1. Optical Design for an Infrared Multi-Object Spectrometer R. Winsor, J.W. MacKenty, M. Stiavelli Space Telescope Science Institute M. Greenhouse, E. Mentzell, R. Ohl NASA – Goddard Space Flight Center R. Green National Optical Astronomy Observatories

2. Multi-Object Spectrometers Punch Plate –Take image –Make punch plate based on image –Install punch plate –Take observation data –Does not work well for spacecraft Robotically positioned optical fibers Integral field

3. Multi-Object Spectrometers Punch Plate Robotically positioned optical fibers –Mechanically complex and expensive –Limits ability to get spectra on neighboring objects simultaneously –Difficult to apply to an Infrared instrument Cryogenic vacuum environment Integral field

4. Multi-Object Spectrometers Punch Plate Robotically positioned optical fibers Integral field –Small field of view –Maps fiber bundle to a vertical arrangement for imaging

5. Micro-Mirror Array (MMA)

6. Using a Texas Instrument’s Digital Micromirror Device (DMD) for IRMOS Mirrors are individually addressable into one of two tilt configurations (on or off) Slit lists can be generated and implemented quickly –Flexibility in geometry of slit (good for galaxies) 16  m square mirrors, 17  m mirror spacing Allows for input focal ratios as fast as f/3.0 848 x 600 Mirror array

7. Micro-Mirror Array (MMA) Design Challenges: –Tilted Focal Plane Clocked at 45 degrees –Discontinuous surface Interference effects? Wavefront error from spillover –Requires a User-Defined Surface for modeling in optical design software

8. Optical Design – Stage One Designed for the Kitt Peak Mayall Telescope (3.8m) Convert F/15 from telescope to f/4.6 –Plate scale = 0.2 arcsec/pixel Seeing is typically 0.8”, and can be as good as 0.6” Create tilted focal plane Angle of incidence ~10 degrees at MMA Spot sizes: FWHM better than 0.6” Entirely Reflective

9. Merit Function Start with axial design Optimize for RMS spot radius “Unfold” by adding angle of incidence operands Require a minimum angle of incidence rather than an exact angle –RAID is exact angle of incidence –Use OPGT to set the minimum Allows configurations that might not be expected to work well

10. Optical Design - Spectrometer Resolutions (  ) of 300, 1000, and 3000 in the J (1.1  m ), H (1.6  m ) and K (2.2  m ) bands Gratings have 50mm diameter active area Spot sizes better than 0.6” FWHM F/5.0 beam to detector –Rockwell HAWAII-I detector, 18.5  m pixels Maintains 1:1 mapping from MMA to Detector Compact size Entirely Reflective

11. Merit Function Multiple Configuration –5 different grating groove densities were used 0, 36, 150, 333, 600 –Grating without grooves is a mirror for imaging purposes Coordinates of Optics downstream of the gratings had to be fixed –Different gratings require different substrate tilts –Zemax does not have built in solves for coordinate break tilts to deal with different grating configurations

12. Merit Function No operands were used to encourage a pupil or a collimated beam at the grating –Multiple configurations were made to guarantee good performance across all grating configurations Especially important due to different grating tilts –Not clear that use of such operands would be a better strategy Is time saved? Only if a merit function with fewer configurations can be developed. Will solution work after entering new grating information?

13. Merit Function Angle of incidence at detector was allowed to be variable, but not exceed 30 degrees RMS spot radius optimized

14. Implementing Merit Function Start with axial design –Optimize for good spot sizes Modify Merit function to slowly “Pull” apart the design –So that light paths are realistic –Increase angles of incidence and re- optimize Repeat until a real solution is found

15. Add Folds Fold the design into a size that can be packaged into a “compact” dewar Performed after optimization of each stage –More time consuming to optimize with the other optics Global coordinates must remain constant during multiple configurations

16. Correcting for Astigmatism Traditionally, Toroidal surfaces are used. –Relatively easy to fabricate –Different radii of curvature (different power) in the “x” and “y” directions Use conic values with toroidal shape to correct higher order wavefront error. –In “x” or “y” direction, but not both –Still straightforward to fabricate Biconic –Different radii and conic values in both “x” and “y” directions

17. Biconic Mirror Allows compact design Difficult to Fabricate –Only a handful of vendors are capable of making such a surface –Requires a minimum of a 4-axis diamond machine Process referred to as “Diamond Machining”

18. Testing Profilometry –Contact Method (discontinuous mapping) Several hundred contact points –Relatively Low cost –Adequate? Perhaps. Computer Generated Holograms (CGH) –Interferometric – continuous surface mapping –Relatively high cost Complex setup –How are the CGH’s tested? –More than adequate

19. Front End Optics – Side View

20. Front End Optics – Top View

21. Spectrometer Optics – Side View

22. Spectrometer Optics – Top View

23. IRMOS Optics – Side View

24. IRMOS Optics – Top View

26. Acknowledgements Moore’s Nanotechnology division Werner Preu , and the Labor Fur Mikrozerspanung (Institute for Micromachining), University of Bremen, Bremen, Germany Focus Software (Zemax-EE) Texas Instruments (DMD)