IGRINS Immersion GRating INfrared Spectrograph: Current Design

1. IGRINSImmersion GRating INfrared Spectrograph:Current Design Sungho Lee Korea Astronomy and Space Science Institute (KASI) / Univ. of Texas at Austin (UT) In-Soo Yuk (KASI), Moo-Young Chun (KASI), Soojong Pak (KHU), Hanshin Lee (UT), Chan Park (KASI), Joseph Strubhar (UT), Weisong Wang (UT), Casey Deen (UT), Michael Gully-Santiago (UT), Jared Rand (UT), Jung-Hoon Kim (SET), Won-Kee Park (SNU/KHU), Haingja Seo (KHU), Kang-Min Kim (KASI), Heeyoung Oh (KASI), Sang-On Lee (KASI), Marc Rafal (UT), Stuart Barnes (Univ. of Canterbury/UT), John Goertz (UT), John Lacy (UT), Tae-Soo Pyo (Subaru), Daniel T. Jaffe (UT)

2. Instrument Team

3. IGRINS • High resolution IR spectrograph which can cover a broad wavelength range in a single exposure • IGRINS will be commissioned at the McDonald 2.7-m telescope, and also designed to be compatible with 4-8 m telescopes. • Spectral resolution • R=40,000 (3.66 pixel sampling) • Wavelength coverage • H-band : 1.49~1.80 µm (25 orders) • K-band : 1.96~2.46 µm (22 orders) • Slit dimension

4. Design Concept Model of IGRINS on 2.7m • Cross-dispersed echelle spectrograph • Main disperser : silicon immersion grating (R3, 36.5 l/mm) • Cross disperser : VPH gratings (H: 650 l/mm, K: 400 l/mm) • High sensitivity • Silicon immersion grating • VPH gratings • HAWAII-2RG (2048x2048) detectors • Compact (0.9 x 0.6 x 0.4 m) • Silicon immersion grating • VPH gratings • White pupil optical design • Simple and reliable operation • No cold moving parts in the spectrograph • Only switching mechanism for the calibration sources

5. Optical Design Layout • Collimated beam size = 25 mm • Slit size = 0.13 mm x 1.94 mm

6. H-Band Spectral Format

7. K-Band Spectral Format

8. Spectrograph Optical Performance H-band K-band • Geometric spot diagram across the spectra • Squares: 2 x 2 pixels (36 x 36 micron) • Circles: Airy disk size • Optical quality does not degrade spectral resolution

9. Input Relay Optics Circle: Airy disk size • 1 arcsec seeing disk image through the input optics at the slit mirror • 4.4 arcsec per size • Left: Center of the slit • Middle: One edge of the slit • Right: One corner of the 2 x 2 arcmin field Convert a telescope f-ratio (f/9-f/16) to f/10 Provide a cold stop to prevent thermal radiation Deliver 2 x 2 arcmin FOV to the slit-viewing/guiding camera

10. Slit-Viewing Camera • 1 arcsec seeing disk image through the input optics • and slit viewer • 4.4 arcsec per size • Left: Center of the slit • Middle: One edge of the slit • Right: One corner of the 2 arcmin x 2 arcmin field Target acquisition and slit monitoring Offset guiding in a 2 x 2 arcmin FOV at the 2.7-m telescope Use 1024 x 1024 clean area of an Engineering Grade H2RG Ks-band filter

11. Immersion Grating • Outstanding capability of IGRINS in the compact design comes from the silicon immersion gratings. • The high refractive index (n=3.4)of silicon keep the high spectral resolution with a much smaller beam size. • Silicon lithography can make a very coarse grating which enables continuous spectral coverage.

12. IGRINS Immersion Grating • Silicon R3 grating (10 cm) • Spectral ghost < 0.3% (~5 nm periodic error) • Spectral grass ~10-5 (scattering at groove surfaces) • Will make another grating • Choose and cut into the shape

13. VPH Gratings • Volume Phase Holographic grating • Cross-dispersers in each H and K band spectrograph • Advantages of VPH gratings over conventional gratings • Higher efficiency by less scatters • Enabling compact optical systems by transmission configuration • High durability and easy handling • Has been used in optical and NIR (H-band) spectrographs • We have purchased H-VPHGs which show good performances.

14. IR VPH Grating Test Performance verification K-band grating development in collaboration with KOSI Thermal cycling tests

15. Mechanics – Cyrostat Input relay optics Slit-viewing camera Size of the cryostat : 900 x 600 x 400 mm Total mass : 210 kg Compactness minimizes the flexure issue All access from the bottom of the cryostat Optical bench is mounted on the bottom plate and thermally isolated by G10 supports

16. Cyrostat – Structual Analysis • Mostly looking upward to the straight Cassegrain focus • Deflection is < 10 um at the G10 supports • Corrected out by focussing and guiding

17. Mechanics – Camera Barrels

18. Mechanics – Camera Barrels Radial spring 3 Baffle Plates Axial spring Radial spring Radial Spring Built in Baffle Axial spring Axial Spring M3 screws with helicoil Precision Pin Bent Holes • Camera is the most sensitive • Design it first to minimize risk • 3+1 baffle vanes • 3-point kinematic mount • Springs for thermal expansion

19. Mechanics – Detector Mounts Thermal insulation Cryo ASIC Board Flex Cable Cold Strap from ASIC board • Flex cable to the outside electronics • H2RG & ASIC thermally isolated from the optical bench Cold Strap from H2RG H2RG

20. Mechanics – Telescope Mount • 4-point structure • Only translation on the FP • Same mount for 2.7 m and Gemini

21. Cryogenics 2 rubber springs Metal bellows Vibration isolation design • Operating temperature • Optical bench and optics : 130 K • Detectors : 77 K • Temperature stability control • Silicon immersion grating : ±0.06 K • Detector : ±0.1 K • Optical bench : ±1 K • The temperature will be monitored at least at six positions • Cold head, optical bench, radiation shield, input optics, two spectrograph cameras

22. Detectors

23. Detector Testing • Collaboration with WIFIS at Univ. of Toronto • ROIC functional test is ongoing • Cryogenic EG detector test in this year • Test at KASI • Test dewar design • Cryogenic test at KASI next year

24. Electronics Architecture • IP based control system (each device has an IP address) • Standard SW protocols and HW devices • System can evolve as needed.

25. Software Architecture • Standard observing scenarios • Software Requirements Document • Software Specification Document: working for each SW package

26. Calibration Unit • Line Calibration : • Th-Ar lamp or Uranium lamp • OH emission lines • Telluric absorption lines • Continuum Calibration : • Tungsten-halogen lamp • Compatible with f/8 ~ f/16 telescopes • Considering an absorption gas cell for future RV programs

27. Integration and Test – Lab Setup and Handling Plan • Clean room, optical bench, interface • Multi-purpose cart • Storage and transportation • Telescope installation

28. Overall Alignment Procedure Module Alignments (Warm) [M1] Input-relay lens module [M3] H & K camera lens module [M2] Slit-viewer lens module [M5] Optical bench assembly Warm Test [Sb1] Input-relay optics [Sb2] Slit-viewer optics [Sb3] Spectrograph reflective optics Cold Test [Sb4] Slit-viewer system [Sb5] Input+Slitviewersystem [Sb6] Spectrograph system [M4] Calibration optics System Alignments (Cold) [S1] Instrument alignment [S2] Telescope alignment

29. Fabrication & Alignment Plan Fabricate and test H & K camera lenses Fabricate and test dispersion part components Correct positions of H & K cameras + detectors Correct design of M2 mirror mount Correct design of H & K camera barrels Fabricate H & K camera barrels Fabricate dispersion part mounts Assemble and align H & K camera barrels Assemble and align dispersion part (compensator: M2 mirror) Assemble and align dispersion part + camera barrels + detectors (compensator: detector assembly)

30. Future Work • Overall timeline • PDR : 2009. 12 • Camera CDR : 2010. 11 • Main CDR : 2011 (TBD) • Commissioning : 2012. 11 (TBD) • Tasks for the Camera CDR • Camera lens barrel design • Scattered light and ghost analysis • Revise engineering requirements: OCDD, FPRD, error budget • Revise I/T plan, acceptance test plan, alignment plan