# Upgrading the high contrast imaging facility SPHERE: science drivers and
instrument choices

A. Boccaletti  LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne
Université, Université de Paris Cité, 5 place Jules Janssen, 92195 Meudon,
France  G. Chauvin  Université Côte d’Azur, Observatoire de la Côte d’Azur,
CNRS, Laboratoire Lagrange, France  F. Wildi  Départment d’astronomie de
l’Université de Genève, 51 ch. des Maillettes Sauverny, 1290 Versoix,
Switzerland  J. Milli  Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble,
France  E. Stadler  Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
E. Diolaiti  INAF Osservatorio di Astrofisica e Scienza dello Spazio di
Bologna, Via P. Gobetti 93/3, 40129, Bologna, Italy  R. Gratton  INAF
Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122, Padova,
Italy  F. Vidal  LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne
Université, Université de Paris Cité, 5 place Jules Janssen, 92195 Meudon,
France  M. Loupias  Univ Lyon, Univ Lyon1, Ens de Lyon, CNRS, Centre de
Recherche Astrophysique de Lyon UMR5574, F-69230, Saint-Genis-Laval, France
M. Langlois  Univ Lyon, Univ Lyon1, Ens de Lyon, CNRS, Centre de Recherche
Astrophysique de Lyon UMR5574, F-69230, Saint-Genis-Laval, France  F.
Cantalloube  Aix Marseille Université, CNRS, LAM, UMR 7326, 13388, Marseille,
France  M. N’Diaye  Université Côte d’Azur, Observatoire de la Côte d’Azur,
CNRS, Laboratoire Lagrange, France  D. Gratadour  LESIA, Observatoire de
Paris, Université PSL, CNRS, Sorbonne Université, Université de Paris Cité, 5
place Jules Janssen, 92195 Meudon, France  F. Ferreira  LESIA, Observatoire de
Paris, Université PSL, CNRS, Sorbonne Université, Université de Paris Cité, 5
place Jules Janssen, 92195 Meudon, France  M. Tallon  Univ Lyon, Univ Lyon1,
Ens de Lyon, CNRS, Centre de Recherche Astrophysique de Lyon UMR5574, F-69230,
Saint-Genis-Laval, France  J. Mazoyer  LESIA, Observatoire de Paris,
Université PSL, CNRS, Sorbonne Université, Université de Paris Cité, 5 place
Jules Janssen, 92195 Meudon, France  D. Segransan  Départment d’astronomie de
l’Université de Genève, 51 ch. des Maillettes Sauverny, 1290 Versoix,
Switzerland  D. Mouillet  Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble,
France  J.-L. Beuzit  Aix Marseille Université, CNRS, LAM, UMR 7326, 13388,
Marseille, France  M. Bonnefoy  Univ. Grenoble Alpes, CNRS, IPAG, 38000
Grenoble, France  R. Galicher  LESIA, Observatoire de Paris, Université PSL,
CNRS, Sorbonne Université, Université de Paris Cité, 5 place Jules Janssen,
92195 Meudon, France  A. Vigan  Aix Marseille Université, CNRS, LAM, UMR 7326,
13388, Marseille, France  I. Snellen  Leiden Observatory, Leiden University,
PO Box 9513, 2300 RA Leiden, The Netherlands  M. Feldt  Max Planck Institute
for Astronomy, Königstuhl 17, D-69117 Heidelberg, Germany  S. Desidera  INAF
Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122, Padova,
Italy  S. Rousseau  Université Côte d’Azur, Observatoire de la Côte d’Azur,
CNRS, Laboratoire Lagrange, France  A. Baruffolo  INAF Osservatorio
Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122, Padova, Italy  C.
Goulas  LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne
Université, Université de Paris Cité, 5 place Jules Janssen, 92195 Meudon,
France  P. Baudoz  LESIA, Observatoire de Paris, Université PSL, CNRS,
Sorbonne Université, Université de Paris Cité, 5 place Jules Janssen, 92195
Meudon, France  C. Bechet  Univ Lyon, Univ Lyon1, Ens de Lyon, CNRS, Centre de
Recherche Astrophysique de Lyon UMR5574, F-69230, Saint-Genis-Laval, France
M. Benisty  Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS,
Laboratoire Lagrange, France  A. Bianco  INAF Osservatorio Astronomico di
Brera, Via Brera 28, 20121, Milano, Italy  B. Carry  Université Côte d’Azur,
Observatoire de la Côte d’Azur, CNRS, Laboratoire Lagrange, France  E. Cascone
INAF Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131
Napoli, Italy  B. Charnay  LESIA, Observatoire de Paris, Université PSL, CNRS,
Sorbonne Université, Université de Paris Cité, 5 place Jules Janssen, 92195
Meudon, France  E. Choquet  Aix Marseille Université, CNRS, LAM, UMR 7326,
13388, Marseille, France  V. Christiaens  Space sciences, Technologies and
Astrophysics Research (STAR) Institute, University of Liège, 19C allée du Six
Août, 4000 Liège, Belgium  F. Cortecchia  INAF Osservatorio di Astrofisica e
Scienza dello Spazio di Bologna, Via P. Gobetti 93/3, 40129, Bologna, Italy
V. de Caprio  INAF Osservatorio Astronomico di Capodimonte, Salita Moiariello
16, 80131 Napoli, Italy  A. De Rosa  INAF Osservatorio di Astrofisica e
Scienza dello Spazio di Bologna, Via P. Gobetti 93/3, 40129, Bologna, Italy
C. Desgrange  Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France  V.
D’Orazi  INAF Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5,
35122, Padova, Italy  S. Douté  Univ. Grenoble Alpes, CNRS, IPAG, 38000
Grenoble, France  M. Frangiamore  INAF Osservatorio Astronomico di Brera, Via
Brera 28, 20121, Milano, Italy  E. Gendron  LESIA, Observatoire de Paris,
Université PSL, CNRS, Sorbonne Université, Université de Paris Cité, 5 place
Jules Janssen, 92195 Meudon, France  C. Ginski  Leiden Observatory, Leiden
University, PO Box 9513, 2300 RA Leiden, The Netherlands  E. Huby  LESIA,
Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université
de Paris Cité, 5 place Jules Janssen, 92195 Meudon, France  C. Keller  Leiden
Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
C. Kulcsár  Université Paris-Saclay, Institut d’Optique Graduate School, CNRS,
Laboratoire Charles Fabry, Palaiseau, France  R. Landman  Leiden Observatory,
Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands  S. Lagarde
Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Laboratoire
Lagrange, France  E. Lagadec  Université Côte d’Azur, Observatoire de la Côte
d’Azur, CNRS, Laboratoire Lagrange, France  A.-M. Lagrange  LESIA,
Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université
de Paris Cité, 5 place Jules Janssen, 92195 Meudon, France  M. Lombini  INAF
Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via P. Gobetti
93/3, 40129, Bologna, Italy  M. Kasper  European Southern Observatory, Karl-
Schwarzschild-Str. 2, 85748, Garching, Germany  F. Ménard  Univ. Grenoble
Alpes, CNRS, IPAG, 38000 Grenoble, France  Y. Magnard  Univ. Grenoble Alpes,
CNRS, IPAG, 38000 Grenoble, France  G. Malaguti  INAF Osservatorio di
Astrofisica e Scienza dello Spazio di Bologna, Via P. Gobetti 93/3, 40129,
Bologna, Italy  D. Maurel  Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble,
France  D. Mesa  INAF Osservatorio Astronomico di Padova, Vicolo
dell’Osservatorio 5, 35122, Padova, Italy  G. Morgante  INAF Osservatorio di
Astrofisica e Scienza dello Spazio di Bologna, Via P. Gobetti 93/3, 40129,
Bologna, Italy  E. Pantin  Université Paris-Saclay, Université Paris Cité,
CEA, CNRS, AIM, 91191, Gif-sur-Yvette, France  T. Pichon  Université Paris-
Saclay, Université Paris Cité, CEA, CNRS, AIM, 91191, Gif-sur-Yvette, France
A. Potier  Jet Propulsion Laboratory, California Institute of Technology, 4800
Oak Grove Drive, Pasadena, CA 91109  P. Rabou  Univ. Grenoble Alpes, CNRS,
IPAG, 38000 Grenoble, France  S. Rochat  Univ. Grenoble Alpes, CNRS, IPAG,
38000 Grenoble, France  L. Terenzi  INAF Osservatorio di Astrofisica e Scienza
dello Spazio di Bologna, Via P. Gobetti 93/3, 40129, Bologna, Italy  E.
Thiébaut  Univ Lyon, Univ Lyon1, Ens de Lyon, CNRS, Centre de Recherche
Astrophysique de Lyon UMR5574, F-69230, Saint-Genis-Laval, France  I. Tallon-
Bosc  Univ Lyon, Univ Lyon1, Ens de Lyon, CNRS, Centre de Recherche
Astrophysique de Lyon UMR5574, F-69230, Saint-Genis-Laval, France  H.-F.
Raynaud  Université Paris-Saclay, Institut d’Optique Graduate School, CNRS,
Laboratoire Charles Fabry, Palaiseau, France  D. Rouan  LESIA, Observatoire de
Paris, Université PSL, CNRS, Sorbonne Université, Université de Paris Cité, 5
place Jules Janssen, 92195 Meudon, France  A. Sevin  LESIA, Observatoire de
Paris, Université PSL, CNRS, Sorbonne Université, Université de Paris Cité, 5
place Jules Janssen, 92195 Meudon, France  F. Schiavone  INAF Osservatorio di
Astrofisica e Scienza dello Spazio di Bologna, Via P. Gobetti 93/3, 40129,
Bologna, Italy  L. Schreiber  INAF Osservatorio di Astrofisica e Scienza dello
Spazio di Bologna, Via P. Gobetti 93/3, 40129, Bologna, Italy  A. Zanutta
INAF Osservatorio Astronomico di Brera, Via Brera 28, 20121, Milano, Italy

###### Abstract

SPHERE+ is a proposed upgrade of the SPHERE instrument at the VLT, which is
intended to boost the current performances of detection and characterization
for exoplanets and disks. SPHERE+ will also serve as a demonstrator for the
future planet finder (PCS) of the European ELT. The main science drivers for
SPHERE+ are 1/ to access the bulk of the young giant planet population down to
the snow line ($3-10$ au), to bridge the gap with complementary techniques
(radial velocity, astrometry); 2/ to observe fainter and redder targets in the
youngest ($1-10$ Myr) associations compared to those observed with SPHERE to
directly study the formation of giant planets in their birth environment; 3/
to improve the level of characterization of exoplanetary atmospheres by
increasing the spectral resolution in order to break degeneracies in giant
planet atmosphere models. Achieving these objectives requires to increase the
bandwidth of the xAO system (from $\sim$1 to 3 kHz) as well as the sensitivity
in the infrared (2 to 3 mag). These features will be brought by a second stage
AO system optimized in the infrared with a pyramid wavefront sensor. As a new
science instrument, a medium resolution integral field spectrograph will
provide a spectral resolution from 1000 to 5000 in the J and H bands. This
paper gives an overview of the science drivers, requirements and key
instrumental trade-off that were done for SPHERE+ to reach the final selected
baseline concept.

###### keywords:

High Contrast Imaging, Adaptive Optics, Coronagraphy, Exoplanets.

##  1 INTRODUCTION

Understanding how and when planets form is a central question in the study of
exoplanets. The 5000+ known exoplanets are all valuable to provide insight to
this question either from an individual point of view, or in a statistical
sense. Among the techniques used to detect and study exoplanets, direct
imaging occupies a special place since it is restrained to the observation of
young stars for contrast reasons, which in turn also has the ability to
provide more straightforward constraints on planet formation than with other
methods. The case of the system PDS 70 where two giant planets are caught at
formation is a good illustration of this advantage . Direct imaging is also
one of the two methods able to directly perform spectral characterization of
exoplanet atmospheres. This has been done at relatively low spectral
resolution so far, but the field is now moving toward medium to high spectral
resolution.

In the context of direct imaging of exoplanets, SPHERE, the Spectro-
Polarimetic High contrast imager for Exoplanets REsearch [1] has been
instrumental to collect very detailed information on exoplanetary systems in a
broad sense, that is exoplanets and their environments. SPHERE provided new
insights on a variety of brown dwarf companions [2, 3, 4], as well as known
planetary mass companions [5, 6, 7, 8], which paved the way towards the first
detection of a cloudy massive Jupiter, HIP 65426 b [9]. Then, the first
unambiguous discovery of a young planet, PDS 70 b, in a formation stage has
followed [10, 11]. Near IR photometry and low resolution spectroscopy of these
young giant planets has allowed to explore the physical processes at play in
their atmospheres, confirming low surface gravities, and the presence of thick
dusty clouds. With the exquisite stability of SPHERE it was also possible to
study these systems dynamically, the follow up of $\beta$ Pic b, the HR 8799
system, as well as 51 Eri b being good examples [12, 13]. One of the greatest
achievements of SPHERE has been to reveal the fine structures within imaged
circumstellar disks, yielding unprecedented information regarding the
structure of planetary systems during or right after the planetary formation
phase. Large field of view, and polarimetric capabilities has been decisive to
resolve a wealth of structures in protoplanetary disks: large spiral arms
[14], multiple rings or gaps [15], variable shadows [16], and central cavities
[17, 18], and similarly for debris disks: sharp Kuiper-like belts [19, 20],
multiple belts in gas-rich debris disks [21, 22], moving structures [23]. The
vast majority of these structures could be explained by the presence of
planets or by massive collisions. Disk science is closely connected to
exoplanet science, as reinforced by the results described above and especially
the case of PDS 70 b, where a planet was discovered right inside the cavity of
a gas-rich disk.

One important feature of the guaranteed time observations is the SpHere
INfrared survey of Exoplanets (SHINE) which observed $\sim$600 stars. The
first analysis of 150 targets demonstrates that giant planets are relatively
rare beyond 10 au [24] (Fig. 1). On the contrary, radial velocity surveys
[25], suggest the bulk of the population being located at $3-10$ au. However,
the current performance of SPHERE [26] in this physical separation range will
not allow us to fully bridge the gap in planet separation in order to image
planets detected via complementary techniques (transit, radial velocity,
$\mu$-lensing and astrometry). Thus, further improvements in contrast at short
angular separation will be necessary to offer a global vision of planet
occurrence at all separations.

Another limitation comes from a low sensitivity (red/faint targets) to planet-
forming disks observed in the submillimeter (Fig. 2), while the combined
analysis of ALMA and SPHERE data has proven promising to diagnose planet
formation conditions [27, 28]. Finally, the analyzing of planet’s atmospheric
properties (chemical compounds, thermal structures, clouds) is strongly
limited by the low spectral resolution (R=50).

The SPHERE+ project has been proposed to circumvent the main limitations of
SPHERE in terms of contrast at short angular separations, sensitivity to faint
and red targets, and spectral resolution for atmosphere characterization. For
that purpose the main new components that are considered for a SPHERE upgrade
are: 1/ a second stage AO system, SAXO+, to achieve faster correction rate
with an IR wavefront sensor, and 2/ a new integral field spectrograph, MEDRES,
to achieve a spectral resolution of about 1000 at near IR wavelengths.

Figure 1: SPHERE (Left) and expected SPHERE+ (Right) detection probabilities
from the SHINE sample of young ($\sim 100$ Myr), nearby ($\sim 50$ pc) stars
compared to the current population of exoplanets detected with all techniques:
transit (yellow dot), radial velocity (red arrow), $\mu$-lensing (black dot),
and direct imaging (pink dot). The Gaia detection limits that will be
available with the Data Release 4 for a solar-type star at 50 pc are overlaid.

Figure 2: Left: histogram of target’s R magnitude for several young star
associations, indicating the performance of SPHERE and those expected with
SPHERE+. Right: Magnitude color diagram of actual targets in star forming
regions observable with ALMA (including the famous HL Tau) overlaid with the
AO sensitivity of SAXO (blue line) and the sensitivity expected with an IR
Pyramid WFS for several assumptions for the spectral range.

##  2 Science drivers and technical requirements

The main motivation for SPHERE+ relies on three key scientific requirements
that are currently driving this project as well as our proposed instrumental
concept. They can be summarized as follows :

  * •

sci.req.1 \- Access the bulk of the young giant planet population down to the
snow line ($3-10$ au), in order to bridge the gap with complementary
techniques (radial velocity, astrometry), taking advantage of the synergy with
Gaia, and to explore for the first time the complete demographics of young
giant planets at all separations in order to constrain their formation and
evolution mechanisms (Fig. 1).

  * •

sci.req.2 \- Observe a large number of fainter (lower mass) stars in the
youngest ($1-10$ Myr) associations (Lupus, Taurus, Chamaeleontis, Scorpius-
Centaurus…), to directly study the formation of giant planets in their birth
environment, building on the synergy with ALMA to characterize the
architectures and properties of young protoplanetary disks, and how they
relate to the population of planets observed around more evolved stars (Fig.
2).

  * •

sci.req.3 \- Improve the level of characterization of exoplanetary atmospheres
by increasing the spectral resolution in order to break degeneracies in giant
planet atmosphere models and to measure abundances and other physical
parameters, such as the radial and rotational velocities.

Overall, the SPHERE+ top level requirements connected to the proposed science
cases can be summarized by going closer, deeper, and fainter. As we understand
very well the limitations of SPHERE, the science requirements can be linked
directly to the following instrumental requirements:

  * •

tech.req.1 \- Deeper/closer: increase the bandwidth of the xAO system
(typically 3kHz instead of 1kHz) and improve the correction of non-common path
aberrations and coronagraphic rejection.

  * •

tech.req.2 \- Fainter: include a more sensitive wavefront sensor to gain 2-3
magnitudes for red stars.

  * •

tech.req.3 \- Enhanced characterization: develop spectroscopic facilities with
significantly higher spectral resolution compared to the current SPHERE
Integral Field Spectrograph (IFS). In this respect, both medium
($R_{\lambda}=1\,000-5\,000$) and high ($R_{\lambda}=50\,000-100\,000$)
resolutions are extremely valuable for the characterization of planetary
atmospheres.

##  3 SAXO+

The current AO system of SPHERE, SAXO[29], is made of a spatially filtered
Shack-Hartmann wavefront sensor with 40x40 sub-apertures, combined to a low-
noise EMCCD detector ($<0.1$ e-) operating at visible wavelengths, and a high
order 41x41 deformable mirror by Cilas (HODM). The Real Time Computer, SPARTA,
is running the loop at a maximum frequency of 1.38 kHz on bright stars. SAXO
has demonstrated Strehl ratio in the 90% range in the H band, up to R
magnitude of 10 [30], and even reaching moderate Strehl ratio up to R=14. The
main constraint for an upgrade is to preserve all the functionalities and the
performances of SPHERE both in the visible and near IR arms, so that any
improvement should be considered as additional modes with the capability to
switch back to the initial configuration. SAXO+ is designed to operate by
default with IRDIS and IFS (in simultaneous observations as in the current
configuration), but also with MEDRES and HIRiSE [31, 32, 33].

The science requirements for SPHERE+ are driving the characteristics of SAXO+.
To gain contrast at short separations around bright stars, the 2nd stage
should run faster, in a typical range of 2 to 3 kHz. The exact frequency is to
be optimized with AO simulations. To allow to observe targets that have J
magnitudes of 10 to 13 but with R-J of 3 to 5, SAXO+ should implement an IR
wavefront sensor (Fig. 2). We choose a Pyramid Wavefront sensor for
sensitivity reasons. As we intend to focus primarily on a narrower field of
view than SPHERE, the deformable mirror can be of lower order compared to the
1st stage, so we are considering a 24x24 or 32x32 MEMS DM.

A key question for SAXO+ is the level of interaction between the two AO stages
for optimal performances. Our baseline, which is essentially constrained by
sci.req.2, is to have an ”integrated solution” where one single new RTC drives
the two loops. That means the wavefront measurements are obtained from the
visible Shack-Hartmann and the IR Pyramid, while the correction is performed
with both the HODM and the new faster DM. The RTC is based on the COSMIC
concept [34] while the current RTC, SPARTA, would be bypassed. An alternative
is to control the full system with just the IR WFS, if it proves to be
efficient enough. This derivative case will also be studied with simulations.
In that integrated version, SAXO+ is more like an extension of the current AO
system than an additional component. The second solution is a ”standalone”,
with no real time interactions between the two loops, the first one still
being controlled by SPARTA, and the second stage by COSMIC. We anticipate that
this solution could be acceptable for bright stars (sci.req.1) but can be
problematic to handle the science case of faint and red targets as long as the
first stage will certainly makes the correction worse. For this reason the
”standalone” solution is only considered as a backup plan.

In terms of photon sharing, we identified that two or three dichroic
beamsplitters will be needed depending on the redness of the targets to
optimize the photon flux on the wavefront sensor. The two first dichroic
beamsplitters are optimized for the spectral bands Y ($0.95-1.2\,\muup$m) and
YJ ($0.95-1.4\,\muup$m), while a third one can extend to the H band with 50%
of transmission ($0.95-1.4\,\muup$m + half of $1.4-1.8\,\muup$m).

Given the aforementioned constraints, we designed SAXO+ has a separate compact
module which fits downstream the 1st stage, in between the Atmospheric
Dispersion Corrector and the coronagraphic mask/Differential Tip Tilt Sensor.
The current status of the SAXO+ opto-mechanical design is fully detailed in
Stadler et al. (this proceeding, paper 12185-165). Here we summarize the main
features of this sub-system. A pick off mirror is installed to send the IR
beam upwards, 40 cm above the SPHERE optical axis. The two active components,
that is the DM and the modulator for the pyramid are both located close to the
enclosure for maintenance purpose. The baseline DM is 10mm in diameter, but we
can accommodate the Boston Micro Machine kilo-DM which is 13mm in diameter.
All components of the science channel are in a single vertical plane with
small incidence angles to preserve polarimetric capabilities with IRDIS. The
wavefront sensing channel after the dichoic beamsplitter is slightly tilted
with respect to this plane. The focal ratio at the pyramid is at least F/20
(goal F/40). The output beam is at F/40 for compliance with the other optics
in SPHERE, in particular the coronagraphic mask in the common path and the
stop in IRDIS. The current coronagraph’s apodizers located on a wheel inside
the ADC would prevent wavefront sensing with SAXO+, so a dedicated pupil plane
is needed inside SAXO+ right upstream of the corongraphic mask. As a baseline
we will implement the very same apodizers [35] for consistency with former
observations but we will also explore new apodizer designs to improve the
contrast at close angular separations as it was identified as a limitation
[36]. Figure 3 shows the optical and mechanical layout of SAXO+.

Preliminary assessment of performances with two AO loops are presented in
Vidal et al. (this proceeding, paper 12185-164). In this first estimation we
assumed typical turbulence conditions (wind speed 15 m/s, r${}_{0}=14$ cm,
bright target), and a first AO loop simulated with COMPASS using the
characteristics of SAXO (frequency 1 kHz, 41$\times$41 actuators DM, 800 modes
corrected, waveferont sensing measurement at 700nm), and a first guess of the
SAXO+ characteristics (frequency 3 kHz, 24$\times$24 actuators DM, 200 modes
corrected, waveferont sensing measurement at 1200nm). Beside, we simulated a
”perfect” coronagraph, which subtracts a perfect PSF normalized to the Strehl
ratio. The result shown in Fig. 4 (left) is promising as we can expect a gain
as large as an order of magnitude on the raw contrast at a separation of 5
$\lambda$/D. This has to be confirmed with more accurate simulations taking
into account temporal effects between the two AO loops, low-wind effect and
non common path aberrations, as well as real coronagraphs.

In addition to a second faster AO loop we foresee the implementation of a
third slower loop using the scientific image as a sensor with the ability to
produce a dark-hole in the corrected AO region. Potier et al., 2020 [37]
demonstrated the capability of the technique on the SPHERE internal source,
which combines pair-wise probing that provides a modulation of the speckle
field with the DM to estimate the post-AO beam splitter wavefront aberrations
and Electric Field Conjugation to calculate the correction. Both phase and
amplitude can be compensated in half of the corrected area. More recently, on-
sky experiment has been proven successful, achieving contrasts as large as
$10^{-6}$ or better (1$\sigma$ contrast) at about 500 mas (Fig. 4, right),
which represents a factor of 5 in improvement of the raw contrast (Potier et
al. 2022, submitted, and this proceeding paper 12185-236). As a comparison,
the median contrast of the SPHERE survey [26] is somewhat equivalent but
requiring about 1 hour of observation with angular differential imaging. The
ability to obtain similar performances in just 1 minute is extremely promising
for the future of the SPHERE observations. Implemented in SPHERE+, the dark-
hole method will be fully integrated in the RTC with potentially even more
stable and robust contrasts. Further improvement is achievable using coherent
differential imaging as a postprocessing technique to reach a few $10^{-6}$
contrast level in the other half of the AO corrected region.

Figure 3: Optical (left) and mechanical (right) design of SAXO+. In the left
panel, the red beam stands for the science path, while the orange one is the
wavefront sensor path. The right panel shows the implementation of the optical
elements in the mechanical frame, inside the green bench which resides on top
on the SPHERE common path. The drawing indicates the optical mechanical
supports, the motors for moving parts and the detector (blue camera).

Figure 4: Left: simulated 5$\sigma$ contrast with SAXO (blue) and SAXO+
(green) for a typical set of atmospheric conditions. Right: 1$\sigma$ contrast
obtained with the dark hole technique on sky with SPHERE (green line) as
compared to the nominal case (grey line).

##  4 MEDRES

IFS, the integral field spectrograph of SPHERE provides a broad spectral range
with two spectral resolutions: the YJ mode at R=50, and the YJH mode at R=30,
in a 1.7′′x1.7′′ field of view. IFS data reduction takes advantage of Angular
and Spectral Differential Imaging to reach very high contrasts. These methods
exploit the angular and spectral diversity between the speckles and the image
of a real companion; they are extremely powerful at apparent separation of a
few $\lambda/D$ from the star, but are inefficient at very short separation.
Recently another method has been proposed to improve the detection of
exoplanet atmospheres which is based on cross correlating the spaxels in IFS
data with molecular templates estimated from exoplanet atmosphere models, also
referred as ”molecular mapping”. This technique is extremely powerful to
disentangle the signature of a planet amongst the halo of speckles which have
different spectral behavior, and it has a lower dependency with the angular
separation compared to ADI and ASDI. It has been successful to study the
atmosphere of $\beta$ Pic b [38] and HIP 65426 [39] at a spectral resolution
of about 4000 with SINFONI.

The new spectro-imager, MEDRES, is intended to improve the atmosphere
characterization of exoplanets but also importantly to boost the detection by
taking advantage of the cross correlation technique. When it comes to
increasing the spectral resolution the noise budget does matter to achieve
large contrasts. A model that includes the most important noise components has
been developed for the SPHERE IFS and checked against real data. Extrapolating
to larger spectral resolution, and plotting the contrast versus target star’s
magnitude clearly demonstrates the necessity of low noise detector and low
thermal background in order to achieve contrast better than $10^{6}$ even for
the faintest stars (Fig 5).

The spectral resolution is a key parameter for MEDRES as it sets the
performances in terms of detection and drives the spectral coverage and FoV. A
first series of numerical simulations were performed to determine the optimal
spectral resolution when using molecular mapping. Simulations are using
Fourier-based formalism developed for the ZELDA experiment in SPHERE [36, 40],
which are representative of a bright star observation in moderate seeing
conditions (0.7′′ seeing, 5 mas coherence time). The simulated wavefront maps
include reconstructed tip and tilt, high order modes, as well as a model of
aliasing and fitting errors. Beside, the coronagraph has the same features as
the apodized Lyot coronagraph (ALC2 mask and STOP_ALC2 diaphragm). Planets
with L and T spectral types were injected at various angular separations from
the inner working angle of the coronagraph (about 0.1′′) out to the edge of
the FoV. Then, we tested several spectral resolution ranging from 500 to
100000 in both the J and H bands. We found that for young giant planets the
spectra of which are mostly H20 dominated, R$\approx$1000 provides the optimal
signal to noise ratio. For other molecules like CO and CH4, even lower
spectral resolution of a few hundreds are sufficient.

The design of MEDRES is based on the same concept as the IFS, that is a BIGRE
integral field unit configuration with two frames of microlenses [41]. With a
spectral resolution of 1000 and a 1k $\times$ 1k detector, MEDRES achieves a
0.56${}^{\prime\prime}\times 0.56^{\prime\prime}$ FoV in about 10% of either
the J or H band. To follow the aforementioned noise requirements, while the
IFU itself is located in a warm part of the instrument, the spectrograph is
cooled down to 220 K and the detector is at 80 K, the two latter being
installed in two cryogenic vessels. We are considering an electron Avalanche
Photodiode Array to achieve very low noise as again driven by the error
budget. The whole assembly is about 30 cm long. Similarly to SAXO+, MEDRES can
be installed on top of the SPHERE optical beam, 40 cm above (Fig. 6). A full
description of MEDRES is provided in Gratton et al. (this proceeding, paper
12184-164). To achieve higher spectral resolution (R=5000) which can be of
interest for characterization purpose (at the expense of a lower detection
level) we will also consider fiber fed IFU based on multi-core fibers with an
exchange mechanism to switch between the 2 designs and a corresponding
disperser inside the spectrograph (see Fig. 6). Finally, we plan also to
explore a possible alternative design based on an image slicer (see Meyer et
al., this proceeding, paper 12184-121).

Figure 5: Expected contrast as a function of the target J magnitude for the 2
modes in the SPHERE IFS and for a higher spectral resolution (R=1000) with
various assumptions of temperature (warm vs cooled), and read out noise (CMOS
vs. eAPD).

Figure 6: Proposed optical design of MEDRES (left) showing the 3 different
parts (IFU, spectrograph, detector), and the MEDRES implantation in SPHERE
(right).

##  5 Conclusion

The two new sub-systems, SAXO+ and MEDRES, which constitute the SPHERE upgrade
are supported by the same consortium but follow two separate development
plans.

SAXO+ is proposed as a technological demonstrator on-sky in the context of the
Planetary Camera and Spectrograph (PCS) roadmap for the Extremely Large
Telescope [42]. Indeed, an efficient extreme AO system is required for PCS to
detect the light reflected by planets, possibly down to Super Earth size.
However, designing this system for such a large telescope remains challenging,
and upscaling of existing systems will not be sufficient. As of today, the
solution to obtain a very high contrast ($10^{9}-10^{10}$ at 0.1′′) with PCS
necessarily involves a two-stage AO operating at high frequency. In mid 2022,
SAXO+ is entering a feasibility study expected to last 18 months in which we
will consolidate the opto-mechanical design. This will be driven by numerical
simulations the purpose of which is to define the main characteristics and the
operating modes of the whole AO system, both SAXO and SAXO+, and to estimate
performance in terms of contrasts then translated into exoplanet yields.
Procurement can start in early 2024 followed with Assembly Integration and
test in Europe in 2025. The on-sky implementation is expected early 2026.

MEDRES is developed in parallel to SAXO+ with the aim to provide an optimized
spectro-imager for both detecting and characterizing young giant planets, as a
sound complement to other existing similar type of instruments, which either
are general purpose facilities, or designed to reach very high spectral
resolution for characterization only. MEDRES will be proposed as a visitor
instrument for SPHERE.

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