ELT-HIRES the High Resolution Spectrograph for the ELT: the IFU-SCAO module

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2018

Publication Year

2021-01-27T14:09:24Z

Acceptance in OA@INAF

ELT-HIRES the High Resolution Spectrograph for the ELT: the IFU-SCAO module

Title

TOZZI, Andrea; XOMPERO, MARCO; OLIVA, Ernesto; BONAGLIA, MARCO; AGAPITO, GUIDO; et al.

Authors 10.1117/12.2311289 DOI http://hdl.handle.net/20.500.12386/30041 Handle PROCEEDINGS OF SPIE Series 10702 Number

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PROCEEDINGS OF SPIE

SPIEDigitalLibrary.org/conference-proceedings-of-spie

ELT-HIRES the High Resolution

Spectrograph for the ELT: the

IFU-SCAO module

Tozzi, A., Oliva, E., Xompero, M., Agapito, G., Bonaglia,

M., et al.

A. Tozzi, E. Oliva, M. Xompero, G. Agapito, M. Bonaglia, G. Di Rico, C.

Giordano, S. Esposito, N. Sanna, "ELT-HIRES the High Resolution

Spectrograph for the ELT: the IFU-SCAO module," Proc. SPIE 10702,

Ground-based and Airborne Instrumentation for Astronomy VII, 107028Q (11

July 2018); doi: 10.1117/12.2311289

Event: SPIE Astronomical Telescopes + Instrumentation, 2018, Austin, Texas,

United States

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ELT-HIRES the High Resolution Spectrograph for the ELT: the IFU-SCAO module

A.Tozzi*

a

, E. Oliva

a

, M. Xompero

a

, G.Agapito

a

, M. Bonaglia,

G. Di Rico

a

, C.Giordano

a

, S. Esposito

a

, N. Sanna

a

a _{INAF-Arcetri Observatory, largo E. Fermi 5, I-50125 Firenze, Italy;}

ABSTRACT

The first generation of ELT instruments will include an optical-infrared High Resolution Spectrograph, conventionally indicated as ELT-HIRES. This paper describes the optical design and overall architecture of the Integral Field Unit (IFU) that will fed the spectrograph. The module have the possibility to change the spaxel dimension thanks to a series of reflection mirrors and using a fast tip tilt mirror the position of the re-imaged foci on the fiber bundles can be adjusted looking at the focus image that is visible using a fiber viewer IR camera.

Keywords: Ground-based instruments, high resolution spectrographs, infrared spectrographs

1. INTRODUCTION

The European Extremely Large Telescope (ELT) will be the largest ground-based telescope at visible and infrared wavelengths. The flagship science cases supporting the successful ELT construction proposal were the detection of life signatures in Earth-like exo-planets and the direct detection of the cosmic expansion re-acceleration. It is no coincidence that both science cases require observations with a high-resolution spectrograph. The HIRES-ELT instrument foresees an observing mode that delivers integral field high resolution spectroscopy with spatial sampling down to the diffraction limit of the ELT telescope. The IFU module presented here is sub-system of the front-end of HIRES-ELT that includes two module, namely:

- SCAO module. It is the wavefront sensor, based on a pyramid beam-splitter, that provides the guiding on the

reference star and the analysis of the incoming wavefront. The expected performances of the SCAO correction are presented in XXXXXXXX.

- IFU module. It is the module that transforms the incoming f/17.7 light beam from the telescope to the

appropriate f/numbers to feed the spectrometer fibers-array with the required spatial scale. The module have the possibility to change the spaxel dimension thanks to a series of reflection mirrors and using a fast tip tilt mirror the position of the re-imaged foci on the fiber bundles can be adjusted looking at the focus image that is visible using a fiber viewer IR camera. The two modules are optically separated by a dichroic window.

2. OVERALL CONCEPT OF THE ELT-HIRES INSTRUMENT

The IFUSCAO module collects the light coming from the ELT and it will injected it into the optical fibers bundle. The module is composed by two different subsystems optically separated by a dichroic window:

• SCAO module. It is the wavefront sensor, based on a pyramid sensor, that will provide the guiding on the reference star and the analysis of the incoming wavefront.

• IFU module. It is the module that will transform the incoming f/17-7 light beam to the appropriate f/# to feed the microlenses placed on the input side of the fibers, arranged in a squared configuration.

The IFU-SCAO is located into the Nasmyth derotator structure as visible in

Figure 1

: it is a gravity variant box of

1250x1150x750 mm where the two subsystems are placed one over the other.

Ground-based and Airborne Instrumentation for Astronomy VII, edited by Christopher J. Evans, Luc Simard, Hideki Takami, Proc. of SPIE Vol. 10702, 107028Q · © 2018 SPIE · CCC code: 0277-786X/18/$18 · doi: 10.1117/12.2311289

Proc. of SPIE Vol. 10702 107028Q-1

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Figure 2

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Figure 3 the IFU-SCAO module is separated into two sub modules by the dichroic.

3. IFU MODULE

The IFU module focuses the light from the ELT onto lenslet arrays connected to the fiber bundles corresponding to different observing modes. To minimize costs and simplify manufacturing, these lenslet arrays will be made using the same type of micro-lenses that are used in the front-end and in the spectrometer slit. The HR mode will include 64 micro-lenses organized on a 8x8 pattern while the UHR will consist of 96 micro-lenses organized on a 10x10 pattern

with empty corners (see

Figure 5

).

Parameter Value Comment

Input focus & FOV f/17.6 D~20” ELT focus. FOV in reflection for SCAO

Wavelength covarege 950-1800 nm Extendible to 2400 nm

Output focus IFU100 f/34 D~2” With 2mm space between lenslets

Scale for mode IFU100 100 mas/spaxel

67 mas/spaxel

IFU100-HR mode (8x8 spaxel) IFU100-UHR mode (10x10 spaxel)

Output focus IFU010 f/340 D~0.2” With 2mm space between lenslets

Scale for mode IFU010 10 mas/spaxel

6.7 mas/spaxel

IFU010-HR mode (8x8 spaxel) IFU010-UHR mode (10x10 spaxel)

FOV of IFU foci 8X8 spaxels 10x10 spaxels - 4

IFU-HR mode IFU-UHR mode

Interfaces to fibers Arrays of micro-lenses

Table 1: IFU scale and modes

In

Table 1

the IFU parameters for the two High-Resolution and Ultra-High-Resolution modalities are reported.

The two f/numbers values, f/34 and f/340, have been selected to obtain the scale defined by the scientific requirements. Their names (IFU100 and IFU010) refer to the scales (mas/spaxel) in the HR modes.

SCAO module

IFU module

LIGHT FROM ELT

VIS to SCAO

IR to IFU

Spectrograph

Dichroic

window

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o o m f/34-340 ca Fold. mirro 2 UH

MI

.a

006

000

IFU sral 2 mera viewer iR bundle 4,300 Ime C17 11,47 0,092 6 734 Note: TOIL Gan Toll.n PmIW lector 340 cam mirrors HR bundle 5,168 6,260

n.S0.5 _{ola. Ea: aimz} PIAF -OexmbrloAsh LargoE.F imi.5-50125(I FF. a390 5 275231 +39 055 2752292 emil: a,mdsetr ichroic corrector ollimator mir hutter (IFU1 us+shutter (IF -ra (IFU100) ra IFU010 1 r 0,646 J I 591/5: + N In

Figure 4

used to select In front to th with a black f Fi

the IFU modu t the IFU010/I Figure 4 I e two fiber bu foil of metal o igure 5 On th On the rig 4. IF ule is shown. IFU100 moda IFU module w undles, one fo one or both bu

e left the UHR ght the HR fib

FU MODUL

The tip-tilt se ality and to ali

with the two op or the HR and undle. R fiber bundle ber bundle: 8x LE PHYSIC elector, based gn the beam o ptical branche d one for the U

e: 10x10 lense 8 lenses of 64 CAL DESCR on a Physik I on the lenslet c es: f/340 (IFU UHR modality es of 430 micr 46 micron side RIPTION. Instrument S3 corresponding U010) and f/34 y, there is a s

ron side, fiber e, fiber core 1 335 piezo stee g to the HR or 4 (IFU100). hutter that pe core 75 micro 13 micron. ering mirror, is r UHR mode. ermits to cover on. s r

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For each IFU focus, f/340 and f/34, it will be placed the fiber termination module where the two UHR and HR bundles

will be placed rearranging them for example as shown in

Figure 5

: the maximum dimension, that is represented by the

diagonal that in case of a space between lenslet of 2 mm is 12,407 mm. The four lenses at the corners in UHR mode, will not be present (96 microlenses).

The switching of the star between the two modalities, HR and UHR, may be done moving the Tip Tilt steering mirror of the appropriate angle to have a displacement on the focal plane of 6.734 mm, that is the center to center distance of the two fiber bundles.

Two beam splitters permit to have a direct image of the pointed object on a single SWIR camera (for example a commercial InGaAs camera identical to those used in the front-end interfaces). Thanks to the folding mirrors it is possible to arrange them to have on the same plane of the SWIR camera the two transmitted focal planes: IFU010 and IFU100.

In

Table 2

the list of motorized axis for IFU module is reported. There are three linear stages for focusing: one for each fiber bundle and one for the camera viewer.

Item Use model

Tip Tilt Mode Selection & guiding 1 Piezo stage S335 PI

ADC Atmospheric dispersion corrector 2 Faulhaber AM1540

Focus Guider Focus stage for the guider camera 1 PLS85 PI

Shutter IFU010 Shutter for selection bundle 1 M111-DG PI

Shutter IFU100 Shutter for selection bundle 1 M111-DG PI

Focus fiber IFU010 Focusing stage for bundle & shutter 1 M112-DG PI

Focus fiber IFU100 Focusing stage for bundle & shutter 1 M112-DG PI

Table 2: IFU Motor axis list

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IFU f/340+f 11/07/2017 folc `/34 ds fold Focus viewer 999 bean IFU100 IFU010 Fo Camera IFU10C cera IFU01 1 3D Layout

Tip -tilt for mode select Internal guic n splitter -1 Focus fiber )cus fiber 00 mm Op EEL ion & ding

ECoI

Zemax >ticStudio _T_IFU_340 +34_s3. Configuration: , / DIC COR ilLll ---i,w

i

Ilimator 15 35_5.zmx All 4 In

Figure 6

For each f/nu some percent The optical d Corrector (AD Figure 6: O • Dich These two wi one: there is o Ele Dic (DI Cor (CO the Zemax op umber there is t to a InGaS IR description is DC), TipTilp Optical layout o

hroic and corre indows work only a focal sh ement chroic IC) rrector OR) 5. I ptical layout i s a beam splitt R camera mod divided into i steering mirro of the IFU mo the ector. together and hift of 5.35 mm Table Description 200 mm bef Material: Inf Physical size Centre Thick 1st_{surface fl} Rmin>98% Tmin>98% 2nd_{surface f} Rmax<1% 65.0 mm fro degrees with Corrector. IFU MODU is shown: ther ter that reflect dule for the gu its functional or, cameras an odule. Differen e feeding, yel the system pe m along the o 3 Dichroic an

fore the ELT f frasil 301 e 120 x 120 m kness is 10 mm lat, λ/10 PV, S % over 700 nm % over 950 nm flat, λ/10 PV, % over 950 nm om 2nd_surface h respect the 2 ULE OPTICA

re are four con ts the major p uiding.

parts: dichroi nd fiber viewe

nt colours are low and green

ermits to have optical axis its nd Corrector o focus, tilted by mm (TBC), cle m, wedged by S/D 40/20 wit m - 950 nm ( m - 2450 nm ( S/D 40/20 wit m - 2450 nm ( e of Dichroic-1 2nd_{surface of D} AL DESCR nfigurations, t part of the ligh ic and its corr er and guiding referred to di n the alignmen e the outcomin elf. optical specifi y 45 degrees ear aperture 11 y 21,7 ± 0,5 a th dichroic coa (AOI = 45±4 d (AOI = 45±4 d th A/R coating (AOI = 45±4 d 1 (centre to ce DIC there is th RIPTION

two for the IF ht to the fiber rector, collima g system. ifferent config nts. ng chief ray p ications 15 x 115 mm arc-min ating as follow degrees) degrees) g as follows: degrees) entre distance) he first surfac FU100 and tw rs bundle, whi ator, Atmosfe gurations. Red perfect aligned (TBC) ws ), tilted by -96 ce of the wo for IFU010 ile it transmits ric Dispersion

d and blue are

d with the free

6.43

0. s n

e

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Material: Infrasil 301

Physical size 50 x 50 mm, clear aperture 45 x 45 mm

Thickness 7,40 mm, wedged by 54,55 ± 0,05 arc-min (in the direction opposite to wedge of Dichroic-1)

Both surfaces flat, λ/10 PV, S/D 40/20 A/R coating on both surfaces as follows:

Rmax<1% over 950 nm - 2450 nm (AOI = 43±4 degrees)

• Collimator.

The collimator if an off-axis parabolic mirror. It generates a collimated beam with the pupil image in between the two triplets of prisms that compose the ADC.

Table 4 Collimator optical specifications.

Element Description Collimator

(COL) 200 mm after the transmitted focus there is the off-axis parabola mirror. Material: Fused Silica (TBC)

Physical size 50 mm diameter (TBC), clear aperture 48 mm (TBC) Centre Thickness is 67 mm

Radius of curvature is 486,4 mm ± 0,5 mm, Conic constant is -1 Parental focal length is 243,2 mm ± 0,5 mm

Zonal radius = 125,0 mm ± 0,1 mm

surface, λ/10 PV, S/D 40/20 with AR coating

Rmin>98% over 950 nm - 2440 nm (AOI = 30±4 degrees)

• ADC.

The ADC consists of two identical groups of three prisms. It provides diffraction limited compensation of the atmospheric dispersion up to a zenith distance of 70 deg.

Table 5 ADC optical specifications.

Element Description Atmospheric

Dispersion Corrector (ADC)

204 mm after the center of COL there is the first surface of ADS. .

Physical size 50 mm diameter (TBC), clear aperture 48 mm (TBC), center thickness for each prism is 30 mm ± 0,1 mm,

in/out surfaces of the two couple of prisms are 90° with respect the chief ray.

1st_{surface prism angle is 7° 24’ 90” ± 30”, material 1}st_{prism is ZnS}

2nd_{surface prism angle is 14° 42’ 07” ± 30”, material 2}nd_{prism is BaF}

2

3rd_{surface prism angle is 21° 51’ 05” ± 30”, material 3}rd_{prism is CaF}

2

4rd_{surface prism angle is 5° 08’ 50” ± 30”}

surface, λ/10 PV, S/D 40/20 with AR coating

Rmin>98% over 950 nm - 2440 nm (AOI = 30±4 degrees)

In

Figure 7

the spot diagrams for the two different zenithal angles 0° and 70° are reported for the case f/34 (IFU100).

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3: 0.0000. 0.0000(am0) IMR: -0.000, 0.000 mm I, 0.0000, -0.0003 (deg) :MA: -0.000. -6.673 m6 09]: 0.0003, 0.0000 (dey) IMA: 7.032, 0.016 mm OW: -0.0003. 0.0000 Cd MA: -7.031. 0.016 e OB); 0.0000, 0.0003 (de IMA: -0.000, 6.727 mm a.0.9 a1.5 ®2.5 OB): 0.0000, 0.0000 (deg) INA. -0.000, 0.00D mm -0.000. -7.011 am OBI: 0.0003, 0.0000 (EN INA: 6.698, 0.014 mm 083: -0.0003, 0.0000 -IN, -6.698, 0.019 063: 0.0000, 0.0003

O

INA: -0.000. 7.06? Figu • TipT Tip Tilt mirr piezo steerin defection ang align the targ closed-loop r • Cam Here as “cam based on a re Ele Col (CA Col (CA • Fibe

ure 7 Spot diag

Tilt

ror is a 25.4 m g platform m gle. It will be get on the corr esolution with mera mera” we are r flective spher ement llimator f/34 AM_100) llimator f/340 AM_010) rs/Guiding gram for Z=0° mm diameter model S-335 t used not only rect position in h a TT angle o

referring to th re. The use of Descript 900 mm based in Material Physical Centre T Radius o Surface, Rmin> 0 1000 mm based in Material Physical Centre T Radius o Surface, Rmin> ° (left) and Z= flat mirror pl that works in y to switch bet n the focal pla over PSF disp

he optical grou one or anothe

Table 6 Cam

tion

after the tran an on axis sp l: Fused Silica l size 55 mm d Thickness is 50 of curvature is λ/10 PV, S/D >98% over 95 m after the tran

an on axis sp l: Fused Silica l size 55 mm d Thickness is 50 of curvature is λ/10 PV, S/D >98% over 95 =70° (right). R laced 100 mm n closed loop tween the two ane, having 0. lacement scal

ups that allow er camera is se mera optical sp smitted focus pherical mirror a (TBC) diameter (TBC 0 mm s 1067 mm ± 1 D 40/20 with A 50 nm - 2440 n nsmitted focu pherical mirror a (TBC) diameter (TBC 0 mm s 6502 mm ± 5 D 40/20 with A 50 nm - 2440 n Represented fi m after the AD up to ±15 m o optical confi .2 micro-radia e that is 9mm w to feed the f elected by the pecifications. (800 mm afte r. C), clear apert 1 mm, Conic c AR coating nm (AOI = 2 us (900 mm aft r. C), clear apert 5 mm, Conic c AR coating nm (AOI = 4 eld are 0,0 an DC and place mrad: that it iguration IFU ans (0.3” of TT m/0.1°. fiber bundles. TipTil mirror er TT) there is ture 52 mm (T constant is 0 20±3 degrees) fter TT) there ture 52 mm (T constant is 0 4±3 degrees) nd (+/-1.1”;+/-ed on a Physi means 4 deg 100 and IFU0 T mechanical

The two cam r. s the collimato TBC) is the collimat TBC) -1.1”). ik Instrumente gree as optica 010 but also to deflection) o

mera optics are

or tor e al o f e

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A flat beam s of f/340 mod camera. The r Ele Bea (BS BS The encircle spot diameter splitter is used de permit to h reflected beam ement am Splitter S010-100) energy value r for the differ

d to have som have on the s m if used to fe Table 7 Description 200 mm bef splitter tilted Material: Inf Physical size Centre Thick 1st_{surface fl} Rmin>98% Tmin<98% 2nd_{surface f} Rmax<1%

for the IFU1 rent incoming

Figure

me percent of l same detector eed the fibers p

7 Slit Viewer B fore the f/360 a d of 10° and 1 frasil 301 e 40 mm of di kness is 6 mm lat, λ/10 PV, S % over 950 nm % over 950 nm flat, λ/10 PV, % over 950 nm 00 case is sh fields. e 8 Encircle en ight projected r the two focu

placed on the Beam Splitter and 250 mm b 5° tilted respe iameter (TBC m S/D 40/20 wit m - 2450 nm m - 2450 nm ( S/D 40/20 wit m - 2450 nm ( own in

Figu

nergy for PSF d onto a IR ca us planes, thi same focal pl r optical specif before the f/36 ectively. ), clear apertu th dichroic coa (AOI = 12.5± (AOI = 12.5±4 th A/R coating (AOI = 12.5±4

re 8

: we hav F f/34 (IFU100 amera used as is to avoid th lane. fications 6 focus, there ure 38 mm (TB ating as follow ±4 degrees) 4 degrees) g as follows: 4 degrees) ve approximat 0) case. viewer: the fo he use of a do is the beam BC) ws tely 90% ener folding mirrors ouble infrared rgy in 320 um s d m

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Name LI t R in mm) (t,R ) L2 t,R in mm) ( ) F# fiber F# lens 0 core (mm) D lens (mm)

Maximum light losses for all fields and wavelengths

SBSL7 SLAM55 t =1.00 t =4.00 4% HR RI flat R1 =0.510 F/3.5 F/20 0.113 0.646 R2 =0.510 K1 =-4.0 K2 =-4.0 R2 =2.407 50517 SLAM55 t =0.80 t =2.60 UHR RI flat R3 =0.350 F/3.5 F /20 0.075 0.430 5% R2 =0.350 K1 =-4.0 K2 =-4.0 R2 =1.71 50517 SLAM55 t =8.0 t =19.0 MR RI flat R1 =2.80 F/3.5 F /20 0.570 3.40 2% R2 =2.80 K2 = -3.0 K2 = -3.0 R2 =15.1

Main parameters of micro -lenses (glued doublets) for the HIRES Zy1H spectrometer module. LI, L2: t= thickness, R= radius of curvature, K =conic constant. F ber is glued to first (flat) surface of 11. F# fiber : nominal aperture of light beam at the fiber; beams are telecentric .

F# lens: nominal aperture of light beam at the micro-lens; beams are telecentric. 0 core: diameter of fiber core.

D lens: size of the lens.

Includes light losses produced by blurring of images, by deviations from telecentricity, and by variations of the beam apertures. Maximum values among all fields and wavelengths; average values are smaller.

6. MICROLENSES DESIGN

In front to each fiber a microlens is placed: on the two f/# foci a double IFU will be placed as visible in

Figure 5

. The

dimension of the lenses in the HR and UHR modes are respectively of 646 um and 340 um. The lenses will be squared to permit the possibility to mold them side by side forming a monolithic microlens array. The scope of these lenses is to collect the desired field of view incoming to the lens into the core fiber dimension that in smaller and is around a commercial value of 113 um diameter. At the same time the fiber will be illuminated not by the focus but by a pupil image: the lenses are calculated to have a pupil image placed on the fiber with the correct diameter.

Figure 9 Layout, rays-tracing and main parameters of the micro-lenses.

7. ACKNOWLEDGEMENTS

This work was financially supported by the Italian Institute of Astrophysics through the grant and “T-REX: Italian Technologies for ELT”.

REFERENCES

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[3] Marconi, A.; Di Marcantonio, P.; Maiolino, R.; et al.; SPIE, 10702-70 (2018); this conference [4] Oliva, E.; Delabre, B.;Tozzi, A.; et al.; SPIE, 9908-7RO (2016)

[5] Fürész, G.; Epps, H.; et al, Barnes, S.; et al.; SPIE 9147-9GF (2014)