1 / 24

Optical Design of an Infrared Multi-Object Spectrometer Utilizing a Free-Form Optical Surface

Optical Design of an Infrared Multi-Object Spectrometer Utilizing a Free-Form Optical Surface. Robert S. Winsor ITT Industries, Advanced Engineering and Sciences 45145 Research Place Ashburn, VA 20147 Raymond G. Ohl, Joseph A. Connelly NASA/Goddard Space Flight Center Greenbelt, MD 20771

mingan
Download Presentation

Optical Design of an Infrared Multi-Object Spectrometer Utilizing a Free-Form Optical Surface

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Optical Design of an Infrared Multi-Object Spectrometer Utilizing a Free-Form Optical Surface Robert S. WinsorITT Industries, Advanced Engineering and Sciences45145 Research PlaceAshburn, VA 20147 Raymond G. Ohl, Joseph A. ConnellyNASA/Goddard Space Flight CenterGreenbelt, MD 20771 John W. MacKenty Space Telescope Science Institute 3700 San Martin Dr., Baltimore, MD 21218

  2. Spetrometer • Schematic depiction of a Spectrometer

  3. Multi-Object Spectrometers • Punch Plate • Take image • Make punch plate based on image • This is a sheet of opaque material with slit openings at the object locations • Install punch plate • Take observation data • Does not work well for spacecraft • Robotically positioned optical fibers • Integral field

  4. 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

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

  6. MOS using a Micro-Mirror Array (MMA) • A programmable Slit MOS using a DMD • Using a Texas Instrument’s Digital Micromirror Device (DMD) for IRMOS • Mirrors are individually addressable into one of two tilt configurations (on or off) ±10º • A set of slits can be generated and implemented quickly • Flexibility in geometry of slit (good for galaxies) • 16mm square mirrors, 17mm mirror spacing • Allows for input focal ratios as fast as f/3.0 • 848 x 600 Mirror array

  7. Micro-Mirror Array (MMA) Texas Instruments DMD GSFC MMAs

  8. Micro-Mirror Array (MMA) • Design Challenges: • 10º 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 • These challenges are in addition to the typical challenges of a Multi-Object Spectrometer • Astigmatism is common in spectrometers of this type

  9. Optical Design – Focal Reducer Front End • 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

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

  11. 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

  12. Biconic Mirror • Sagittal Depth given by the following:

  13. IRMOS Front End Optics – Side View DMD M2 FoldMirror Telescope Focal Plane M1

  14. IRMOS Front End Optics – Top View Dewar Window Cold StopPupil DMD

  15. IRMOS Spectrometer Optics – Side View Biconic HAWAII-I Detector M3 Fold DiffractionGrating DMD

  16. IRMOS Spectrometer Optics – Top View DiffractionGrating Filter M3 Biconic

  17. IRMOS IRMOS Optics – Side View Biconic

  18. IRMOS IRMOS Optics – Top View Biconic

  19. IRMOS During I&T M3 Fold 1 M1 DiffractionGrating Wheel

  20. Biconic Mirror (and Aspheric) Testing • Profilometry • Contact Method (discontinuous mapping) • Several hundred contact points • Relatively Low cost • Adequate accuracy? Perhaps. • Computer Generated Holograms (CGH) • Interferometric – continuous surface mapping • Relatively high cost compared with profilometry • Complex setup • How are the CGH’s tested? • More than adequate accuracy

  21. IRMOS Fabrication Results • Biconic Fabrication Results White Light Interferometer110Ǻ RMS Micro-roughness UCMM Measurement:0.33  RMS ( = 632.8 nm) CGH Measurement:0.219  RMS ( = 632.8 nm)

  22. IRMOS Fabrication Results • IRMOS has been assembled, and is in the cryostat • Spectra seen here in the Z-band (~800nm-1000nm) • Image is on a silicon mux, thus the fade in response • Width of spectra is ~7 pixels here. • This is an initial result. Further improvements are expected as additional tweaks are performed.

  23. Conclusions • Using a free-form surface has made this instrument possible • The use of a free-form surface can be applied to a research-grade astronomical instrument • Trend in technology is making high-quality surfaces like these more accessible and affordable • Multiple metrology solutions are useful for determining surface errors to at least 60nm accuracy or better

More Related