Further author information: (Send correspondence to O.A.)
O.A.: E-mail<EMAIL_ADDRESS>
# METIS high-contrast imaging: from final design to manufacturing and testing

Olivier Absil  STAR Institute, Université de Liège, Allée du Six Août 19c,
B-4000 Liège, Belgium  Matthew Kenworthy  Leiden Observatory, Leiden
University, P.O. Box 9513, 2300 RA Leiden, The Netherlands  Christian
Delacroix  STAR Institute, Université de Liège, Allée du Six Août 19c, B-4000
Liège, Belgium  Gilles Orban de Xivry  STAR Institute, Université de Liège,
Allée du Six Août 19c, B-4000 Liège, Belgium  Lorenzo König  Jet Propulsion
Laboratory, California Institute of Technology, 4800 Oak Grove Dr., Pasadena,
CA 91109, USA  STAR Institute, Université de Liège, Allée du Six Août 19c,
B-4000 Liège, Belgium  Prashant Pathak  Department of SPASE, Indian Institutes
of Technology Kanpur, 208016, UP, India  STAR Institute, Université de Liège,
Allée du Six Août 19c, B-4000 Liège, Belgium  David Doelman  SRON Netherlands
Institute for Space Research, Niels Bohrweg 4, 2333 CA, Leiden, The
Netherlands  Leiden Observatory, Leiden University, P.O. Box 9513, 2300 RA
Leiden, The Netherlands  Emiel Por  Space Telescope Science Institute (STScI),
3700 San Martin Dr, Baltimore MD, 21218, USA  Leiden Observatory, Leiden
University, P.O. Box 9513, 2300 RA Leiden, The Netherlands  Frans Snik  Leiden
Observatory, Leiden University, P.O. Box 9513, 2300 RA Leiden, The Netherlands
Joost van den Born  NOVA Optical and Infrared Instrumentation Group, P.O. Box
2, 7990 AA Dwingeloo, The Netherlands  Faustine Cantalloube  Université
Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France  Alexis Carlotti
Université Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France  Benjamin
Courtney-Barrer  European Southern Observatory, Alonso de Córdova 3107,
Vitacura, Santiago, Chile  Research School of Astronomy and Astrophysics,
Australian National University, Canberra 2611, Australia  Pontus Forsberg
Department of Materials Science and Engineering, Uppsala University,
Lägerhyddsvägen 1, 751 03 Uppsala, Sweden  Mikael Karlsson  Department of
Materials Science and Engineering, Uppsala University, Lägerhyddsvägen 1, 751
03 Uppsala, Sweden  Thomas Bertram  Max-Planck-Institut für Astronomie,
Königstuhl 17, 69117 Heidelberg, Germany  Roy van Boekel  Max-Planck-Institut
für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany  Dennis Dolkens  NOVA
Optical and Infrared Instrumentation Group, P.O. Box 2, 7990 AA Dwingeloo, The
Netherlands  Markus Feldt  Max-Planck-Institut für Astronomie, Königstuhl 17,
69117 Heidelberg, Germany  Adrian M. Glauser  ETH Zürich, Institute for
Particle Physics & Astrophysics, Wolfgang-Pauli-Str. 27, 8093 Zürich,
Switzerland  Eric Pantin  Université Paris-Saclay, Université Paris Cité, CEA,
CNRS, AIM, 91191 Gif-sur-Yvette, France  Sascha P. Quanz  ETH Zürich,
Institute for Particle Physics & Astrophysics, Wolfgang-Pauli-Str. 27, 8093
Zürich, Switzerland  Felix Bettonvil  Leiden Observatory, Leiden University,
P.O. Box 9513, 2300 RA Leiden, The Netherlands  Bernhard Brandl  Leiden
Observatory, Leiden University, P.O. Box 9513, 2300 RA Leiden, The Netherlands

###### Abstract

The Mid-infrared ELT Imager and Spectrograph (METIS) is one of the first-
generation scientific instruments for the ELT, built under the supervision of
ESO by a consortium of research institutes across and beyond Europe. Designed
to cover the 3 to 13 $\mu$m wavelength range, METIS had its final design
reviewed in Fall 2022, and has then entered in earnest its manufacture,
assembly, integration, and test (MAIT) phase. Here, we present the final
design of the METIS high-contrast imaging (HCI) modes. We detail the
implementation of the two main coronagraphic solutions selected for METIS,
namely the vortex coronagraph and the apodizing phase plate, including their
combination with the high-resolution integral field spectrograph of METIS, and
briefly describe their respective backup plans (Lyot coronagraph and shaped
pupil plate). We then describe the status of the MAIT phase for HCI modes,
including a review of the final design of individual components such as the
vortex phase masks, the grayscale ring apodizer, and the apodizing phase
plates, as well as a description of their on-going performance tests and of
our plans for system-level integration and tests. Using end-to-end
simulations, we predict the performance that will be reached on sky by the
METIS HCI modes in presence of various environmental and instrumental
disturbances, including non-common path aberrations and water vapor seeing,
and discuss our strategy to mitigate these various effects. We finally
illustrate with mock observations and data processing that METIS should be
capable of directly imaging temperate rocky planets around the nearest stars.

###### keywords:

ELT, mid-infrared instrumentation, high-contrast imaging, coronagraphy,
performance simulation, manufacturing and testing

##  1 INTRODUCTION

The mid-infrared ELT imager and spectrograph (METIS)[1] is one of the first-
generation instruments of the Extremely Large Telescope. METIS recently passed
its final design review, and is expected to see first light in 2029[2].
Specifically designed to deliver high-contrast imaging (HCI) capabilities,
METIS features a high-performance adaptive optics module and advanced
coronagraphic concepts that can be combined with imaging cameras covering the
3–13 $\mu$m range, as well as with a high-resolution (R=100,000) integral
field spectrograph operating from 3 to 5 $\mu$m. The METIS instrument is
divided into a series of sub-systems, all located inside the METIS cryostat:

  * •

The CFO (common fore-optics) implements a series of important functions such
as derotation and chopping. It gives access to rotating wheel mechanisms in a
pupil plane (CFO-PP1) upstream of the derotator and in a focal plane (CFO-FP2)
downstream of the chopper, where coronagraphic elements can be implemented.
Another focal plane (CFO-FP1) and pupil plane (CFO-PP2) are respectively used
up by a pupil stabilization mirror and by the chopper.

  * •

The SCAO (single-conjugate adaptive optics) implements an infrared pyramid
wavefront sensor operating in the H and K bands, which is used to control the
ELT-M4 and M5 adaptive mirrors[3]. SCAO is also used to control the pupil
stabilization mirror located in the first focal plane of CFO. The beam feeding
SCAO is picked up just after this focal plane, upstream of the chopper.

  * •

The IMG (imager), located after the second focal plane of the CFO, includes
two infrared cameras covering respectively the LM bands (IMG-LM) and the N
band (IMG-N). Both arms include a dedicated pupil plane (IMG-LM-PP1 and IMG-N-
PP1), where cold stops and/or coronagraphic elements can be placed.

  * •

The LMS (LM spectrograph) is fed by sliding a mirror or beamsplitter just
after the second focal plane of the CFO. If the beamsplitter is used, 10% of
the light will simultaneously be fed to the IMG. After passing through a pupil
plane (LMS-PP1), where coronagraphic elements can be placed, the beam enters a
field stop before being presented to the image slicer and propagating through
the rest of the spectrograph.

METIS also includes a warm calibration unit (WCU) outside of its cryostat. A
schematic representation of the various pupil planes and image planes
available in METIS is given in Fig. 1.

---
Figure 1:  Schematic representation the METIS design, and of the placement of
HCI components within METIS for the two baseline coronagraphic concepts
(vortex coronagraph and apodized phase plate). Pupil and focal planes are
highlighted with orange ellipses, and some of the main functionalities of the
instrument are displayed.

METIS will address various science cases, ranging from our solar system to
distant galaxies, but is more specifically optimized to study nearby planetary
systems and planetary formation using a combination of HCI and high-resolution
spectroscopy (HRS).

##  2 THE METIS HIGH-CONTRAST IMAGING MODES

METIS is expected to bring major breakthroughs in exoplanet science thanks to
the exquisite angular resolution and sensitivity enabled by the 39-m ELT
aperture. To take full advantage of this situation, a vortex coronagraph[4] is
implemented in METIS to reach the smallest inner working angle (IWA) while
maintaining a high throughput. A single-grating vector apodizing phase plate
(vAPP)[5], creating two conjugated PSFs with dark holes on either side of the
star, is also implemented for more robustness to aberrations, at the expense
of a larger IWA and lower throughput.

###  2.1 Vortex coronagraphy

Our implementation of vortex coronagraphy is based on the annular groove phase
mask (AGPM)[4] concept, which uses the form birefringence of a concentric
subwavelength grating to create a spatially variant half-wave plate defining a
helical phase ramp through geometric (Pancharatnam-Berry) phase. The
implementation of this specific vortex phase mask (VPM) using reactive ion
etching on a diamond substrate has already demonstrated its performance in the
thermal infrared both in the lab[6, 7] and on sky[8]. The METIS CFO-FP2 focal-
plane wheel will host three different types of VPMs, covering respectively the
L band (VPM-L), the M band (VPM-M), and the N band (VPM-N), with typical
operating bandwidths ranging from about 30% (L and M bands) to 50% (N band).
The VPM gratings have been specifically designed to cover these wide bands
with high coronagraphic performance, following the same methodology as in
previous works[9]. In order not to drive the overall HCI performance, the VPMs
need to achieve rejection rates higher than 500:1 on average over the L-M
bands, and higher than 100:1 on average over the N band. We also require the
rejection rates to be better than 100:1 at any given wavelength in L-M bands,
and better than 20:1 at any given wavelength in N band. Such performance is
compatible with the proposed operating bandwidths. In the case of N-band
observations, three identical VPM-N will be placed side-by-side in the CFO-FP2
focal-plane wheel to enable three-point chopping patterns for efficient
background subtraction.

In order to perform coronagraphic observations, the VPMs need to be associated
with Lyot stops in a downstream pupil plane, resulting in what we call the
classical vortex coronagraph (CVC) observing mode in METIS. The Lyot stops are
implemented inside the scientific cameras, respectively in the IMG-LM-PP1,
IMG-N-PP1, and LMS-PP1 pupil plane wheels. They are designed to account for
the effects of pupil instabilities and pupil blurring, which require their
outer edge to be undersized, while their central obscuration and spiders need
to be oversized, compared to the input ELT pupil. The amount of
undersizing/oversizing is driven both by the stellar extinction performance
and by need to properly reject the background emission within the considered
field of view (the Lyot stops also act as cold stops). The undersizing of the
outer pupil edge is computed relative to the largest inscribed regular
dodecagon within the ELT-M1 pupil (38177 mm in diagonal in ELT-M1 space),
while the oversizing of the central obscuration is computed relative to the
smallest circumscribed hexagon around the ELT-M1 central obscuration (11608 mm
in diagonal in ELT-M1 space). The size difference of the Lyot stops relative
to the ELT pupil typically corresponds to about 2% to 3% of the ELT nominal
pupil diameter (38542 mm in ELT-M1 space). In addition, because a significant
amount of residual stellar light is located around the central obscuration in
a pupil plane downstream the VPM, the size of the central obscuration has been
increased by 9% of the ELT nominal diameter – a value that results from a
trade-off between throughput and stellar light cancellation efficiency. The
design of the “classical” Lyot stop (referred to as CLS) for the CVC mode is
represented in Figure 2 in the case of IMG-LM-PP1, where the ELT nominal pupil
diameter corresponds to about 45 mm. In practice, five of the six spider arms
will be made slightly thinner thanks to the latest evolution in the design of
the ELT spider arms. Also, as explained in Section 4.1, the thickness of one
or two of these spider arms (or part thereof) will be made thicker to enable
focal-plane wavefront sensing using an asymmetric Lyot sensor concept. The
overall throughput of the CVC mode relative to the input ELT pupil at 4
$\mu$m, taking into account the VPM transmission, the Lyot stop transmission,
and the loss in encircled energy in a 1-FWHM aperture due to the Lyot stop
shape, amounts to about 53% at 3.8 $\mu$m, not taking into account the off-
axis transmission profile of the CVC mode. It would reduce to about 44% for an
actual off-axis companion at $5\lambda/D$ from the optical axis. This is to be
compared with a typical throughput of 70% for the standard imaging mode (this
value accounts for the throughput of the cold stop and associated loss in
encircled energy).

---
Figure 2:  Left. ELT input pupil, illustrating our dodecagonal/hexagonal all-
glass approximation (white) compared to the full input pupil (gray). Middle.
Design of the Lyot stops associated with the classical (middle left) and ring-
apodized (middle right) vortex modes, as implemented at IMG-LM-PP1. Sizes are
in given in mm, with the ELT nominal diameter corresponding to 45 mm in this
pupil plane. Right. Design of the ring apodizer (RAP). Sizes are in given in
mm, with the ELT nominal diameter corresponding to 68 mm in this pupil plane.
The relative dimensions of all masks with respect to the input ELT pupil are
respected to illustrate the amount of oversizing or undersizing in the various
edges.

In addition to the CVC mode, METIS includes a more aggressive coronagraphic
mode also using the VPM as a phase mask, which aims to further reduce the
amount of stellar leakage compared to the CVC mode at the expense of
throughput. This mode is based on the concept of the ring-apodized vortex
coronagraph (RAVC)[10], which compensates for the harmful diffractive effect
of the central obscuration on the vortex coronagraph rejection rate by
introducing a grayscale apodizer (referred to as ring apodizer, or RAP) in an
upstream pupil plane. When properly designed in terms of throughput and
geometry, this compensation would be perfect for an annular pupil in the
absence of optical aberrations. In our case, the performance of the RAVC is
limited by the dodecagonal and hexagonal shapes of the outer and inner edges
of the ELT-M1 pupil, as well as by the presence of the spiders. Because it is
placed in a pupil plane (CFO-PP1) upstream of the derotator, where the pupil
is not stabilized in terms of clocking, the RAP cannot be designed to (even
partly) compensate for these non-radial features of the ELT input pupil.
Nevertheless, the theoretical rejection rate provided by the RAVC can be
improved by a factor around 10 compared to the CVC mode, which will be
particularly useful for the brightest targets where speckle noise is driving
the performance budget. The higher rejection rate and lower throughput of the
RAVC have been specifically designed to allow the observations of $\alpha$ Cen
A (magnitude $L=-1.6$) without the need of a neutral density. The design of
the RAP is illustrated in Figure 2. The grayscale area, producing a 62.5%
transmission, is oversized with respect to the input ELT pupil, to make sure
that the ELT pupil image always remains fully inscribed inside the outer edge
of the RAP even in presence of pupil drifts. The overall throughput of the
RAVC in terms of encircled energy at 3.8 $\mu$m for an off-axis companion at
$5\lambda/D$ is about 14% relative to the input ELT pupil.

###  2.2 Apodizing phase plate coronagraphy

While the CVC is supposed to be the workhorse HCI mode in METIS, the
achievable performance of the vortex coronagraph strongly depends on the
quality and stability of the wavefront that will be delivered by the ELT
combined with the METIS adaptive optics system. In particular, a charge-2 VPM
(like the ones used in METIS) is known to be particularly sensitive to
pointing errors, including the high-frequency contribution of wind shake on
the ELT structure. While these effects have been taken into account in the
end-to-end simulations used to assess the performance of the various HCI modes
(see Sect. 5), the early performance of the ELT and of the METIS adaptive
optics system remains a significant risk for the early operations of the METIS
HCI modes. For this reason, a more robust HCI concept based on a single pupil
plane element has been added to the METIS observing mode. The performance of a
pupil-plane coronagraph is (to the first order) insensitive to tip-tilt errors
caused by vibrations or residual wavefront errors from the adaptive optics
system. Moreover, tip-tilt insensitivity is very relevant for thermal infrared
observations, where chopping/nodding are required to remove the background.
Considering its technological maturity and high performance, the vector
apodiziding phase plate (vAPP)[5] was selected as a baseline for pupil-plane
coronagraphy in METIS.

Figure 3: APP-IMG phase pattern and resultant monochromatic PSF. Left. Phase
of the vAPP as a function of position in the METIS cold stop pupil. Right. The
two (monochromatic) coronagraphic PSFs formed by this phase pattern, located
on either side of a central leakage term of about 1% intensity relative to the
input on-axis starlight.

The vAPP is a transparent pupil plane element that modifies the phase in the
pupil plane to create regions in the PSF where the star light is suppressed,
so-called dark zones. The vAPP uses the geometric (Pancharatnam-Berry) phase
to generate two coronagraphic PSFs that have dark zones on opposite sides of
the point spread function, for opposite circular polarization states. Adding a
polarization grating to this device enables to spatially separate the two PSFs
in the focal plane, as illustrated in Figure 3. Because their technology has
never been tested at wavelengths beyond 5 $\mu$m, and because their lower
throughput (compared to the CVC) is not very appropriate for N-band
observations, vAPPs are only implemented in the IMG-LM and the LMS. The vAPPs
will be placed in the first pupil plane of these two sub-systems, where a
high-quality, derotated and stabilized pupil image is available. As for the
Lyot stops, the vAPPs will also serve as cold stops, by integrating an opaque
metal mask.

The design of the vAPPs was optimized as a trade-off between various competing
performance metrics: depth of the dark zones, Strehl ratio, inner working
angle (IWA), and outer working angle (OWA). This trade-off also had to take
into account the wavelength smearing due to the wavelength-dependent behaviour
of the polarization grating, and the specific geometry of the METIS pupil and
of the focal-plane detector. The main design guidelines are summarized below
in the case of the vAPP that will be installed in the IMG-LM (referred to as
APP-IMG):

  * •

the center of the two APP-IMG PSFs shall be offset by $10\lambda/D$ from the
on-axis leakage term, with an angle of 50∘ to reduce the effect of wavelength
smearing as much as possible, and ensure that both coronagraphic PSFs are
located within the well-corrected zone by the ELT-M4 deformable mirror;

  * •

the dark holes shall be extended along the Hawaii 2RG detector rows
(perpendicular to the read-out channels), and the maximum PSF intensity in the
detector rows including the dark holes shall be less than $5\times 10^{-3}$ of
the peak in the non-coronagraphic PSF;

  * •

the extension of the dark holes along the detector columns (read-out channels)
shall be defined by an IWA of $2.5\lambda/D$ and an OWA of $20\lambda/D$;

  * •

the dark hole depth shall be $<10^{-3.5}$ at the IWA, and gradually go down to
a dark hole depth of $10^{-5}$ beyond $5\lambda/D$ (note: this also includes
light from the conjugated PSF, which should not perturb the considered dark
hole);

  * •

the APP-IMG PSF shall have a Strehl ratio larger than 60%.

The resulting design of the phase pattern is illustrated in Figure 3, together
with the resulting PSF. One spider arm of the vAPP pupil mask was purposely
made thicker to enable focal-plane wavefront sensing[11] (see also Sect. 4.1).
In the case of the LMS, the dark zone of the vAPP was designed to match the
size of the image slicer as much as possible, so that the integral field
spectrograph receives as few stellar photons as possible. The overall
throughput of the APP modes at 3.8 $\mu$m, taking into account the intrinsic
transmission of the vAPP, the reduction in encircled energy, and the splitting
of the beam into two parts, amounts to about 14% relative to the input ELT
pupil.

###  2.3 HCI back-up modes

Manufacturing high-technology coronagraphic devices such as the VPMs and the
vAPPs comes with its intrinsic risks. In order to mitigate these risks as much
as possible, backup plans for both observing modes have been integrated into
the METIS final design. The backup plan for the vortex observing modes
consists of a classical Lyot coronagraph (CLC), while the backup plan for the
APP observing mode consists of a binary amplitude apodizer referred to as
shaped pupil plate (SPP).[12] The design of the Lyot occulting masks (LOM) was
optimized together with their dedicated Lyot stop in a trade-off between IWA,
throughput, and coronagraphic performance, resulting in an on-sky occulter
size of 85 mas for L- and M-band operations, and of 250 mas for N-band
operations. The associated Lyot stops, which had to be significantly
undersized in their outer edge to mitigate starlight residuals, have a
throughput of 63% compared to the input ELT pupil, leading to an overall
throughput around 44% when taking into account the focal-plane mask throughput
and the loss in encircled energy. The design of the SPP was optimized with
similar constraints as the vAPP, in a trade-off between IWA, OWA, throughput,
and depth of the dark zones. The final design of the SPP, together with the
associated PSF, is illustrated in Figure 4. In this design, two half spiders
have been made thicker in the pupil stop to enable asymetric pupil focal-plane
wavefront sensing[13] (see Section 4.1). The overall SPP throughput, taking
into account the transmission of an AR-coated ZnSe pupil-plane substrate, is
about 22% in terms of encircled energy, compared to the input ELT pupil.

Figure 4: Left. Design of the shaped pupil plate (SPP), considered as a backup
option for the vAPP mode. Middle & right. Illustrations of the associated
monochromatic PSF, at two levels of zoom.

The coronagraphic elements needed to implement the two backup modes will be
installed inside the instrument, but will only be tested during assembly,
integration and test at system level (system AIT) and deployed on sky in case
one of the two HCI baseline modes (vortex and APP) does not deliver according
to expectation, due to the intrinsic performance of the as-built coronagraphic
components. If the baseline modes are shown to perform as planned during
system AIT and commissioning, the backup modes will not be offered to the
community nor supported in terms of pipeline processing.

##  3 MANUFACTURE, ASSEMBLY, INTEGRATION, AND TEST

With its system-level final design review (FDR) passed in late 2022, METIS has
entered its manufacture, assembly, integration and test (MAIT) phase in
earnest. This phase starts at component and subsystem level, followed by the
system-level AIT that will consist in integrating and testing all subsystems
inside the METIS cryostat at the integration hall in Leiden. Here, we describe
the progress with the MAIT of HCI-specific components, which will later be
integrated into the appropriate subsystems (CFO, IMG, and LMS).

###  3.1 Vortex phase masks

At the preliminary design review (PDR), the vortex phase masks (VPM) were
identified as long-lead items. They were therefore the subject of an early FDR
procedure with a series of other long lead items, which took place in
mid-2021. A procurement contract was then signed with Uppsala University in
Spring 2022, and the manufacturing started in earnest in Fall 2022. The
manufacturing started with the N-band VPMs, because their testing requires
more time due to the need for a specific cryogenic facility at CEA Saclay.[14]
A first N-band VPM, referred to as VPM-N4, was manufactured in February 2023,
and –after having to handle some issues with diamond substrate quality– two
additional ones, referred to as VPM-N8 and N9, were manufactured in June 2023.
All three VPMs, which have a diameter of 15 mm and include an anti-reflective
(AR) grating on their back side, were tested at CEA Saclay in a series of
three cryogenic runs, with constant improvements in the coronagraphic test
setup. It turned out that VPM-N8 and N9 did not meet the performance
requirement at first, while VPM-N4 proved to be compliant with the 100:1
average rejection rate across the N band (see Figure 5). VPM-N8 and N9 were
thus sent back to Uppsala for re-etching, based on an updated estimation of
the exact grating parameters (especially grating depth, which is impossible to
measure in a non-destructive way) informed by the measured coronagraphic
performance for a series of five wavelengths across N band. After re-etching
in January 2024, which made the grating about 800 nm shallower, both VPMs were
re-tested at CEA in February 2024, demonstrating exquisite performance (well
beyond the requirement, see Figure 5). All the details of the manufacturing,
testing, and re-etching procedures can be found elsewhere.[7, 14] The three
N-band VPMs have now been delivered to ULiège, where they will be soon
installed and centered into their custom-designed cartridge, before being
integrated into the CFO-FP2 focal-plane wheel.

Figure 5: Measured performance of two of the three N-band VPMs manufactured
for METIS (left: VPM-N4, right: VPM-N8). Not displayed here is VPM-N9, which
shows very similar performance to VPM-N8. The final, consolidated measurements
are displayed in orange, with ealier results in shades of gray. In the case of
VPM-N8, the gray-colored measurements were obtained before the re-etching of
the VPM, showing poor early performance. The requirement in terms of average
rejection rate across the N band is displayed with a green dashed line for
reference. Figure 6: Measured performance of VPM-L (blue) and VPM-M (orange),
using the VODCA coronagraphic bench of ULiège. The black lines highlight the
width of the filters used for the measurements, while the crosses represent
monochromatic (QCL laser) measurements. Both have vertical error bars of about
10% (not shown here). The requirement on the average rejection rate on the
operating waveband (500:1) is highlighted in green. The VPMs should also
provide a rejection rate better than 100:1 at any given wavelength (red dashed
line). These requirements apply to a wavelength range of 2.9–4.1 $\mu$m for
VPM-L, and of 3.9–5.3 $\mu$m for VPM-M.[15]

After the N-band VPMs, the L- and M-band VPMs were manufactured in parallel,
using 20-mm diameter diamond substrates. The initial etching of the two
concentric gratings and of the square AR gratings on their back side was
performed in December 2023 at Uppsala, followed by coronagraphic testing on
the ULiège VODCA bench[16] in February 2024. While VPM-L showed excellent
performance, well above the requirement of 500:1 over the 2.9–4.1 $\mu$m range
(see Figure 6), VPM-M showed poor performance across its target operating
range (3.9–5.3 $\mu$m), which was expected as the etching process lasted too
long, leading to a deeper grating than originally targeted. VPM-M was
therefore sent to Uppsala for re-etching, based on a more precise estimation
of the grating depth retrieved from the measured rejection rate. VPM-M was
then tested again at ULiège in May 2024, showing an average performance
compliant with the average rejection rate requirement from about 4.3 to 5.3
$\mu$m (see Figure 6). While it was originally designed to cover wavelengths
down to 3.9 $\mu$m in order to ensure a good overlap with VPM-L, the re-etched
version of VPM-M is deemed to be sufficient good for integration into METIS,
since the 3.9–4.2 $\mu$m region is very well covered by VPM-L, and since the
4.2–4.4 $\mu$m region is not prone to high-quality ground-based observations
due to the very low transparency of the Earth atmosphere at these wavelengths.
More details about the MAIT of the METIS vortex phase masks can be found in a
dedicated SPIE paper.[15]

###  3.2 Apodizing phase plates

The vAPPs are produced with a liquid-crystal technology, where a direct-write
system is used to print the desired fast-axis orientation pattern in a liquid-
crystal photo-alignment layer deposited on a substrate. Multiple layers of
self-aligning birefringent liquid-crystals are deposited on top of each other
with varying thickness and twist, carefully designed to generate the required
half-wave retardance.[5] This liquid crystal layer is then sandwich together
with an opaque metal mask between two AR-coated CaF2 substrates, as
illustrated in Figure 7. A wedge is introduced in one of the two substrates to
offset any ghost arising from internal reflections inside the substrates. The
two substrates are glued together with a very thin, IR-transparent glue layer.

The manufacturing of the vAPPs is currently on-going at ColorLink Japan Ltd.
In order to prepare the manufacturing of the final components, several
intermediate steps have been taken, including the printing of a visible-
wavelength version of the METIS vAPP phase patterns (one for the APP-IMG, one
for the APP-LMS), and infrared transmission measurements of the glue layer on
a representative CaF2 substrate. The monochromatic PSFs created at visible
wavelengths by the two prototypes are illustrated in Figure 7. These test
components demonstrate that the patterning quality is high and that no defects
are present in the active areas. Their associated PSFs show a high-quality
dark zone except for the presence of an optical ghost not related to the vAPP,
and a leakage term of the order of 1%, demonstrating good control of the
liquid-crystal recipes. While the final vAPPs cannot be fully tested in a
cryogenic environment ahead of installation into their respective subsystem,
we are planning to test the high-contrast performance of APP-LMS on the VODCA
bench at ULiège in Fall 2024. This will not be possible for APP-IMG, which has
a too large pupil (about 45 mm in diameter compared to 20 mm for APP-LMS) to
fit on the VODCA bench.

Figure 7: Left. Practical implementation of the vAPP, illustrated here for
APP-IMG. Right. In-lab PSFs measured at visible wavelengths for the two vAPP
prototypes, respectively for APP-IMG (left) and APP-LMS (right).

###  3.3 Other HCI components

An additional HCI component required a dedicated development: the ring
apodizer (RAP). Following previous experiments with similar apodizers[17, 18],
we designed the grayscale region using a microdot pattern. The details related
to the design of the microdot pattern are presented in a separate paper.[19]
The microdot pattern and the other opaque parts of the RAP (see Figure 2) are
manufactured by Opto-Line Inc through chromium deposition onto an SiO-
protected, AR-coated ZnSe substrate, procured from G&H Artemis Optical. This
technology was previously used to manufacture custom Lyot stops for the vortex
coronagraph of the NEAR experiment on VLT/VISIR.[20] Before printing the final
component, we printed in Summer 2023 a series of microdot patterns of various
sizes, thicknesses, and filling factors, in order to select the microdot
parameters that provide the most appropriate effective throughput. These
patterns were characterized on the VODCA bench, and tested for robustness to
cryogenic conditions. A detailed report on these tests is given elsewhere.[19]
Finally, the RAP was printed in June 2024 by Opto-Line Inc, and will be
further tested in Liège in Summer 2024.

The coronagraphic elements needed to implement the two backup modes (i.e., the
LOMs and SPP) follow the same manufacturing process as the RAP, based on
chromium deposition on a SiO-protected, AR-coated ZnSe substrate. The Lyot
stops, on the other hand, will be manufactured by drilling holes into a well-
chosen substrate, which will then be coated with a black painting such as
Acktar BlackTM.

##  4 HCI OPERATION STRATEGY AND SYSTEM-LEVEL TESTING

In addition to the need for dedicated components, reaching optimal HCI
performance also requires specific operation strategies, especially in terms
of wavefront control. Here, we summarize our approach to mitigate the effect
of non-common path aberrations (NCPA), and briefly describe our plans for
testing the METIS HCI performance in representative conditions ahead of
commissioning.

###  4.1 NCPA & focal-plane wavefront sensing

Beyond the design of the coronagraph and the residual wavefront errors from
the AO system, one of the main drivers in the performance of any HCI
instrument comes from NCPAs.[21] The requirements on an appropriate mitigation
strategy strongly depend on the level and evolution time scale of NCPAs in the
instrument. The first order of business is therefore to identify the main
sources of NCPAs. The contribution from optical surfaces in the non-common
path between the SCAO pyramid wavefront sensor and the first pupil plane
inside the imager (where Lyot stops and vAPPs are located) is expected to
amount to about 120 nm rms, and to be mostly static since it is fully
contained in a gravity-invariant temperature-control vacuum cryostat. The main
sources of variable NCPAs are associated with chromatic effects between the
near infrared (K band) where SCAO operates, and the mid-infared (L to N bands)
where the scientific cameras operate. These chromatic NCPAs have two main
origins:

  * •

Chromatic beam wander (CBW) is due to variations in the optical propagation of
slightly offset beams through the METIS optical train, due to differential
atmospheric refraction between the operating wavelengths of SCAO and
scientific cameras. This contribution has been fully modeled using Fourier
optics propagation through the instrument, and will be the subject of a
dedicated publication (Boné et al., in prep). This effect creates NCPA
variations on a typical timescale of a few minutes, resulting in about 35 nm
rms of differential wavefront error variation over an hour.

  * •

Water vapor seeing (WVS) is related to the strong chromaticity of water vapor
at mid-infrared wavelengths, which adds a wavelength-dependent contribution to
dry-air seeing. This contribution is described in detail in a previous
publication[22]. In terms of wavefront error, it typically ranges from about
30 nm rms at L band to 350 nm rms at N band, with a temporal behavior driven
by the wind speed under a frozen flow hypothesis for Kolmogorov-like
turbulence.

If not corrected, these NCPA variations appearing at typical timescales
ranging from a fraction of a second to tens of minutes would severely degrade
the HCI performance of METIS, as demonstrated in previous works.[23, 22, 24]
This calls for a tailored mitigation strategy. In order to reduce as much as
possible the impact of this mitigation strategy on the METIS hardware, we
elected to make use of focal-plane wavefront sensing (FP-WFS) based on the
data stream from the IMG-LM or IMG-N camera. Non-common path pointing errors
at the level of the VPMs will be taken care of with the QACITS algorithm.[25]
Higher-order modes will be sensed thanks to a custom implementation of the
asymmetric pupil wavefront sensor (AP-WFS)[13], which enforces some level of
asymmetry in the input pupil to lift the ambiguity on even Zernike modes in
the reconstruction of the input wavefront from the measured focal-plane
images. While this concept works well for a pupil-plane coronagraph like the
vAPP, where one only needs to create a custom asymmetry level in the pupil
mask,[11] the situation becomes more complicated for the vortex coronagraph
modes, as their performance would be degraded by introducing an asymmetry in
the input pupil. Creating such an asymmetry in the input pupil plane would
actually not be practical in the case of METIS anyway, as the only available
pupil plane upstream of the VPMs (CFO-PP1) is not stabilized in clocking and
translation. We will therefore implement the pupil asymmetry downstream of the
VPMs, at the level of the Lyot stops, resulting in a new concept referred to
as Asymmetric Lyot waveFront sensor (ALF). Because of the nonlinear nature of
this FP-WFS concept, the implementation of the wavefront reconstruction will
be based on supervised deep learning. This concept is presented in detail in a
dedicated paper.[26]

Our two FP-WFS algorithms (QACITS and AP-WFS/ALF) will only be implemented
with the IMG cameras. When performing high-contrast observations with the LMS,
the structure of the raw data and the associated integration times make it
much more challenging to reconstruct the input wavefront in real time. This is
one of the reasons why 10% of the light will be sent to the IMG-LM during LMS
observations, allowing to use QACITS for precise centering on the VPM (the
same VPM is simultaneously feeding the IMG and LMS). Whether higher-order
modes will also be controlled through the AP-WFS/ALF concept during LMS
observations will be decided later as a trade-off between the expected
performance gain and the operational challenges. In particular, we anticipate
that the high-spectral resolution observations obtained with the LMS will not
be limited by the same instrumental effects as standard HCI modes, and that
the effect of NCPAs might be easier to mitigate in post-processing using
techniques based on spectral cross-correlation, for instance[27].

###  4.2 System-level testing

While most of the various HCI components have or will be tested on dedicated
test facilities ahead of installation into their respective METIS subsystem,
the global performance of the HCI modes can only be assessed once the
instrument is fully assembled, including the science cameras (IMG, LMS) that
contain among others the vAPPs, Lyot stops, and detectors. These HCI
performance tests also require SCAO to work in closed-loop in order to provide
a high wavefront quality, which cannot be delivered by the warm calibration
unit. This in turn requires a telescope simulator, which needs to include a
sufficiently representative deformable mirror, as well as the capabilities to
mimic the beam that will be delivered to METIS by the ELT. The design of the
METIS telescope simulator, which will be used both for subsystem-level testing
of the SCAO module, and for HCI performance testing at system level, is
described in a separate paper.[28] In addition to testing the system-level
performance of the HCI modes, this setup will also be used to validate the two
FP-WFS algorithms (QACITS and AP-WFS/ALF), which will be implemented inside
the SCAO soft real-time computer.

##  5 END-TO-END PERFORMANCE SIMULATIONS

Now that the design of the METIS HCI modes has been finalized, and that the
performance of the individual HCI components is better understood, we can
update the end-to-end performance simulations that were presented just after
the preliminary design review.[23]

###  5.1 Sensitivity limits

Our simulation strategy to estimate the sensitivity limits of METIS was
presented in detail in previous publications[23, 24], and will not be repeated
here. Instead, we will just give a summary of the main steps in our end-to-end
simulations. These simulations start by producing a representative 1h-long
sequence of SCAO residual phase screens, using a METIS-specific version of the
COMPASS simulation software.[29] This sequence is typically sub-sampled at a
frame rate of 300 ms (commensurate with the typical frame rate of IMG-LM),
resulting in a total of 12,000 phase screens. These phase screens are then
propagated through our instrument simulator using the HEEPS package[24], based
on the PROPER Fresnel propagation software.[30] During this propagration
process, other instrumental errors such as NCPA (including the variable
contributions of CBW and WVS), amplitude errors due to the Talbot effect[31],
pupil drifts upstream of the pupil stabilization mirror, imperfections in the
VPM performance, and imperfections in the ELT-M1 pupil (non-uniform
reflectivity, or even misaligned segments), are taken into account. This leads
to a sequence of 12,000 instantaneous, monochromatic PSFs. This process can be
repeated at a series of wavelengths to simulate the effect of the finite
spectral filter width, and to include the effect of atmospheric refraction on
(differential) pointing errors, in order to build a sequence of polychromatic
PSFs. These instantaneous PSFs are then scaled to the appropriate stellar
photo-electron rate at the considered detector, and the thermal background is
added. We finally add photon noise to these various sources to produce a mock
data set, which is then associated with a parallactic angle vector
representative of field rotation in a 1h-long observation in pupil tracking
mode for a given target. We also produce an off-axis PSF for flux calibration
purposes, and present this mock data set to a classical angular differential
imaging (cADI) algorithm implemented in the vortex image processing (VIP)
package.[32, 33] The final product of the VIP-based processing is a $5\sigma$
contrast curve, which represents the sensitivity limit of the observation in
terms of planet/star flux ratio.

Figure 8: Raw contrast (top) and post-ADI contrast curves (bottom) for the
classical vortex coronagraphy mode at N band, under various levels of water
vapor seeing (colored lines), compared to SCAO-only simulations (black line).
Wavefront correction based on FP-WFS (QACITS and ALF algorithms) is performed
on the first 20 Zernike modes at a loop repetition rate of 10 Hz, assuming a
realistic noise level in both sensors.

An important feature in our simulations, which was already introduced in a
previous publication[24], is our implementation of the effect of closed-loop
wavefront control based on our FP-WFS algorithms. For both CBW and WVS, we
generate representative time sequences of NCPA phase screens, which we feed to
a time-domain closed-loop simulation that takes into account the estimated
noise in the FP-WFS as a function of the input star magnitude. These
estimations of the actual amount and variability of residual NCPA at the level
of the coronagraph are key to obtain a realistic prediction of the post-ADI
sensitivity limits, as NCPAs are expected to be one of the main drivers in the
HCI performance budget.[23] In this context, it is particularly interesting to
investigate the influence of the strength of WVS on the HCI performance at N
band, where WVS largely dominates the performance budget. Based on archival
data collected with the GRAVITY fringe tracker at the Very Large Telescope
Interferometer (VLTI), we have been able to obtain a first rough estimate of
the variability in the strength of WVS, using the strategy presented in a
previous work.[22] Based on this preliminary analysis, the median conditions
are expected to correspond to about 6 $\mu$m of rms differential precipitable
water vapor (PWV) on a 30-m baseline, which corresponds to an rms variation in
differential WV column density of about $2\times 10^{19}$ cm-2. These median
conditions lead to an additional wavefront error of about 30 nm rms at L band
and about 350 nm rms at N band, as already mentioned in Sect. 4.1. The best
WVS conditions (percentile 10) may be as good as 1.5 $\mu$m rms differential
PWV (i.e., four times lower variability than the median conditions), while the
worst conditions (percentile 90) are estimated to be around 12 $\mu$m rms
differential PWV (i.e., twice as much as the median conditions). These
preliminary values still need to be confirmed with more data, and will be
fully reported in a future publication. Based on these preliminary estimates,
one can update the end-to-end simulations at N band, taking into account the
closed-loop correction provided by QACITS and ALF. The results of these
simulations are displayed in Figure 8, where we assume a 10-Hz loop repetition
rate, which is expected to be achievable on bright targets (N-band magnitude
lower than $N\simeq 2$). These simulations show how critical it will be to
identify the episodes of lowest WVS to reach the best possible HCI performance
at N band. During these episodes, the HCI performance could be close to being
SCAO-limited – not considering other instrumental effects, such as CBW or
amplitude aberrations due to the Talbot effect, which may start to dominate
the error budget. It is expected that these episodes of very low WVS are
correlated with the PWV content in the atmosphere, as well as to dry air
seeing to some level, although the statistical behavior and correlation of WVS
with respect to other atmospheric effects still need to be further studied.

Our latest estimation of the L- and M-band HCI performance in terms of post-
ADI contrast curves does not significantly differ from our 2022 SPIE
publication[24], as WVS seeing is not the main contribution in the error
budget at those wavelengths, and therefore need not be repeated here.

###  5.2 Prospects for high-contrast exoplanet science

Figure 9: End-to-end simulations of a 5h ADI observing sequence on $\alpha$
Cen A at N band with the classical vortex coronagraph. Two Earth-like planets
(i.e., one Earth radius with an Earth-like atmosphere) were injected into
these simulations (orange circles), one at 1.1 au where it would receive the
same irradiation as the Earth from the Sun, and another (warmer and hence
brighter) one at half that separation. Left. Post-ADI image in the absence of
WVS, which is representative of the best WVS conditions that should be
reachable at the ELT site. Right. Post-ADI image assuming the worst WVS
conditions, as described in Sect. 5.1.

With its high-contrast and high-spectral resolution capabilities, METIS is
expected to enable new breakthroughs in the field of exoplanet science, among
others. It is beyond the scope of this paper to provide a comprehensive
overview of all the HCI-related science cases of METIS. Yet, based on the new
N-band simulations presented above, it is useful to reconsider the capability
of METIS to detect potential Earth-like planets around our nearest neighbors,
and around $\alpha$ Cen A in particular, which is the most promising target
for N-band imaging of an Earth twin.[34] The final ADI images obtained for a
5-h ADI observing sequence with two extreme levels of WVS are displayed in
Figure 9, where we injected two Earth twins respectively in the middle of the
habitable zone (0.82” separation at quadrature), and twice closer. In the
absence of WVS, the two planets are recovered with an SNR of 6 and 11,
respectively, while both of them are below the detection threshold
($\text{SNR}=5$) in the case of strong WVS. These simulations highlight two
important points: (a) in the absence of WVS, the habitable zone around
$\alpha$ Cen A lies in the background-limited and not contrast-limited regime,
and (b) METIS has the potential of imaging an Earth twin within a 5h
integration time, under excellent WVS conditions. METIS also has the potential
of revealing warmer rocky planets at N band around this most favorable target
star, even though in general warmer planets may benefit from observations at
shorter wavelengths (L and M bands), which also provide a significant boost in
angular resolution.

##  6 CONCLUSION & PERSPECTIVES

Over the last couple of years, the METIS project has moved from the final
design phase to the manufacture, assembly, integration, and test phase. The
various components dedicated to the implementation of the HCI modes are now
either fully manufactured, or making good progress towards being manufactured
and qualified for integration into METIS. In the coming months, the HCI
activities will therefore focus more on the finalization of HCI-specific
software for focal-plane wavefront sensing, and of its integration into the
adaptive optics control architecture. Finally, once METIS is fully assembled
in the integration hall at Leiden (2027–2028), the HCI activities will focus
on the validation of the HCI modes at system level, including end-to-end
performance tests under closed-loop adaptive optics conditions, and including
system-level testing of the FP-WFS algorithms.

###### Acknowledgements.

This project has received funding from the European Research Council (ERC)
under the European Union’s Horizon 2020 research and innovation programme
(grant agreement No 819155), and from the Wallonia-Brussels Federation (grant
for Concerted Research Actions).

## References

  * [1] Brandl, B., Bettonvil, F., van Boekel, R., Glauser, A., Quanz, S., Absil, O., Amorim, A., Feldt, M., Glasse, A., Güdel, M., Ho, P., Labadie, L., Meyer, M., Pantin, E., van Winckel, H., and METIS Consortium, “METIS: The Mid-infrared ELT Imager and Spectrograph,” The Messenger 182, 22–26 (Mar. 2021).
  * [2] Brandl, B., Absil, O., Feldt, M., Garcia, P., Glasse, A., Guedel, M., Labadie, L., Meyer, M., Pantin, E., Quanz, S., Wang, S.-Y., van Winckel, H., Bettonvil, F., Glauser, A. M., van Boekel, R., Salo, C., Scheithauer, S., Stuik, R., Haupt, C., and Siebenmorgen, R., “Final design and status of the Mid-Infrared ELT Imager and Spectrograph, METIS,” in [Ground-based and Airborne Instrumentation for Astronomy X ], Bryant, J. J., Motohora, K., and Vernet, J. R. D., eds., Proc. SPIE 13096 (2024).
  * [3] Bertram, T., Absil, O., Bizenberger, P., Brandl, B., Brandner, W., Briegel, F., Cardenas Vazquez, M. C., Coppejans, H., Correia, C., Feldt, M., Glauser, A. M., Häberle, M., Huber, A., Kulas, M., Laun, W., Mohr, L., Mortimer, D., Naranjo, V., Obereder, A., Orban de Xivry, G., Rohloff, R.-R., Scheithauer, S., Steuer, H., and van Boekel, R., “How to make METIS SCAO work,” in [Adaptive Optics Systems IX ], Jackson, K. J., Schmidt, D., and Vernet, E., eds., Proc. SPIE 13097 (2024).
  * [4] Mawet, D., Riaud, P., Absil, O., and Surdej, J., “Annular Groove Phase Mask Coronagraph,” Astrophys. J. 633, 1191–1200 (Nov. 2005).
  * [5] Doelman, D. S., Snik, F., Por, E. H., Bos, S. P., Otten, G. P. P. L., Kenworthy, M., Haffert, S. Y., Wilby, M., Bohn, A. J., Sutlieff, B. J., Miller, K., Ouellet, M., de Boer, J., Keller, C. U., Escuti, M. J., Shi, S., Warriner, N. Z., Hornburg, K., Birkby, J. L., Males, J., Morzinski, K. M., Close, L. M., Codona, J., Long, J., Schatz, L., Lumbres, J., Rodack, A., Van Gorkom, K., Hedglen, A., Guyon, O., Lozi, J., Groff, T., Chilcote, J., Jovanovic, N., Thibault, S., de Jonge, C., Allain, G., Vallée, C., Patel, D., Côté, O., Marois, C., Hinz, P., Stone, J., Skemer, A., Briesemeister, Z., Boehle, A., Glauser, A. M., Taylor, W., Baudoz, P., Huby, E., Absil, O., Carlomagno, B., and Delacroix, C., “Vector-apodizing phase plate coronagraph: design, current performance, and future development [Invited],” Appl. Opt. 60, D52 (July 2021).
  * [6] Delacroix, C., Absil, O., Forsberg, P., Mawet, D., Christiaens, V., Karlsson, M., Boccaletti, A., Baudoz, P., Kuittinen, M., Vartiainen, I., Surdej, J., and Habraken, S., “Laboratory demonstration of a mid-infrared AGPM vector vortex coronagraph,” Astron. Astrophys. 553, A98 (May 2013).
  * [7] Forsberg, P., Karvinen, P., Ronayette, S., Kuittinen, M., Absil, O., König, L., Delacroix, C., Orban de Xivry, G., Barrière, J.-C., Pantin, E., and Karlsson, M., “Making the diamond vortex phase masks for the METIS instrument,” Diam. Relat. Mater. 146 (June 2024).
  * [8] Absil, O., Mawet, D., Karlsson, M., Carlomagno, B., Christiaens, V., Defrère, D., Delacroix, C., Femenía Castella, B., Forsberg, P., Girard, J., Gómez González, C. A., Habraken, S., Hinz, P. M., Huby, E., Jolivet, A., Matthews, K., Milli, J., Orban de Xivry, G., Pantin, E., Piron, P., Reggiani, M., Ruane, G. J., Serabyn, G., Surdej, J., Tristram, K. R. W., Vargas Catalán, E., Wertz, O., and Wizinowich, P., “Three years of harvest with the vector vortex coronagraph in the thermal infrared,” in [Ground-based and Airborne Instrumentation for Astronomy VI ], Evans, C. J., Simard, L., and Takami, H., eds., Proc. SPIE 9908, 99080Q (Aug. 2016).
  * [9] Vargas Catalán, E., Huby, E., Forsberg, P., Jolivet, A., Baudoz, P., Carlomagno, B., Delacroix, C., Habraken, S., Mawet, D., Surdej, J., Absil, O., and Karlsson, M., “Optimizing the subwavelength grating of L-band annular groove phase masks for high coronagraphic performance,” Astron. Astrophys. 595, A127 (Nov. 2016).
  * [10] Mawet, D., Pueyo, L., Carlotti, A., Mennesson, B., Serabyn, E., and Wallace, J. K., “Ring-apodized Vortex Coronagraphs for Obscured Telescopes. I. Transmissive Ring Apodizers,” Astrophys. J. suppl. ser. 209, 7 (Nov. 2013).
  * [11] Bos, S. P., Doelman, D. S., Lozi, J., Guyon, O., Keller, C. U., Miller, K. L., Jovanovic, N., Martinache, F., and Snik, F., “Focal-plane wavefront sensing with the vector-Apodizing Phase Plate,” Astron. Astrophys. 632, A48 (Dec. 2019).
  * [12] Carlotti, A., Vanderbei, R., and Kasdin, N. J., “Optimal pupil apodizations of arbitrary apertures for high-contrast imaging,” Opt. Express 19, 26796 (Dec. 2011).
  * [13] Martinache, F., “The Asymmetric Pupil Fourier Wavefront Sensor,” Publ. Astron. Soc. Pac. 125, 422 (Apr. 2013).
  * [14] Ronayette, S., Mouzali, S., Barrière, J.-C., Pantin, É., Orduna, T., Gallais, P., Lortholary, M., Dumaye, L., Absil, O., Delacroix, C., Ives, D., and Karlsson, M., “An N-band test bench for the METIS coronagraphic masks,” in [Ground-based and Airborne Instrumentation for Astronomy VIII ], Evans, C. J., Bryant, J. J., and Motohara, K., eds., Proc. SPIE 11447, 114472U (Dec. 2020).
  * [15] Delacroix, C., König, L., Absil, O., Orban de Xivry, G., Forsberg, P., Karlsson, M., Ronayette, S., Pantin, E., and Barrière, J.-C., “The ELT/METIS Annular Groove Phase Masks,” in [Advances in Optical and Mechanical Technologies for Telescopes and Instrumentation VI ], Navarro, R. and Jedamzik, R., eds., Proc. SPIE 13100 (2024).
  * [16] Jolivet, A., Orban de Xivry, G., Huby, E., Piron, P., Catalan, E. V., Habraken, S., Surdej, J., Karlsson, M., and Absil, O., “L- and M-band annular groove phase mask in lab performance assessment on the vortex optical demonstrator for coronagraphic applications,” J. Astron. Telesc. Instrum. Syst. 5, 025001 (Apr. 2019).
  * [17] Zhang, M., Ruane, G., Delorme, J.-R., Mawet, D., Jovanovic, N., Jewell, J., Shaklan, S., and Wallace, J. K., “Characterization of microdot apodizers for imaging exoplanets with next-generation space telescopes,” in [Space Telescopes and Instrumentation 2018: Optical, Infrared, and Millimeter Wave ], Lystrup, M., MacEwen, H. A., Fazio, G. G., Batalha, N., Siegler, N., and Tong, E. C., eds., Proc. SPIE 10698, 106985X (July 2018).
  * [18] Llop-Sayson, J., Ruane, G., Mawet, D., Jovanovic, N., Coker, C. T., Delorme, J.-R., Echeverri, D., Fucik, J., Eldorado Riggs, A. J., and Wallace, J. K., “High-contrast Demonstration of an Apodized Vortex Coronagraph,” Astron. J. 159, 79 (Mar. 2020).
  * [19] König, L., Delacroix, C., Absil, O., and Orban de Xivry, G., “Design, manufacturing and prototype testing of the METIS ring apodizer,” in [Advances in Optical and Mechanical Technologies for Telescopes and Instrumentation VI ], Navarro, R. and Jedamzik, R., eds., Proc. SPIE 13100 (2024).
  * [20] Kasper, M., Arsenault, R., Käufl, U., Jakob, G., Leveratto, S., Zins, G., Pantin, E., Duhoux, P., Riquelme, M., Kirchbauer, J. P., Kolb, J., Pathak, P., Siebenmorgen, R., Soenke, C., Fuenteseca, E., Sterzik, M., Ageorges, N., Gutruf, S., Kampf, D., Reutlinger, A., Absil, O., Delacroix, C., Maire, A. L., Huby, E., Guyon, O., Klupar, P., Mawet, D., Ruane, G., Karlsson, M., Dohlen, K., Vigan, A., N’Diaye, M., Quanz, S., and Carlotti, A., “NEAR: First Results from the Search for Low-Mass Planets in $\alpha$ Cen,” The Messenger 178, 5–9 (Dec. 2019).
  * [21] Guyon, O., “Extreme Adaptive Optics,” Annu. Rev. Astron. Astrophys. 56, 315–355 (Sept. 2018).
  * [22] Absil, O., Delacroix, C., Orban de Xivry, G., Pathak, P., Willson, M., Berio, P., van Boekel, R., Matter, A., Defrère, D., Burtscher, L., Woillez, J., and Brandl, B., “Impact of water vapor seeing on mid-infrared high-contrast imaging at ELT scale,” in [Adaptive Optics Systems VIII ], Schreiber, L., Schmidt, D., and Vernet, E., eds., Proc. SPIE 12185, 1218511 (Aug. 2022).
  * [23] Carlomagno, B., Delacroix, C., Absil, O., Cantalloube, F., Orban de Xivry, G., Pathak, P., Agocs, T., Bertram, T., Brandl, B., Burtscher, L., Feldt, M., Glauser, A., Hippler, S., Kenworthy, M., Stuik, R., and van Boekel, R., “METIS high-contrast imaging: design and expected performance,” J. Astron. Telesc. Instrum. Syst. 6, 035005 (July 2020).
  * [24] Delacroix, C., Absil, O., Orban de Xivry, G., Shinde, M., Pathak, P., Cantalloube, F., Carlomagno, B., Christiaens, V., Boné, A., Dolkens, D., Kenworthy, M., and Doelman, D., “The High-contrast End-to-End Performance Simulator (HEEPS): influence of ELT/METIS instrumental effects,” in [Modeling, Systems Engineering, and Project Management for Astronomy X ], Angeli, G. Z. and Dierickx, P., eds., Proc. SPIE 12187, 121870F (Aug. 2022).
  * [25] Huby, E., Baudoz, P., Mawet, D., and Absil, O., “Post-coronagraphic tip-tilt sensing for vortex phase masks: The QACITS technique,” Astron. Astrophys. 584, A74 (Dec. 2015).
  * [26] Orban de Xivry, G., Absil, O., Delacroix, C., Pathak, P., Quesnel, M., and Bertram, T., “ALF: an asymmetric Lyot wavefront sensor for the ELT/METIS vortex coronagraph,” in [Adaptive Optics Systems IX ], Jackson, K. J., Schmidt, D., and Vernet, E., eds., Proc. SPIE 13097 (2024).
  * [27] Snellen, I., de Kok, R., Birkby, J. L., Brandl, B., Brogi, M., Keller, C., Kenworthy, M., Schwarz, H., and Stuik, R., “Combining high-dispersion spectroscopy with high contrast imaging: Probing rocky planets around our nearest neighbors,” Astron. Astrophys. 576, A59 (Apr. 2015).
  * [28] Mortimer, D., Cárdenas Vázquez, M. C., Bertram, T., Bizenberger, P., Häberle, M., Huber, A., and Scheithauer, S., “Building an ELT simulator in the lab for subsystem testing of METIS/SCAO,” in [Adaptive Optics Systems IX ], Jackson, K. J., Schmidt, D., and Vernet, E., eds., Proc. SPIE 13097 (2024).
  * [29] Feldt, M., Steuer, H., Correia, C., Obereder, A., Raffetseder, S., Bertram, T., Shatokhina, J., and Cantalloube, F., “Simulating METIS’ SCAO System,” in [Adaptive Optics for Extremely Large Telescopes (AO4ELT7) ], 6 (June 2023).
  * [30] Krist, J. E., “PROPER: an optical propagation library for IDL,” in [Optical Modeling and Performance Predictions III ], Kahan, M. A., ed., Proc. SPIE 6675, 66750P (Sept. 2007).
  * [31] Boné, A., Agócs, T., Absil, O., Delacroix, C., Amorim, A., and Lynn, J., “The Talbot effect’s impact on the high contrast imaging modes of METIS,” in [Advances in Optical and Mechanical Technologies for Telescopes and Instrumentation IV ], Navarro, R. and Geyl, R., eds., Proc. SPIE 11451, 114514D (Dec. 2020).
  * [32] Gomez Gonzalez, C. A., Wertz, O., Absil, O., Christiaens, V., Defrère, D., Mawet, D., Milli, J., Absil, P.-A., Van Droogenbroeck, M., Cantalloube, F., Hinz, P. M., Skemer, A. J., Karlsson, M., and Surdej, J., “VIP: Vortex Image Processing Package for High-contrast Direct Imaging,” Astron. J. 154, 7 (July 2017).
  * [33] Christiaens, V., Gonzalez, C., Farkas, R., Dahlqvist, C.-H., Nasedkin, E., Milli, J., Absil, O., Ngo, H., Cantero, C., Rainot, A., Hammond, I., Bonse, M., Cantalloube, F., Vigan, A., Kompella, V., and Hancock, P., “VIP: A Python package for high-contrast imaging,” J. Open Source Softw. 8, 4774 (Jan. 2023).
  * [34] Bowens, R., Meyer, M. R., Delacroix, C., Absil, O., van Boekel, R., Quanz, S. P., Shinde, M., Kenworthy, M., Carlomagno, B., Orban de Xivry, G., Cantalloube, F., and Pathak, P., “Exoplanets with ELT-METIS. I. Estimating the direct imaging exoplanet yield around stars within 6.5 parsecs,” Astron. Astrophys. 653, A8 (Sept. 2021).