11institutetext:  LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne
Université, Univ. Paris Diderot, Sorbonne Paris Cité, 5 place Jules Janssen,
92195 Meudon, France 22institutetext: Université Paris-Saclay, Université
Paris Cité, CEA, CNRS, AIM, 91191, Gif-sur-Yvette, France 33institutetext:
UKATC, The Royal Observatory, Blackford Hill, Edinburgh, EH9 3HJ, Scotland
44institutetext: Institute for Astronomy, University of Edinburgh, Royal
Observatory, Blackford Hill, Edinburgh EH9 3HJ 55institutetext: Space
Telescope Science Institute, 3700 San Martin Dr, Baltimore, Maryland 21218
66institutetext: Max-Planck-Institut für Astronomie (MPIA), Königstuhl 17,
69117, Heidelberg, Germany 77institutetext: Steward Observatory and the
Department of Astronomy, The University of Arizona, 933 N Cherry Ave, Tucson,
AZ, 85721, USA 88institutetext: Telespazio UK for the European Space Agency,
ESAC, Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Spain
99institutetext: Centro de Astrobiología (CSIC-INTA), Carretera de Ajalvir,
28850 Torrejón de Ardoz, Madrid, Spain 1010institutetext: NASA Goddard Space
Flight Center, Exoplanets & Stellar Astrophysics Laboratory, Code 667,
Greenbelt, MD 20771, USA 1111institutetext: European Space Agency, 3700 San
Martin Drive, Baltimore, MD21218

# JWST/MIRI coronagraphic performances as measured on-sky

A. Boccaletti  0000-0001-9353-2724 JWST/MIRI coronagraphic performances as
measured on-skyJWST/MIRI coronagraphic performances as measured on-sky C.
Cossou JWST/MIRI coronagraphic performances as measured on-skyJWST/MIRI
coronagraphic performances as measured on-sky P. Baudoz JWST/MIRI
coronagraphic performances as measured on-skyJWST/MIRI coronagraphic
performances as measured on-sky P. O. Lagage JWST/MIRI coronagraphic
performances as measured on-skyJWST/MIRI coronagraphic performances as
measured on-sky D. Dicken JWST/MIRI coronagraphic performances as measured on-
skyJWST/MIRI coronagraphic performances as measured on-skyJWST/MIRI
coronagraphic performances as measured on-skyJWST/MIRI coronagraphic
performances as measured on-sky A. Glasse JWST/MIRI coronagraphic performances
as measured on-skyJWST/MIRI coronagraphic performances as measured on-
skyJWST/MIRI coronagraphic performances as measured on-skyJWST/MIRI
coronagraphic performances as measured on-sky D. C. Hines  0000-0003-4653-6161
JWST/MIRI coronagraphic performances as measured on-skyJWST/MIRI coronagraphic
performances as measured on-sky J. Aguilar JWST/MIRI coronagraphic
performances as measured on-skyJWST/MIRI coronagraphic performances as
measured on-sky O. Detre JWST/MIRI coronagraphic performances as measured on-
skyJWST/MIRI coronagraphic performances as measured on-sky B. Nickson
JWST/MIRI coronagraphic performances as measured on-skyJWST/MIRI coronagraphic
performances as measured on-sky A. Noriega-Crespo JWST/MIRI coronagraphic
performances as measured on-skyJWST/MIRI coronagraphic performances as
measured on-sky A. Gáspár JWST/MIRI coronagraphic performances as measured on-
skyJWST/MIRI coronagraphic performances as measured on-sky A. Labiano
0000-0002-0690-8824 JWST/MIRI coronagraphic performances as measured on-
skyJWST/MIRI coronagraphic performances as measured on-skyJWST/MIRI
coronagraphic performances as measured on-skyJWST/MIRI coronagraphic
performances as measured on-sky C. Stark JWST/MIRI coronagraphic performances
as measured on-skyJWST/MIRI coronagraphic performances as measured on-sky D.
Rouan JWST/MIRI coronagraphic performances as measured on-skyJWST/MIRI
coronagraphic performances as measured on-sky J. M. Reess JWST/MIRI
coronagraphic performances as measured on-skyJWST/MIRI coronagraphic
performances as measured on-sky G. S. Wright JWST/MIRI coronagraphic
performances as measured on-skyJWST/MIRI coronagraphic performances as
measured on-sky G. Rieke JWST/MIRI coronagraphic performances as measured on-
skyJWST/MIRI coronagraphic performances as measured on-sky M. Garcia Marin
JWST/MIRI coronagraphic performances as measured on-skyJWST/MIRI coronagraphic
performances as measured on-sky

###### Abstract

Context. Characterization of directly imaged exoplanets is one of the most
eagerly anticipated science functions of the James Webb Space Telescope. MIRI,
the mid-IR instrument has the capability to provide unique spatially resolved
photometric data points in a spectral range never achieved so far for such
objects.

Aims. We aim to present the very first on-sky contrast measurements of the
MIRI’s coronagraphs. In addition to a classical Lyot coronagraph at the
longest wavelength, this observing mode implements the concept of the four
quadrant phase mask for the very first time in a space telescope.

Methods. We observed single stars together with a series of reference stars to
measure raw contrasts as they are delivered on the detector, as well as
reference subtracted contrasts.

Results. MIRI’s coronagraphs achieve raw contrasts greater than $10^{3}$ at
the smallest angular separations (within $1^{\prime\prime}$) and about
$10^{5}$ further out (beyond $5\sim 6^{\prime\prime}$). Subtracting the
residual diffracted light left unattenuated by the coronagraph has the
potential to bring the final contrast down to the background and detector
limited noise floor at most angular separations (a few times $10^{4}$ at less
than $1^{\prime\prime}$).

Conclusions. MIRI coronagraphs behave as expected from simulations. In
particular the raw contrasts for all four coronagraphs are fully consistent
with the diffractive model. Contrasts obtained with subtracting reference
stars also meet expectations and are fully demonstrated for two four quadrant
phase masks (F1065C and F1140C). The worst contrast, measured at F1550C, is
very likely due to a variation of the phase aberrations at the primary mirror
during the observations, and not an issue of the coronagraph itself. We did
not perform reference star subtraction with the Lyot mask at F2300C, but we
anticipate that it would bring the contrast down to the noise floor.

###### Key Words.:

Exoplanets – Techniques: image processing – Techniques: high angular
resolution

date | filter | object | type | obs id | Ngroup | Nint | dither | Texp
---|---|---|---|---|---|---|---|---
UT |  |  |  |  |  |  |  | per dither (s)
06/09/2022 | F1140C | BD +30 2990 | REF 1 ON | 1037 / obs 4 | 100 | 21 | 9 | 508.122
06/09/2022 | F1140C | HD 158165 | TARG ON | 1037 / obs 5 | 100 | 21 | 9 | 508.122
06/09/2022 | F1140C | HD 158165 | TARG OFF | 1037 / obs 6 | 10 | 25 | 4 | 65.672
06/09/2022 | F1140C | HD 158896 | REF 10 ON | 1037 / obs 7 | 100 | 21 | 9 | 508.122
06/13/2022 | F1140C | $-$ | BGD | 1045 / obs 65 | 100 | 21 | 4 | 508.122
06/20/2022 | F1140C | BD +30 2990 | REF 1 ON 2nd | 1037 / obs 30 | 100 | 21 | 9 | 508.122
06/18/2022 | F1550C | $-$ | BGD | 1037 / obs 8 | 100 | 95 | 4 | 2299.49
06/18/2022 | F1550C | HD 163113 | TARG OFF | 1037 / obs 9 | 10 | 25 | 4 | 65.672
06/18/2022 | F1550C | HD 163113 | TARG ON | 1037 / obs 10 | 100 | 95 | 9 | 2299.49
06/18/2022 | F1550C | HD 162989 | REF 1 ON | 1037 / obs 11 | 100 | 95 | 9 | 2299.49
06/20/2022 | F1065C | $-$ | BGD | 1037 / obs 26 | 100 | 21 | 4 | 508.122
06/20/2022 | F1065C | HD 158165 | TARG OFF | 1037 / obs 27 | 10 | 25 | 4 | 65.672
06/20/2022 | F1065C | HD 158165 | TARG ON | 1037 / obs 28 | 100 | 21 | 9 | 508.122
06/20/2022 | F1065C | BD +30 2990 | REF 1 ON | 1037 / obs 29 | 100 | 21 | 9 | 508.122
06/23/2022 | F2300C | $-$ | BGD | 1037 / obs 32 | 100 | 90 | 4 | 2944.836
06/23/2022 | F2300C | HD 163113 | TARG OFF | 1037 / obs 33 | 10 | 25 | 4 | 88.776
06/23/2022 | F2300C | HD 163113 | TARG ON | 1037 / obs 34 | 100 | 90 | 1 | 2944.836
Table 1: Main parameters of the observations during MIRI coronagraph’s
commissioning: date, filter, name of the object, type of object (target or
reference, degrees separation on-sky from the target, on or off the center of
the coronagraph, or background image), id of the program, number of group,
number of integration, number of dither position (9 is for the SGD, 4 or 1 is
a classical dither), total exposure time per dither.

##  1 Introduction

Exoplanet characterization is entering a new era with the James Webb Space
Telescope (JWST), in particular with the coronagraphs of the MIRI instrument
(Rieke et al. 2015; Wright et al. 2015) designed to obtain high contrast
imaging of exoplanetary systems at mid-IR wavelengths. To date, several
exoplanets have been directly imaged at near-IR, mostly from the ground with
adaptive optics facilities, but no observations have been obtained beyond
$\sim 5\,\muup$m. The few observations performed with ground-based instruments
at the M band are potentially affected by large photometric uncertainties due
to the sky brightness and variability.

The MIRI coronagraphic mode is a suite of four focal plane masks (permanently
mounted in the imager field of view), each paired with a dedicated filter and
an optimized Lyot stop. They were designed to offer large contrasts, and,
importantly, small inner working angles (IWA)111The IWA, although sometimes
ill-defined, is the angular separation at which an off-axis point source will
have its transmission reduced to 50%. at mid-IR. Despite longer operating
wavelengths than NIRCAM, MIRI’s coronagraphs are delivering similar IWAs.
Three of these coronagraphs use four-quadrant phase masks (4QPM, Rouan et al.
2000), manufactured with reactive ion etching in a Germanium substrate.
Details about the 4QPM manufacturing for MIRI can be found in Baudoz et al.
(2006). Since the 4QPMs are chromatic, they are used in conjunction with the
narrow band filters F1065C ($\lambda_{0}=10.575\muup$m,
$\Delta\lambda=0.75\muup$m), F1140C ($\lambda_{0}=11.30\muup$m,
$\Delta\lambda=0.8\muup$m), and F1550C ($\lambda_{0}=15.50\muup$m,
$\Delta\lambda=0.9\muup$m), mounted in the filter wheel together with a Lyot
stop transmitting 62% of the telescope aperture. Because the stop is not just
a downsized version of the JWST pupil but has been optimized to attenuate the
starlight diffraction, the PSF of an off-axis object is slightly broader than
with the full pupil. The field of view (FOV) of the 4QPM subarrays is
$24^{\prime\prime}\times 24^{\prime\prime}$. Another coronagraph using a
classical Lyot mask with a diameter of $3\lambda/D$, representing
$2.1^{\prime\prime}$, is dedicated to longer wavelength observations with the
F2300C filter ($\lambda_{0}=22.75\muup$m, $\Delta\lambda=5.5\muup$m). This
coronagraph, with a FOV of $30^{\prime\prime}\times 30^{\prime\prime}$, uses a
Lyot stop with a different shape and a transmission of $72\%$. Pre-flight
details about the MIRI coronagraphs can be found in Boccaletti et al. (2015).

The central wavelengths of the filters were chosen to characterize the
atmospheres of directly imaged, young giant exoplanets, essentially to
complement near-IR photometric and spectroscopic measurements. The F1065C and
F1140C filters are meant to provide photometric measurements of exoplanets in
and out of the ammonia absorption band. The F1550C filter, in combination with
F1140C, is important to constrain models of the thermal balance of the planet
and of the behaviour of any atmosphere. The detection performance was first
estimated in Boccaletti et al. (2005), Boccaletti et al. (2015). Some recent
simulations of planet detection with MIRI’s coronagraphs can also be found in
Danielski et al. (2018) and Hinkley et al. (2022). The Lyot mask, by design,
is not intended to directly image planets (except a few at wide separations
like HD 106906b), but rather is optimized to detect cold material around
bright stars, such as debris disks, which are reminiscent of Kuiper belts in
young systems (see for instance Lebreton et al. 2016). The larger FOV in this
channel was adopted to include so far as possible the outer debris belts of
even the most extended systems.

The MIRI coronagraphs were previously tested on the ground but the
performances were strongly disturbed by the strong thermal background and the
test bench features, which limit the achievable contrast to a few hundreds
(Cavarroc et al. 2008a). The purpose of this paper is to present the actual
performance of the MIRI coronagraphs as measured during JWST commissioning,
and to compare these results to simulations, which were recently revised to
include the most relevant knowledge of the telescope and the instrument.

The paper is organized as follows: Section 2 describes the observations, and
Section 3 the simulations. The measured contrasts on-sky are presented in
Section 4.

Figure 1: Images of the background obtained in the coronagraphic subarrays showing the ”glow sticks” visible at the horizontal transitions of the 4QPMs, and around the disk of the Lyot mask, but also at the edges of the coronagraphs’ support structure. The intensity scale is adapted for each image for visualization purpose. | OPD tel. | OPD MIRI | OPD Frill | OPD IEC | OPD TD | TA | jitter | pupil shear
---|---|---|---|---|---|---|---|---
| [nm] | [nm] | [nm] | [nm] | [nm] | [mas/axis] | [mas/axis] | [%]
best | 73 | 32 | 0.017 | 1.6 | 0.016 | 6.25 | 2.5 | 0
nominal | 73 | 32 | 0.07 | 2.8 | 0.07 | 8.76 | 3.8 | 2
requirement | 73 | 32 | 1.75 | 4.6 | 1.67 | 12.51 | 5.8 | 4
Table 2: Three input cases used to simulate the performance of the MIRI
coronagraphs. Wavefront errors (OPD) are provided in nanometers RMS, TA and
jitter are in milliarcsec per axis, and the shear is in % of the telescope
pupil.

##  2 Observations

Observations were carried out in June 2022. Table 1 provides the list of data
that are used for estimating the contrasts delivered by the MIRI coronagraphs.
We selected the targets to avoid saturation when observed off-axis (a few
arcsec away to avoid the attenuation of the coronagraphs) to measure the PSF
for a photometric reference, as well as to provide a sufficient flux level
compatible with the sensitivity at each filter when the star is on-axis, hence
masked and attenuated by the coronagraph (attenuation on-axis can be as large
as a factor of $\sim$400 as measured in F1550C). We end up with two series of
objects, one set optimized for F1065C and F1140C, and another one for F1550C
and F2300C. The targets are HD 158165 (K=4.07) of which the flux densities are
0.550 Jy and 0.450 Jy, for respectively F1065C and F1140C, and then HD 163113
(K=2.75), for F1550C and F2300C, with flux densities 1.445 Jy and 0.532 Jy,
respectively.

The observations recorded during the commissioning of MIRI coronagraphs are
intended primarily to measure the raw contrasts, which correspond to the
starlight attenuation at the detector as a function of angular separation. Raw
coronagraphic images in this particular case are entirely dominated by the
diffraction of the telescope pupil leaking through the Lyot stop. The un-
attenuated starlight can be further calibrated and subtracted with the use of
reference stars. At this second step, the dominant terms are expected to come
from the telescope wavefront errors and their variations in time, as well as
the telescope pointing repeatability onto the coronagraph. While a long-term
strategy will certainly involve a library of coronagraphic images and
dedicated algorithms for post-processing (Choquet et al. 2014), the
commissioning procedure makes use of one or two reference stars observed back-
to-back with the targets, or a few days apart.

Reference stars were chosen to have similar flux densities in the
coronagraphic filters, and are located at several distances on the sky to test
the effect of telescope slews. The 4QPM/F1140C coronagraph is the one
identified to perform most of these tests with reference stars. In the F1140C
filter, we observed two reference stars, BD +30 2990 and HD 158896, located
respectively at 0.7∘ and 20.7∘ from the target. The former was also observed
in the F1065C filter. In the F1550C filter, the reference star HD 162989 is at
1.16∘ from the target. No reference star was observed for the Lyot
coronagraph.

Figure 1 shows images of each of the coronagraphic fields when observing the
background sky. All four show an unexpected stray light feature, known
familiarly as ’glow sticks’, which appear as increased signal along the
structural edges in the MIRI imager entrance focal plane. They are most
apparent across the centre of the F1550C image where light is being scattered
into the ’science’ optical path by the raised edge of the phase boundary. In
the F2300C image, the bright glow stick marks scattering at the lower edge of
the aluminium Lyot aperture. The stray light is visible, but fainter for the
shorter wavelength 4QPM coronagraphs in Figure 1. Stray light is also seen
along the upper edge of the Lyot spot.

Two key observations were important in determining the root cause of the glow
sticks. First, their shape and brightness were independent of the observatory
pointing direction to within a few percent, ruling out an astronomical origin.
Second, photometric analysis of the glow sticks determined that the
illuminating source was well fitted by a grey body spectrum with an effective
temperature of $120\pm 20K$, characteristic of the region where the sunshield
approaches the deployable tower assembly (Lightsey et al. 2012). Non-
sequential optical path analysis (S. Rohrbach, private com.) has identified a
possible path from this warm region to the MIRI entrance focal plane via a
reflection from the secondary mirror (SM) followed by scattering from the
hinged SM support strut. Although the cosmetic appearance of the modelled glow
sticks is not identical to those observed, with the along column edges also
predicted to be bright, we regard it as sufficiently close to be consistent
with the observations within the fidelity of the observatory plus MIRI solid
model and the as-built reality.

To mitigate the ’glow stick’ effect (which could be brighter than the observed
source itself) it is necessary to subtract a background image obtained in the
same filter and for identical exposure time, until the variability of this
pattern is understood and an alternative approach is proposed. If this
procedure is followed, the effect of the glow sticks on the final data is
virtually completely removed except for the expected modest increase in photon
noise at their positions.

To precisely center the star on the coronagraph axis, we need both sub-pixel
estimation of the coronagraph position and a precise target acquisition
procedure (Cavarroc et al. 2008b). The glow sticks prevented us from using the
dark transitions of the 4QPMs to estimate the coronagraph center, as planned.
Instead, we developed a method based on comparison with a diffraction model
after subtracting them out, to be presented in a separate paper (Baudoz et
al., in prep.). These measurements were used in the target acquisition
procedure to reach a pointing accuracy of about 5 to 10 mas in full agreement
with the requirement (Rigby et al. 2022).

Figure 2: Observed (top) versus simulated (bottom) PSFs (off-axis) in the four
coronagraphic filters. The pixel scale is 110 mas. Some PSFs are cropped by
the edges of the coronagraph’s mechanical support. Simulated data are using
the nominal scenario (see sec. 2) Figure 3: Observed (top) versus simulated
(bottom) coronagraphic images (on-axis) in the four coronagraphic filters. The
pixel scale is 110 mas. Dark spots in the F2300C image corresponds to
background stars recorded during the background observation. Simulated data
are using the nominal scenario (see sec. 2) Figure 4: One to one (top) versus
PCA (bottom) subtraction of coronagraphic images (TARG and REF1) in the three
4QPMs filters. Several background sources (either positive or negative) are
observed around the target and reference stars. The pixel scale is 110 mas.

##  3 Simulations

The simulations presented in Boccaletti et al. (2015) were recently reassessed
by the JWST Coronagraph Sensitivity Working Group to incorporate the up-to-
date telescope and instrument parameters. We assumed a temporal sampling of
$\pi/5\approx 0.63$ minutes per frame, for a total sequence of about 56
minutes on the target (90 frames) followed by the same amount of telescope
time on a reference star, which is dithered on 9 positions ($9\times 10$
frames, i.e., each dither position has a total of 1/9 the exposure time as the
target source.). This so-called Small Grid Dither (SGD) allows a diversity in
the observations of the reference star to further reduce the starlight making
use, for instance, of Principle Component Analysis (Lajoie et al. 2016;
Soummer et al. 2012). The SGD is a square grid with 10 mas steps. Although the
error on the positioning of the star onto this grid is estimated to 1 or 2
mas, this has no effect in the estimation of the starlight which only relies
on the variations of the intensity of speckles around the mask center, but not
on absolute knowledge of the pointing.

Static aberrations include the telescope wavefront aberration as measured in
the early phase of commissioning (73 nm RMS), and the MIRI instrument
aberrations measured on the ground (32 nm RMS). The former wavefront map is
made of mid spatial frequencies and is expected to evolve along the life of
the mission, while the latter contains mostly low spatial frequencies.
Additional dynamical components in the wavefront on a $\sim 1-2$ hours
timescale are also taken into account with various spatial and temporal
frequencies like the Thermal Distortion (TD) of the telescope backplane, the
fast oscillation in the heaters in the ISIM Electronics Compartment (IEC), and
the Frill around the primary mirror designed to stop the straylight, all being
relatively small in terms of wavefront errors for an instrument like MIRI
(Tab. 2).

In addition, the simulation accounts for misalignments at the focal plane and
pupil plane in the coronagraph. First of all, the offset between the star’s
position, and the center of the mask corresponds to the Target Acquisition
(TA) error. For convenience, in the simulations, the target star is perfectly
centered while the reference star is offset by this TA error. Then, we
included line of sight jitter that is the motion of the star’s position during
the observation. Moreover, the Lyot stop located at the MIRI filter wheel, can
be slightly misaligned with the telescope pupil. This error is expressed in %
of the telescope pupil diameter, assuming a shear along the diagonal. Three
distinct scenarios were considered: ’best’, ’nominal’ and ’requirement’. The
values for all parameters of the simulations are provided in Tab. 2. Finally,
we included a spectral shift of 3% of the F1140C filter with respect to the
operating wavelength of the corresponding 4QPM, causing a chromatic leakage in
the images visible as a central peak in the coronagraphic image.

The simulations are time averaged, so we are left with one single image for
the target and nine for the reference. We applied PCA using 9 components to
built a reference frame then subtracted out from the target image. We provide
a comparison of PSF images (off-axis) and coronagraphic images (on-axis)
actually observed during commissioning, to simulated images in Fig. 2 and Fig.
3. They are virtually identical and the surrounding field on the observed
image is very clean, with no evidence of residual latent images.

Figure 5:  Measured raw and 3$\sigma$ contrasts for all MIRI coronagraphic
filters as compared to the simulations and to the ETC prediction. Subtracted
contrasts are shown for the one to one and PCA algorithms. Reference stars
were not observed at F2300C.  Figure 6: Measured 3$\sigma$ contrasts with PCA
in the F1140C filter for the single epoch and the two epochs cases, and
compared to the simulation taking into account the wavefront error maps (OPD)
before and after rephasing of the primary mirror segments.

##  4 Measured contrasts

The data are processed with the dedicated JWST pipeline up to level 2 (but
without Flat Field correction), then we used our own specific procedures for
subtracting reference star images. Contrasts are measured azimuthally as a
function of the angular separations first in the raw coronagraphic image
(azimuthal mean), and then with the subtraction of a reference image
(azimuthal standard deviation). The reference-subtracted image is created
either by subtracting a single reference exposure from the target exposure
(one-to-one subtraction), or by subtracting an image constructed from a
combination of multiple reference exposures (PCA subtraction). In
commissioning we used the SGD mode for both the target and the reference, so
we have 9 images for each, which means 81 possible one-to-one subtractions and
9 PCA subtractions. We only display a resulting contrast curve which takes the
best value of the contrasts for each separation, independently (consistent
with the best subtraction producing the largest contrast at all separations).
Examples of the subtracted images for the three 4QPMs are given in Fig. 4. The
measurements are compared to the noiseless simulated contrasts for the three
scenarios mentioned in section 3, and to the ETC estimations using the actual
flux of the observed stars and exposure times, together with a ”medium
background” configuration. The ETC assumes that the reference star subtraction
is limited by photon and detector noises, so does not capture a possible
variability in the optical wavefront aberrations, or pointing.

Figure 5 displays the contrast curves for all four coronagraphic filters. The
raw contrasts (brown lines) are in good agreement with the simulations of the
4QPMs coronagraphs. The F1065C and F1550C show the characteristic dip in the
center, while the central region with F1140C is clearly affected by the
chromatic leakage. Raw contrasts are usually better than $\sim 10^{-3}$, and
reach the background limit at $\sim 10^{-5}$ (beyond 6′′ or so). The one-to-
one subtraction (orange lines) only provides a small gain with respect to the
residual diffraction left behind the coronagraph, while PCA (red lines) offers
a more substantial attenuation, reaching the background floor at all angular
separations in F1065C and F1140C, which is in line with ETC expectations
(green lines). The two references (REF1 and REF2 in Fig. 6) observed in F1140C
back to back with the target produce very comparable contrast limits (dotted
red line) indicating a small dependence with telescope slew (at least for a
slew amplitude lower than $\sim 20^{\circ}$). In the case we apply PCA using
the reference star observed 11 days later (REF1 2nd epoch in Fig. 6) the
contrast significantly degrades (dashed red line). We interpret it as a result
of telescope re-phasing during this time frame. At the end of May 2022, a
micro meteorite impacted the segment C3, causing a significant departure of
the wavefront with respect to the initial state. The first part of the F1140C
observations (TARG, REF1 and REF2) were observed in this configuration with
approximately 86 nm RMS of aberrations in total on the primary mirror, while
the 2nd epoch of REF1 was obtained after mirror rephasing leading to a reduced
amount of 65 nm RMS aberrations. Such a difference is not visible in the raw
coronagraphic images when comparing the 2 epochs. However, the subtracted
contrast in PCA if using these 2 epochs is reduced by as large as an order of
magnitude at a separation of $0.66^{\prime\prime}$ compared to the case with a
single epoch. The overall contrast is affected up to a distance of about
$4^{\prime\prime}$. We retrieved the wavefront measurements bracketing these
data, assigning the first map to the target and the second to the reference,
in order to model the loss of contrast. We obtained almost the same contrast
curve as the one measured (blue dotted line compared to red dashed line in
Fig. 6) giving credit to our hypothesis.

Overall, for F1065C and F1140C, PCA subtraction using reference stars achieves
a contrast of 2 to $4.10^{-5}$ inside 1′′. This is compatible with the best
case scenario in the noiseless simulation showcasing the very good performance
met by the observatory in terms of pointing, repeatability and stability. In
fact, we estimate that the main parameters defining the subtracted contrast
are even better than the best case with a line of sight jitter likely in the
1-2 mas range, and TA at a level of 5 mas in total. A qualitative comparison
with simulated images indicates a pupil shear of about 2-3% (responsible for
the strip at $\sim 45^{\circ}$ as seen in the coronagraphic images in Fig. 3).
This value will be refined for each coronagraph configuration in Baudoz et al.
(in prep.).

In the F1550C filter, the raw contrasts are in perfect agreement with the
simulation, but both the one-to-one, and the PCA subtraction are much worse
than the ETC or simulated predictions. The 3$\sigma$ contrast achieves only
$\sim 4.10^{-4}$ at $1^{\prime\prime}$ while we could expect $2.10^{-5}$.
Since the starlight rejection by the 4QPMs itself is coherent with the model,
the only plausible explanation is again a mismatch in terms of wavefront
errors between the target and the reference star. In fact, a ’tilt event’
(change in segment position) on one of the primary mirror segment has
presumably occurred in between the 16th and the 19th of June 2022, and could
be the cause of such a reduced contrast. The project has committed to making
telescope OPDs available on roughly 2-day centers, which could identify if it
is plausible that such an event occurred. There is currently no approach to
locate the time of a tilt even more accurately.

Finally, we obtained raw contrast measurement with the F2300C Lyot mask. We
find that the level of contrast matches qualitatively the model prediction but
does not agree perfectly. The discrepancy can be as large as a factor of 2 to
3 between 2′′ and 3′′ (if we omit distances that are inside the mask hence
irrelevant). The observed image itself features a stronger diffraction in the
direction perpendicular to the bar holding the Lyot spot, producing an
asymmetrical image in contrast to the simulation (Fig. 3). The reason of this
disagreement is still under investigation, but we can confidently assume that
like the other coronagraphs, the Lyot mask performance will be set by the
background level when using a reference subtraction. In fact, this is even
more the case at F2300C since the background is stronger. Indeed, while the
exposure times are similar in F1550C and F2300C, the achievable contrast is
$\sim$4 times worse with the Lyot mask (as predicted with ETC).

##  5 Conclusion

Commissioning observations are the first opportunity to actually measure the
performance of the MIRI coronagraphs in real conditions, because high contrast
imaging was not practical during the ground testing phase. The pointing
accuracy and reproducibility have been proved excellent, meeting the
specifications of 5 mas at best, and definitely less than 10 mas (1/10th to
1/20th of a pixel). All four coronagraphs, the 4QPMs and the Lyot, behave
satisfactorily on point sources, in the sense that the coronagraphic images
and the raw contrasts are almost identical to the models, and meet the
contrast specifications. A small difference is found with the Lyot mask, for
which an additional diffraction is superimposed on the predicted image,
perpendicularly to the bar. Calibrating the residual diffraction left
unattenuated by the 4QPMs with reference stars brings the contrast to the
limit imposed by thermal background and detector noises, at least for the two
shortest wavelength filters, F1065C and F1140C. The reference star subtracted
contrast with the F1550C is likely limited by variations of the wavefront
errors of the primary mirror which may have occurred during the observations
(a ’tilt’ event). Similarly, subtracting images taken a few days apart shows
significant deterioration of the contrast due to primary mirror re-phasing
between the two epochs. At the moment, it is recommended that each
coronagraphic observations with MIRI include: a background image of the same
duration as the science exposure, a 9-point small grid dither on the reference
star (5-point SGD was not tested during commissioning), and an off-axis image
of the star for photometric calibration purpose. Alternatively, one can use
the Target Acquisition images for photometry, but these are obtained with
different filters (usually a neutral density filter, FND).

The commissioning of the MIRI Coronagraphs reported here has demonstrated
excellent performance, that the use of the 4QPM technique provides the
expected small inner working angle, and rejection factors and sensitivity in
excess of pre-launch expectations. We can therefore anticipate that with the
observing recommendations in this paper, the MIRI coronagraphs will have a key
role in direct imaging of exoplanets to constrain atmospheric properties for
the very first time at mid IR wavelengths.

###### Acknowledgements.

The work presented is the effort of the entire MIRI team and the enthusiasm
within the MIRI partnership is a significant factor in its success. We would
like to thank Scott Rhorbach for his invaluable assistance and modelling to
resolve the cause of the glowsticks issue, and the JWST and MIRI commissioning
teams for their support in the execution of MIRI commissioning. The following
National and International Funding Agencies funded and supported the MIRI
development: NASA; ESA; Belgian Science Policy Office; Centre Nationale
d’Etudes Spatiales (CNES); Danish National Space Centre; Deutsches Zentrum fur
Luft-und Raumfahrt (DLR); Enterprise Ireland; Ministerio De Economiá y
Competividad; Netherlands Research School for Astronomy (NOVA); Netherlands
Organisation for Scientific Research (NWO); Science and Technology Facilities
Council; Swiss Space Office; Swedish National Space Board; and UK Space
Agency. MIRI draws on the scientific and technical expertise of the following
organizations: Ames Research Center, USA; Airbus Defence and Space, UK; CEA-
Irfu, Saclay, France; Centre Spatial de Liège, Belgium; Consejo Superior de
Investigaciones Cientficas, Spain; Carl Zeiss Optronics, Germany; Chalmers
University of Technology, Sweden; Danish Space Research Institute, Denmark;
Dublin Institute for Advanced Studies, Ireland; European Space Agency,
Netherlands; ETCA, Belgium; ETH Zurich, Switzerland; Goddard Space Flight
Center, USA; Institute d’Astrophysique Spatiale, France; Instituto Nacional de
Técnica Aeroespacial, Spain; Institute for Astronomy, Edinburgh, UK; Jet
Propulsion Laboratory, USA; Laboratoire d’Astrophysique de Marseille (LAM),
France; Leiden University, Netherlands; Lockheed Advanced Technology Center
(USA); NOVA Opt-IR group at Dwingeloo, Netherlands; Northrop Grumman, USA; Max
Planck Institut für Astronomie (MPIA), Heidelberg, Germany; Laboratoire
d’Etudes Spatiales et d’Instrumentation en Astrophysique (LESIA), France; Paul
Scherrer Institut, Switzerland; Raytheon Vision Systems, USA; RUAG Aerospace,
Switzerland; Rutherford Appleton Laboratory (RAL Space), UK; Space Telescope
Science Institute, USA; Toegepast-Natuurwetenschappelijk Onderzoek (TNOTPD),
Netherlands; UK Astronomy Technology Centre, UK; University College London,
UK; University of Amsterdam, Netherlands; University of Arizona, USA;
University of Bern, Switzerland; University of Cardiff, UK; University of
Cologne, Germany; University of Ghent; University of Groningen, Netherlands;
University of Leicester, UK; University of Leuven, Belgium; University of
Stockholm, Sweden; Utah State University, USA. A portion of this work was
carried out at the Jet Propulsion Laboratory, California Institute of
Technology, under a contract with the National Aeronautics and Space
Administration.

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