Raven, a harbinger of Multi–Object Adaptive Optics basedinstruments at the Subaru telescope

Rodolphe Conana, Colin Bradleya, Olivier Lardierea, Celia Blaina, Kim Venne, DavidAndersenb, Luc Simardb, Jean–Pierre Veranb, Glen Herriotb, David Loopb, Tomonori Usudac,

Shin Oyac, Yutaka Hayanoc, Hiroshi Teradac, and Masayuki Akiyamad

a AO Laboratory, MECH, University of Victoria, Victoria, BC, V8W 3P6, Canadab NRC–HIA, 5071 W. Saanich Rd, Victoria, BC, V9E 2E7, Canada

c Subaru Telescope, NAOJ, 650 North A’ohoku Place, Hilo, HI 96720, USAd Astronomical Institute, Tohoku University, Sendai 980-8578, Japan

e Astronomy Department, University of Victoria, Victoria, BC, V8W 3P6, Canada

ABSTRACT

In the context of instrumentation for Extremely Large Telescopes (ELTs), an Integral Field Spectrographs(IFSs), fed with a Multi–Object Adaptive Optics (MOAO) system, has many scientific and technical advantages.Integrated with an ELT, a MOAO system will allow the simultaneous observation of up to 20 targets in a severalarc–minute field–of–view, each target being viewed with unprecedented sensitivity and resolution. However,before building a MOAO instrument for an ELT, several critical issues, such as open–loop control and calibration,must be solved. The Adaptive Optics Laboratory of the University of Victoria, in collaboration with the HerzbergInstitute of Astrophysics, the Subaru telescope and two industrial partners, is starting the construction of aMOAO pathfinder, called Raven. The goal of Raven is two–fold: first, Raven has to demonstrate that MOAOtechnical challenges can be solved and implemented reliably for routine on–sky observations. Secondly, Ravenmust demonstrate that reliable science can be delivered with multiplexed AO systems. In order to achieve thesegoals, the Raven science channels will be coupled to the Subaru’s spectrograph (IRCS) on the infrared Nasmythplatform. This paper will present the status of the project, including the conceptual instrument design and adiscussion of the science program.

1. INTRODUCTION

The dawn of Extremely Large Telescopes (ELT) is upon us. The Thirty–Meter–Telescope1 (TMT), the European–ELT2 (E–ELT) and the Giant Magellan Telescope3 (GMT) are all approaching their construction phase. Near IRspectrographs with deployable Integral Field Spectrographs (IFSs) assisted by Adaptive Optics are instrumentsof particular interest for ELTs. This type of instruments is well suited to the study of e.g. the evolution ofgalaxies from first light to the era of peak star formation. Fitted with ∼20 deployable IFSs and a Multi–ObjectAdaptive Optics (MOAO) system, it will allow the simultaneous spectro–imaging of several targets at very highspectral and spatial resolution. IRMOS4 and EAGLE5 are two examples of conceptual MOAO+IFS instrumentfor, respectively, the TMT and the E–ELT.

To achieve the multiplexing advantage, a MOAO system uses several deformable mirrors (DMs). Lightfrom individual scientific targets are directed into separate optical paths, each containing a DM thus enablingsimultaneous multi-wave-front correction. In essence, this means that for a large field of view, the turbulence-induced aberrations are compensated only within a few smaller fields. This is quite different from multi- conjugateadaptive optics (MCAO), for example, because in a MCAO system the wave-front aberration is corrected acrossthe whole field of regard. For an IRMOS-type instrument integrated into a large optical telescope, such as theTMT, there will be approximately 20 scientific objects targeted by the ELT. The MOAO system will be requiredto deliver near diffraction limited images to the approximately 2 arc second integral field units (IFSs) situatedthroughout the five arc minute field of regard.

Send correspondence: rconan@uvic.ca

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The wave-front sensing necessary for an MOAO system is tomographic; i.e. the AO system uses severalwave-front sensors. Each one is fed by a different guide star in a different direction. The guide stars can, inprinciple, be either natural or laser based, but in order to use a MOAO instrument anywhere on the sky, ELTMOAO instruments will most likely use laser guide stars. The measurements of all the WFS, once combined,allow the reconstruction of the 3D volume of wave-front aberrations above the telescope and within the largefield of regard of the MOAO system. From this volume map of wave- front distortions, the shape applied to theDMs (in order to correct for the aberrations in the direction of the astronomical objects they are looking at) isoptimally computed.

In MOAO, the WFSs and DMs are looking in different directions and are located at different positions inthe field of regard. Therefore, WFSs and DMs are in open–loop meaning there is no feedback from the WFS tocontrol the DM shape as in a classical closed-loop AO system.

The registration of DMs and WFSs is also a new challenge6 with respect to closed loop AO. The WFSs beingnot able to see the DMs, the calibration of one with respect to the other is a sizable task on which depends theoverall performance of the system. This may involve the use of one or more truth wavefront sensors (TWFs)and a calibration strategy yet to be defined. Moreover both the WFS and the DMS are deployable meaning thatthe calibration may change from time to time during a single night and the system will have to be re-calibratedeach time as fast as possible in order to reduce telescope idle time.

The tomography algorithm which estimates the wavefront aberrations in the volume and then derives theDM shapes is still in its infancy for MOAO.7 Further investigation is needed to understand all the peculiaritiesassociated to this specific algorithm. Moreover the real time implementation of the algorithm can be a dauntingtask. For such a large system with several 1000 degrees of freedom, the computational power to perform thenecessary billions of operation per second will require the development of dedicated super computer based onstate of the art components like last FPGA/DSP/GPU boards.

The performance of AO system is also reduced due to non common path aberrations (NCPA). These areaberrations not seen by the WFS meaning that every element in the optical train after the WFS is susceptibleto degradation in the image quality. The NCPA are usually static or quasi-static aberrations during the scienceexposure and they are carefully calibrated in classical AO systems. In MOAO, there are multiple science outputsis multiple, each one with its own NCPA which will have to be calibrated too. It may even happen that theNCPA may vary when WFSs and DMs are re-configured. A fast and efficient method to asses the NCPA ofMOAO systems is also a brand new challenge.

In the Raven project, it is proposed to design and implement an MOAO demonstrator, initially in the UVicAOLab and then to undertake on-sky testing of the MOAO test bed on the Subaru Telescope. Subaru is a8m meter optical telescope located atop the summit of Mauna Kea (altitude 4200 m). The purpose of Ravenwill be to act as a pathfinder experiment for instruments (such as IRMOS or EAGLE) that are currently beingproposed for the next generation of extremely large, ground-based optical telescopes (e.g. TMT, E-ELT). It willbe specifically designed to address the major technological issues facing the successful implementation of MOAO.

2. SCIENCE CASES

We believe that it is important that Raven demonstrate MOAO while simultaneously producing astronomicalresults. Performing real astronomical observations will undoubtedly uncover more core MOAO issues and willfinally produce a more rewarding and convincing demonstration of MOAO. In spirit, Raven is much like MAD,8–10

which demonstrated to the wider astronomical community that MCAO could work on-sky and produce interestingscientific results. Because Raven will be feeding the IRCS slit spectrograph with both grism (low spectralresolution) and echelle (high resolution) modes, the Raven science cases are not typical of those cited by theproposed ELT MOAO instruments. Raven + IRCS is well-suited to science cases involving high resolutionspectroscopy of bright stars in high stellar density environments and to lower resolution spectroscopy of elongatedextended objects such as proplyds and inclined galaxies. While we are developing multiple science cases and areevaluating the total system sky coverage, here, we provide two examples of science cases which will be appropriatefor Raven + IRCS.

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Figure 1. Raven & Subaru

Figure 2. Raven sky coverage for magnitude from -1 to 12

2.1 Sky Coverage

Raven is using 3 NGS to probe the atmospheric turbulence in a 2’ field-of–view above the telescope. We assessedthe Raven sky coverage using the USNO B1 catalog using the NASA IR Science Archive∗. All stars brighterthan a limiting magnitude of R< 12 were extracted from the catalog. We mapped these stars onto a 2’ squaregrid to assess the fraction of the sky available for Raven observations. Figure 2 shows the resultant sky coveragemap above Subaru that is available at airmasses less than 1.5. As expected the greatest concentration of guidestars is found on the galactic plane, but numerous out of plane fields exist as well. While the sky coverage isquite small (∼ 1%), there are sufficient fields available to carry out our proposed science cases. In addition, wehave identified numerous fields available at all hour angles in which 5 NGS brighter than R<12 exist . These“engineering fields” can be used to verify Raven’s open loop performance using the 3 NGS WFS and 2 sciencearms directing light to the TWFSs instead of IRCS.

2.2 Metal poor stars

As the Universe evolves, the total amount of metals increases due to stellar evolution effects. We usually findthat the oldest stars in a galaxy are the most metal-poor objects. Often the search for the first generation ofstars, those that formed in the early Universe and that contributed to reionization, is equated to the search for

∗http://irsa.ipac.caltech.edu/applications/Gator/

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the most metal-poor stars. However, star formation can occur at later times in low mass halos, such as dwarfgalaxies, forming metal-poor stars, but where the conditions for star formation are quite different from those inthe early Universe (e.g., see Aquarius project simulations by Gao et al.11). The metal-poor Galactic halo starsmay even be remnants of dwarf galaxy mergers, and thus not related to the first generation of star formation inthe Galaxy. Metal-poor stars can also be formed when pristine HI gas dilutes the interstellar medium, or whenmetal depletions due to dust formation occur such as seen in post-AGB and other chemically peculiar stars.12

Remnants of the first generation of stars in the Local Universe, i.e., those that formed at the earliest epochs,are expected to occur in the Galactic Center and Bulge. These would be the objects that formed first, or werepolluted by the first generation of stars. These objects are expected to form at the centers of potentials of largehalos. Examination of the physical properties of these stars (mass, radius, luminosity, chemical composition) areimportant observational constraints for understanding metal-free stars and star formation in the early Universe.The chemistry could differ from that of other metal-poor stars due to environmental effects and timescales forenrichment from previous (if any) generations.

Its is proposed that RAVEN could be used to examine metal-poor stars in the Galactic Center/Bulge. UsingRAVEN with IRCS at Subaru, two targets reaching H∼ 17th could be observed simultaneously within 2′ of eachother in the Galactic Center/Bulge. With available resolution of R∼ 20, 000, detailed model atmospheres analysiscould yield the abundances of several elements, ranging from light elements such as CNO (carbon, nitrogen, andoxygen), to the iron-group elements, and a few heavy neutron capture elements (such as strontium). The GalacticCenter/Bulge has the added advantage that there are many natural guide stars for AO. This H magnitude limitis ∼ 5 magnitudes fainter than the APOGEE Galactic Center survey proposed as the bright time component forthe SDSS-3 survey. The remaining critical challenge for this RAVEN project is the target pre-selection.

The two most promising methods of target selection are (i) using HST WFC3 (Wide Field Camera 3) filtercombinations to constructing reddening-free indices of temperature and metallicity,13 or (ii) examine the IRspectra of previously micro lensed objects in the Galactic Center where optical imaging is prohibitive to determinestellar parameters (especially distance, thus intrinsic luminosities and surface gravities) to be compared with themicrolensing event results.

2.3 Kinematic Asymmetries in High Redshift Disk Galaxies: Impact of HierarchicalFormation on Tully-Fisher Scatter

One of the fundamental scaling relations for disk galaxies is the Tully-Fisher14 (TF) relation between luminosityand rotation speed. The mass of the dark matter halo is thought to be the underlying property that produces thistight correlation, and cosmological simulations seem to reproduce the observe TF relation15 even out to higherredshift16 . However, in this heirarchical picture of galaxy formation, the increased frequency of galaxy mergersat higher redshift should induce scatter in the TF relation17,18 . Kinematic asymmetries are currently difficultto measure in high redshift (z ∼ 1) TF samples due to the use of wide slits and seeing-limited observations19 .

We therefore propose to obtain the J-band imaging and 0.15′′ slit rotation curves for ∼ 10 (2 at a time)inclined, 0.7 < z < 1 galaxies using Raven and IRCS (in imaging and grism mode) on Subaru. The MOAOcorrection will enable us to both resolve photometric and kinematic asymmetries in these high redshift diskgalaxies and concentrate the light sufficiently to obtain spectra for two galaxies in a reasonable amount of time.We will choose our sample from the CFHTLS wide survey based on photometric redshift and axis ratio20 . Weare confident of finding targets because it is known that some pointings in the wide survey have a high stellardensity and low extinction, we are now working on identifying specific Raven targets.

3. RAVEN CONCEPTUAL DESIGN

Raven will be built in the Adaptive Optics Laboratory of the University of Victoria. A first phase, planned tolast 3 to 4 years, will consist in the design, integration and test in the laboratory. At the end of the first phase,Raven will be moved to Hawaii. It will be reassembled at the Subaru Hilo Headquarters where the readinessof Raven for the on–sky tests will be assessed. Then Raven will be relocated on the nasmyth platform of theSubaru Telescope for on–sky performance evaluation.

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Figure 3. Dual–feeding of the IRCS slits. RAVEN will use the IRCS 0.14”–wide slits #2, 3 and 4 for high resolution echellespectroscopy in bands K, H and Iz–J respectively. The 0.1”–wide slit #14 will be used for low resolution spectroscopywith a grism. The two science FoV of RAVEN must be arranged side by side horizontally. The gap between the two FoVmust be lower than 0.4” (i.e. 216µm in the image plane). The minimal diameter of the science FoV is 4.0” in order to fullyfeed the slits. There is no constrains about the maximum extent and the shape of each FoV. Larger FoV can help acquirethe targets on the IRCS imaging camera, and the footprint of each science FoV can be either circular or semicirculardepending of the optical design. The targets will be finely centred on the slit by adjusting the pickoff position.

The Subaru telescope21 is an optical-infrared telescope at the 4,200m (13,460ft.) summit of Mauna Keaon the island of Hawaii. Subaru has a large 8.2m diameter primary mirror. The primary mirror is made ofsingle piece mirror with a computer controlled system of support holding it in shape at an accuracy of 0.012m (1/5,000,000 inch). Raven will be position on the near IR nasmyth platform at the nasmyth focus after thetertiary mirror. In this configuration, Raven will have access to a field–of–view diameter of 3.5’ at the nasmythfocus. Fig. 1 is an illustration describing Raven within the Subaru telescope.

Raven will re–image two AO corrected 4” fields in the Infrared Camera and Spectrograph (IRCS) instrument.IRCS22 is a workhorse infrared spectrograph for the Subaru telescope, providing high angular resolution andsensitivity. IRCS incorporates two 10242 ALADDIN III arrays which are sensitive from 0.9-5.6 um. IRCS hasbeen designed to deliver diffraction limited images from 2-5 um, as well as providing spectroscopy with grismsand a cross-dispersed echelle. The camera can also be used as a slit viewer for the echelle spectroscopy.

3.1 Instrument architecture

RAVEN must feature at least 3 NGS WFS and 2 science arms patrolling a 2–3.5’ diameter field–of–regard (FoR).The science requirements are listed in Tab. 1. For the science, the main requirements is to deliver at least 30% oflight from a point source into the 150mas slit of IRCS after AO correction (in H band with median r0 at zenith).Raven consists in 7 main components (Fig. 4):

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1. Off–axis calibration source unit : Five off–axis sources are needed simultaneously (i.e. 3 stars + 2targets) to calibrate the AO system for different locations in the FoR and measure the field dependentNCPA during daylight time. A minimum of 5 deployable sources is required, although an array of fixedholes (at least a 7×7 square grid) illuminated by a single bright broadband LED may be an alternativesolution. In addition, it is highly desirable to place two or more phase screens in the common beam inrepeatable positions in terms of both conjugated altitude and position in the plane perpendicular to theoptical axis.

2. Fore–optics: This unit includes a woofer and the lenses or mirrors required to collimate and refocusthe beam. The woofer is a common DM conjugated to the pupil plane used both for calibration and forcorrecting the ground–layer turbulence. During science observations, the woofer will be driven in closed–loop with the 3 WFSs. The baseline is to use a 40mm–diameter 17×17–actuator ALPAO DM241. Thefore–optics provides a focal plane to the pick–offs.

3. Pick–Off System (POS) : The POS is responsible for picking–off the incoming light from 2 sciencetargets and 3 NGSs. The POS must be deployable in the whole FoR.

4. WFSs : Three WFS receiving the light of the 3 different NGSs within a 2’ FoV are required. Each WFSwill be a SH–WFS featuring a maximum of 16×16 spots with around 8×8 pixels per subaperture. Theexact number of subapertures and pixels are still TBD; the total number of pixels available for the wholepupil being likely 128×128, i.e. the size of the ANDOR iXon 860 EMMCCD camera sensor that we plan touse. The FoV of the WFSs is still TBD too, but will exceed 2”. The lenslet array of each WFS must be ina pupil plane whatever the pick–off location. The maximum wavelength range available for the wavefrontsensing is 0.6–1.0µm, the lower wavelengths being blocked by the gold coating of the telescope IMR, andthe higher wavelengths exceeding the camera spectral range.

5. Science arms : Two science arms are required for RAVEN. Each science arm must include a DM and aTWFS, both conjugated to the pupil plane whatever the pick–off position. The ALPAO DM241 is plannedto be used for both science arms too. The TWFS will be a SH–WFS (identical to the 3 WFSs) used forcalibration with the off–axis sources, for monitoring the DM figure during science observations (with the“DM figure” source), and possibly for closed–loop AO correction on bright point–like science targets. Thepreferred wavelength range for the TWFS is 0.5–0.9µm. A pair of dichroic beam splitters (BS) can be usedboth to combine the “DM figure” source beam with the science beam and to send the visible science lightto the TWFS. Also, it is a strong goal to include an image rotator (IMR) in at least one science arm to beable to align independently each science objects with the slit. The first target could be aligned with thetelescope IMR (if there is no other orientation constraint coming from the POS), and the second targetcould be aligned with the IMR of the RAVEN science arm.

6. Beam combiner : The beam combiner is responsible for sending two 4” diameter science FoV into theIRCS spectrograph. The 2 science FOV must be packed side–by–side horizontally in order to feed the IRCSslit, with no overlap. The gap between the 2 science FoV must be lower than 0.4” (Fig. 3). The exit pupilsof both science sub–field must be superimposed.

7. Acquisition camera: The inclusion of an acquisition camera, displaying the whole FoR with the shadowsof the POS, is an option. This camera could help acquire the NGSs and the science targets with the POS.A slit viewer is already available in IRCS to accurately center each science target on the slit.

3.2 Fore–optics design options

The main functions of the fore–optics unit are to form a 40–mm diameter pupil plane on the woofer, and torefocus the beam afterwards, with a 2’ FoV. Due to the 200×magnification ratio, a 2’ diameter FoV correspondsto ±5.8◦ in terms of incidence angle range for the fore–optics.

A possible layout for the fore–optics transmitting a 2’ FoV is shown in Fig. 5. From the nasmyth focus afterthe tertiary mirror, a pair of OAPs relay the focus to the science pick–offs image plane. The woofer DM isinserted in the pupil plane between the two OAPs.

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DM

SH

WFS

SH

WFS

SH

WFSOff-axiscalib src

VRI

SHWFS

Truth WFS

JHKRot

BS

ϕ

screen

DM figuresource

BS

R

Sta

rP

icko

ffs

DM

Woofer

JHK

Scie

nce

Pic

ko

ffs

SHWFS

IRCSTelescope+ IMR

Beamcombiner

Truth WFS

Acq.

Camera?

VRI

Rot?

BS

Flip mirror

BS

R

DM figuresource

Figure 4. Optical block diagram of RAVEN.

Table 1. Baseline RAVEN science requirements.

Parameter Requirement

Field of Regard � 2’Science FoV (slit) 4” per channel# of science channels 2# of WFSs 3

Delivered EE ≥30% in 150mas slit after AO correction (in H band with median r0 at zenith)

Throughput ≥ 0.32 in H band, telescope and IRCS excluded (=80% of AO188 throughput)

Wavelength coverage 0.9–2.5µm, with a goal to extend to 5µmImage Rotation Ability to align each source with the slit (goal)

3.2.1 WFS path

The WFS path pick–off system has to pick up the light of 3 NGS within the 2’ FoR. The FoV of each WFS isstill TBD. Figure 7 shows, as an example, the NGS WFS system designed for IRMOS, the MOAO instrumentplanned for TMT.4 The WFS path of Raven uses the same design (Fig. 6) with a beam–splitter diverting thevisible light towards the WFSs and transmitting the NIR beam to the science path. Each pair of WFS andpick–off mirror will translate and rotate together to patrol a fraction of the FoV.

3.2.2 Science path

The science path of Raven uses two small deployable pick–off mirrors, each followed by a steering mirror and atrombone. Three degrees of freedom are required to keep the optical path constant, This design is similar to thedesign proposed for Eagle5 . The size of the pick–off mirrors is defined by the science FoV.

4. END–TO–END MODELING

Several numerical analysis of Raven are underway to help defining the main specifications of the instrument. Weare using two software packages to assess the performance of the instrument. The first one is LAOS, a Matlabprogram developed at the TMT project office to simulate NFIRAOS, the TMT AO facility. The second one isObject–Oriented Matlab AO (OOMAO) written by the UVic AO lab to simulate and to control AO systems.

The first simulations were run with the following parameters for a 8m diameter telescope with a 3.5’ fov. Forthe simulation, 3.5’ fov was the assumed baseline fov. A 7 layers atmospheric model derived from TMT Mauna

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Figure 5. Conceptual design for the Fore–optics and the science path involving a pair of OAP to image a 40mm pupilonto the woofer and to refocus the beam in the science pick–offs image plane. The maximum un–vignetted FoV is 2’.

Figure 6. Raven WFS path conceptual design.

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Figure 7. IRMOS individual NGS WFS probe arm (left): all of the optics and WFS cameras are on a common mountand are rotated and moved together. IRMOS NGS WFS assembly (right): the six probe arms can each sweep out alarge fraction of the FOR, giving the astronomer increased flexibility when selecting science and NGS targets (Courtesyof Andersen et al. 2006).4

Kea site testing campaign is used with r0 = 15cm and L0 = 30m. The wavefront correction is done in H band.No detector noise is added initially. The goal of these noiseless simulation is to study Raven performance withinthe 3.5’ field–of–view available to Raven for an asterism of 3 or 4 guide stars. In the subsequent figures, theguides stars (black dots) are evenly positioned on 6 rings of increasing radius (0.25’, 0.5’, 0.75’, 1’, 1.25’ and1.5’). A modified modal tomography algorithm23 is used to derive the optimal correction applied to the DM.The residual phase rms is computed for two cuts in the fov, one along the zero azimuth (Fig. 8 and Fig. 9) andthe other at the mid–point between the guide stars (Fig. 10) and Fig. 11. Each curve corresponds to a differentradius of the GS asterism ring.

For Fig. 8 and Fig. 9, each curves reaches a minimum equal to the fitting error. The minimum is reached atthe GS location. Outward from the minimum, the residual error increases rapidly whereas inward the increaseis slower thanks to the 2 or 3 other GSs. In Fig. 10 and Fig. 11, the residual errors are almost flat from the fovcenter to the ring radius and they increase rapidly toward the edge of the field. These plots shows there is verylittle gain in performance going from 3 to 4 GSs on a ring. The results tend also to indicate that restricting thefov diameter to 2’ may be the best compromise to keep the residual error low enough in the whole field for thescience runs with IRCS.

Fig. 12 shows residual wavefront error as a function of the fov for different system orders. The system orderis defined as the number of actuators or the number of lenslets across the telescope diameter. The 3 GSs arelocated on a 45” radius ring and the MOAO performance is measured at 25 positions in 1 quadrant of the fieldof regard. As expected, at the location of the GS the error decreases when the system order increases. But overthe majority of the field of regard the performance is as good or better for lower order MOAO systems becausethe tomographic error is a more dominant error term than fitting error. The slight improvement is due to severalfactors (i) to the fastest angular decorrelation of the high order mode of the system compared to the low ordermode that remains strongly correlated on large angular distance and (ii) to the cross–anti–correlation betweenmodes of different orders that degrades the correction. It is interesting to note that reducing the system orderto 8 will make the system performing more evenly in the whole fov with the exception of the GS location. Thisis very relevant for NGS based systems where the science targets and the GSs never overlap.

5. CONCLUSION

Raven is a pathfinder for the future MOAO–assisted NIR IFSs on the ELTs. Raven will feature 3 WFSs in a 2’FoV and two science arms, each one fitted with a DM. Raven will be mounted on the NIR nasmyth platform of

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3.5’

Figure 8. inGs–3GS-15rd-6rings-15dirs

3.5’

Figure 9. inGs–4GS-15rd-6rings-15dirs

10

3.5’

Figure 10. midGs–3GS-15rd-6rings-15dirs

3.5’

Figure 11. midGs–4GS-15rd-6rings-15dirs

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Figure 12. Raven order

the Subaru telescope and will deliver diffraction limited images to IRCS. Raven design is driven by two goals.The first one is to leverage the technical challenges associated to a MOAO system e.g. open–loop control andcalibration procedures. The second goal is to demonstrate that a MOAO instrument can operate routinelyon–sky performing relevant science observations thanks to a design fulfilling the requirements derived from thescience cases. Raven has started its conceptual design phase. A preliminary opto–mechanical design is close tocompletion. The end–to–end modeling of the instrument has already shown that an ensquared energy betterthant 30% is achievable with Raven within a 2’ FoV. Requirements for the real–time–computer have been releasedand a conceptual design study is underway. Our objective is to help enable the next generation of ELT MOAOinstruments using the lessons we learn from Raven.

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