Optical Design for an Infrared Multi-Object Spectrometer

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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

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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

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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

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Multi-Object Spectrometers

• Punch Plate
• Robotically positioned optical fibers
• Integral field
• Small field of view
• Maps fiber bundle to a vertical arrangement for
  imaging

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Micro-Mirror Array (MMA)
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Micro-Mirror Array (MMA)

• Using a Texas Instruments
• 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)
• 16mm square mirrors, 17mm mirror spacing
• Allows for input focal ratios as fast as f/3.0
• 848 x 600 Mirror array

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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

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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

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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

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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 11 mapping from MMA to Detector
• Compact size
• Entirely Reflective

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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

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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?

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Merit Function

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

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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

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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

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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

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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

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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 CGHs tested?
• More than adequate

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Front End Optics Side View
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Front End Optics Top View
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Spectrometer Optics Side View
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Spectrometer Optics Top View
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IRMOS Optics Side View
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IRMOS Optics Top View
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Acknowledgements

• Moores Nanotechnology division
• Werner Preub, and the Labor Fur Mikrozerspanung
  (Institute for Micromachining), University of
  Bremen, Bremen, Germany
• Focus Software (Zemax-EE)
• Texas Instruments (DMD)