.

Figure 8: Module A grism photon conversion efficiency derived from
observations of P330-E. Figure 9: Module B grism photon conversion efficiency
derived from observations of P330-E.

Figure 10: Module A grism continuum and line sensitivities for a $10\sigma$
detection in 10,000 seconds.

During testing of NIRCam before launch two types of ghost images were
identified in grism data. Figure 11 shows examples of these ghosts. The
“tadpole” ghost appears in both Module A and Module B although it is much
stronger in Module B ($>100\times$) as a result of the grisms only being anti-
reflection coated on one side. The “tadpole” ghost is mostly seen with Module
B column grism and appears at a fixed location relative to the 0-order image.
The morphology of tadpole ghost weakly depends on the source position and
filter being used. The “shell” ghost has only been seen with Module B so far
when very bright sources are observed. Either type of ghost is usually not a
concern in module A unless very bright sources are observed.

Figure 11: On the left, the “shell” grism ghost. The mechanism for this ghost
was never definitively determined but likely involves multiple reflections
between the grism, filter, and camera lense(s). On the right, a “tadpole”
ghost is seen and is likely the 0th order grism image reflected off the
filter, back to the grism, and then appearing as a 0th order image on the
detector. “Tadpoles” appears at a fixed location relative to an object’s
0-order image.

###  4.3 Coronagraphy Mode

NIRCam’s coronagraphy is based on a standard Lyot-type coronagraph with five
focal plane masks available. The coronagraphic focal plane masks are not in
the imaging field of view unless the pupil wheel is rotated to a Lyot stop
with a wedge that deflects the field of view to include the masks. Figure 12
shows the stop used with round masks. The holes are undersized to minimize
diffracted light from the edges of the mirror segments. The throughput of the
Lyot stops is $\sim 20\%$ so throughput has been traded for better contrast.
This optical train is shown schematically in Girard et al (2022). Figure 13
shows the masks in a flight image against sky background. There are three
round masks sized to match $6\lambda/D$ when combined with F210M ($2.1\mu m$),
F335M ($3.35\mu m$), or F430M ($4.3\mu m$). The choice of mask essentially
defines the inner working angle. The two bar masks provide more flexibility in
terms of choice of wavelength with the mask widths varying linearly with
widths corresponding to $4\lambda/D$ from 1.7 to 2.2 $\mu m$ or 2.5 to 5 $\mu
m$. The transmission of the mask substrate drops below 1.7 $\mu m$ so
coronagraphy is not recommended at the shortest wavelengths. The masks are not
hard-edged but rather Gaussian-tapered. Fabrication details are given in Krist
et al (2009). The masks were fabricated at JPL’s Microdevices Lab. Girard et
al (2022) present results of coronagraphy tests performed during commissioning
with performance exceeding the expectations given in Perrin et al (2018) with
a firm detection at 3.35 $\mu m$ of the white dwarf companion HD 114174 B at a
separation of 0.5 arc seconds and at a contrast of 10 magnitudes ($10^{4}$
fainter than the K$\sim$5.3 host star). One of the first coronagraphic
observations after the start of science operations achieved contrast of better
than $10^{-5}(5\sigma)$ at 0.5 arc sec separation using the long wavelength
bar mask at wavelengths from 2.5 to 4.6 $\mu m$ and as low as $2\times
10^{-6}$ at 3 $\mu m$ and 1 arc sec (Greenbaum et al 2022). There were no
performance requirements defined for the design of the NIRCam coronagraph, but
as mentioned above, in-flight performance exceeds pre-flight simulations in
Perrin et al (2018). As more experience is gained in the use of the
coronagraphic mode, performance improvements are likely.

Figure 12:  An image of the Lyot stop used with round focal plane masks taken
using the pupil imaging lens. Figure 13:  View of Module A’s masks through the
short wavelength channel. The squares along the edge of the field are neutral
density spots used for target acquisition.

###  4.4 Time Series Mode

The NIRCam long wavelength grisms enable transit spectroscopy. In principle,
JWST can point at a single target for up to 10 days with only brief breaks to
reposition the downlink antenna. This capability is very different from
transit work performed using the Hubble Space Telescope which has to interrupt
observations every few tens of minutes unless the target is in the continuous
viewing zone. Commissioning transit observations have shown that JWST is a
very good platform for observing exoplanet transits (Schlawin et al 2022). See
Beichman et al (2014) for an overview of all of JWST’s transit capabilities.

One module of NIRCam can be used to study objects where high time resolution
data is needed. This mode can use any of NIRCam’s filters with one choice in
the short wavelength arm and another choice in the long wavelength arm. The
time resolution depends on whether full frame or subarray data are requested.
In either readout choice, there is overhead from resetting the pixels used so
in a typical observation the reset - read - read pattern means in one third of
the time when resetting, no data are collected (this inefficiency is present
also for grism time series). The fastest sampling uses a 64 pixel x 64 pixel
subarray which can be read out every 50 milliseconds.

####  4.4.1 Grism Time Series

NIRCam’s grism time series capability was successfully demonstrated during
commissioning. The current implementation of this mode uses a weak lens in the
short wavelength arm and a grism with R$\sim 1500$ bandlimited with either the
F277W, F322W2, F356W, or the F444W filter in the long wavelength arm. The
grism time series mode only uses Module A. The sensitivity curve in Figure 8
is relevant also for this mode and shows that the F322W2 spectral range is
$2.4\mu m$ to $4\mu m$ and is $3.9\mu m$ to $5.0\mu m$ for F444W. This mode
can take advantage of “stripe” mode where the SCA is read out using all four
output amplifiers present in the SCA but only enough rows to capture the grism
spectrum. This scheme uses the row grism only. The shortest read out time is
provided by using a 64x2048. This enables observing significantly brighter
stars than the usual subarray mode, which reads out using only one of the SCA
output amplifiers. Table 3 gives the Vega magnitudes at K(2.2 $\mu m$) for
several wavelengths in the spectrum using either F322W2 or F444W. The table
assumes that the data are taken using a reset followed by clocking through
pixels to read the pixel charge followed by a second read (RAPID mode with two
groups). Using a reset followed by a single read could make the saturation
limit 0.75 magnitudes brighter at the expense of the readout noise being
increased by the kTC noise of$\sim 35$ electrons.

Table 3: Vega K Magnitude for Stars Reaching 80% of Full Well Using Stripe Mode and Reset-Read-Read Filter |  $\lambda(\mu m$) | $K_{mag}$
---|---|---
F322W2 | 2.40 | 3.99
F322W2 | 2.75 | 4.37
F322W2 | 3.23 | 4.10
F322W2 | 4.00 | 3.90
F444W | 3.90 | 3.96
F444W | 4.41 | 3.33
F444W | 5.00 | 1.99

Several important lessons from commissioning indicate that this mode may be
more user friendly than the modes used for transit observations on the Hubble
Space Telescope. First, the time for detectors to settle after the start of
time series observation is nearly instantaneous with a time scale of less than
4 minutes to stabilize to a level of less than 150 ppm variation ( Schlawin et
al 2022) rather than the few tens of minutes required by WFC3IR detectors.
Second, the data analysis is more straightforward than for Hubble transit
data. The noise performance for a region of less than 30 pixels is at the
expected level for photon-limited statistics. The broadband light curve
averages down as $1/\sqrt{N}$ to $\sim 20$ ppm after correction for a downward
trend in the signal level. Section 5 describes an enhancement to this mode
where the short wavelength arm could use the 0-degree DHS to acquire spectra
in the 1 to 2$\mu m$ region simultaneously with the grism spectrum.

###  4.5 In Flight Anomalies

####  4.5.1 Cosmic-Rays

Cosmic-rays are an expected annoyance for space instrumentation. Figure 14
shows a ramp from a pixel with a large cosmic-ray hit that saturated the pixel
between reads three and four. The last reads show a slight decrease in charge
as the charge migrates to adjacent pixels. Most hits like the one shown here
affect only one pixel and rarely a short streak of pixels. The pipeline takes
these hits into account and only uses good data to derive a signal. The
pipeline inserts a flag in the data quality plane in the FITS file for an
image to indicate that a cosmic-ray hit was detected in a given pixel. The
rate of hits matches the pre-launch estimates.

Figure 14:  Ramp from a pixel hit by a cosmic-ray.

What caught some by surprise was the detection of “snowballs” as shown in
Figure 15. The snowballs have appreciable spatial extent with some affecting
as many as 100 pixels. Typically many of these pixels receive a saturating
level of charge as illustrated in the surface plot in Figure 15. Similar
events had been seen in earlier space missions using HgCdTe detectors, but
because the rate is quite low, they were seen rarely. Webb has a much larger
number of 2Kx2K arrays so these events are more noticeable. Approximately 170
events per hour per array are seen in NIRCam arrays with roughly similar rates
observed in NIRSpec and NIRISS. There is no definitive explanation for these
events. HgCdTe arrays are comprised of higher atomic weight materials than
CCDs, and this suggests that a cosmic-ray could hit a heavy element nucleus
and create energetic high atomic number nuclei by spallation, which then
deposit significant charge in a set of pixels (G. Rieke, private
communication).

Figure 15: The right-hand image show a snowball as seen in a single frame in a
ramp. The green box outlines the region shown in the surface plot at left.

####  4.5.2 Persistence

Three of NIRCam’s SCAs, NRCA3, NRCB3, and NRCB4, exhibit noticeable
persistence even several 1000s of seconds after illumination as shown in
Figure 16. A typical time between the end of one program and the start of the
next is $\sim$1800 seconds so scheduling very bright source observations
immediately prior to programs with long, low-signal observations should be
avoided. Saturating cosmic-ray hits, and snowballs in particular, can result
in persistence images of the hit.

Figure 16: Persistence seen in NRCA3 . The blue points are from a dark
exposure while the black points are the brightest pixel in a dark taken
immediately after an on-sky image.

####  4.5.3 Scattered Light and Glints

Occasional glints were expected from ground tests, but some glints and some
scattered light features were a surprise. The scattered light features are
described in Rigby et al, (2022), and are caused by sneak paths through the
aft optics baffle and by scattering off reflective material on the upper
secondary mirror support strut. During ground tests glints (sometimes called
“Dragon’s Breath”) were discovered. Similar artefacts have been observed in
HST instruments, and result from a relatively bright point source scattering
and diffracting off sharp edges. A commissioning test collected images with a
star moved across the edges of the focal plane masks on the focal plane
assemblies. Figure 17 shows a typical glint coming off a sharp edge in the
focal plane mask, and is similar to those seen in ground test. Another type of
glint is shown in Figure 18 where light glances off a sharp edge inside the
NIRCam focal plane housing.

Figure 17: An example of a glint caused by star light scattering off a sharp
edge in the focal plane mask.

‘

Figure 18: An example of a glint caused by star light scattering off a sharp
edge inside the focal plane housing. Adjacent short wavelength SCAs are shown
with the glint from a single star extending across both.

##  5 Future Improvements

Several enhancements to the modes described in Section 4 will improve
observing efficiencies. These include using the DHS in parallel with the long
wavelength grism for time series observations and improving coronagraphy to
permit simultaneous short and long wavelength coronagraphy. Combining DHS
observations with grism observations yields simultaneous wavelength coverage
from $1.0\mu m$ to $2.0\mu m$ and either $2.4\mu m$ to $4.0\mu m$ or $3.9\mu
m$ to $5\mu m$ depending on the choice of long wavelength filter (Schlawin et
al 2017, see DHS + grism data dample in this paper). This mode would not
provide the continuous $1\mu m$ to $5\mu m$ coverage afforded by NIRSpec but
would provide some short wavelength spectral coverage on bright ($J<11$)
sources which saturate in NIRSpec’s prism mode. The NIRISS Single Object
Slitess Spectroscopy mode would provide more complete coverage for
$\lambda<2.5\mu m$ but does not provide any longer wavelength coverage.
Schlawin et al (2017) describe the benefits for molecular abundance retrievals
using this NIRCam mode.

Acquisition of simultaneous short wavelength and long wavelength coronagraphy
would use a long wavelength coronagraphic mask such as MASK335R. At 0.5 arc
second separation, this mask would provide good contrast similar to the $\sim
10^{-5}$ already achieved in the long wavelength arm and better then the
contrast of $5\times 10^{-5}$ at achieved at $2.1\mu m$ using MASK210R (Girard
et al 2022). These contrasts would come at the expense of loss of some inner
working angle at short wavelengths compared to using MASK210R. However, for
some observations such as studying debris disks, the loss of some area in the
short wavelength image is far outweighed by the time savings of observing two
wavelengths at once.

Simultaneous short and long wavelength coronagraphy will be available to Cycle
2 proposers, and DHS transit use will be available in the future.

NIRCam and JWST have been demonstrated to work very well, and have enabled
exciting observations as revealed by several Early Release Observations and
Early Science Observations.

Many people at Lockheed Martin’s Advanced Technology Center contributed to the
NIRCam Project. We especially want to thank Alison Nordt, Eric Dixon, Tony
Magoncelli and Liz Osborne for support during commissioning. The development,
testing, and commissioning of NIRCam was funded by NASA Contract NAS5-02105. A
portion of this research was carried out at the Jet Propulsion Laboratory,
California Institute of Technology, under a contract with the National
Aeronautics and Space Administration (80NM0018D0004). D.J. is supported by NRC
Canada and by an NSERC Discovery Grant. We also want to acknowledge the
contributions of three NIRCam team members who passed away during the
development of NIRCam. Chad Engelbracht analyzed much of the detector test
data that were used to select the flight parts. John Stauffer devised the
original calibration plan for NIRCam. Don Hall provided advice on many issues
and detector production in particular. The referee is thanked for helpful
suggestions.

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