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Hyper-Spectral Imaging with Image Slicers

Hyper-Spectral Imaging with Image Slicers. Prof. Stephen Eikenberry University of Florida 19 April 2012. HSI: Why bother?. Easy answer: when you want spectroscopic info over a 2D field … Harder question: when do you use dispersed spectroscopy instead of narrowband filters?

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Hyper-Spectral Imaging with Image Slicers

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  1. Hyper-Spectral Imaging with Image Slicers Prof. Stephen Eikenberry University of Florida 19 April 2012

  2. HSI: Why bother? • Easy answer: when you want spectroscopic info over a 2D field … • Harder question: when do you use dispersed spectroscopy instead of narrowband filters? • Answer: for a given detector format, you have a limited number of pixels (i.e. 2Kx2K) • IFUs have ~1000 spatial elements with 1000 spectral elements each, simultaneously • Narrowband imagers have ~1M spatial elements and 1 spectral element

  3. Integral Field Spectroscopy: fibers

  4. IFS: Slicers

  5. IFS: slicers

  6. IFS: slicers

  7. FISICA: IFS Slicer Example • FISICA is a fully-cryogenic large-format, seeing-limited image-slicing integral field unit (IFU) for the FLAMINGOS spectrograph, designed for f/15-ish telescopes • Advanced Image Slicer (“Content”) concept • Led by S. Eikenberry, R. Elston, and R. Guzman at University of Florida • 22-slices, field-of-view 15x32 arcsec (KPNO 4-m f/15), or 5x11 arcsec (GTC f/17 focus) • Spatial sampling 0.70” (0.35”/pix) on KPNO & 0.23” (0.12”/pix) on GTC; 960 spatial resolution elements • R~1300 spectroscopy over 1-2.4 microns (select J+H or H+K band for individual spectra)

  8. FISICA Concept • FLAMINGOS is a fully-cryogenic near-IR multi-object spectrograph • Build an IFU which fits inside a clone of the “MOS” dewar • FLAMINGOS will “think” it is observing through a strange MOS slit pattern • Very well-defined (and tight) constraints on opto-mechanical envelopes

  9. Optical Design Layout

  10. Opto-Mechanical Approach • Strong desire to use “monolithic” mirror arrays (following the UF “bolt-and-go” approach) – robust, and no alignment needed • 66 mirrors in 3 pieces of material • All-aluminum 6061-T6 construction (provides homologous contraction, thus can test alignment/focus warm/optical) • All-spherical surfaces (aspheric possible, but this was a first try) • Careful iteration between optical design and mechanical design, including tool path for diamond-turning fabrication

  11. Mechanical Layout

  12. FISICA Fabrication • Slicer mirror • 22 slices • 0.4x19-mm each

  13. FISICA Fabrication • Pupil mirror • 2x11array • ~9-mm dia. each • Integrated with fold flat

  14. FISICA Fabrication • Field mirror • 22x1 array; non-constant radii of curvature (by design!) • ~9-mm dia. each

  15. FISICA Fabrication

  16. FISICA Integration

  17. FISICA Integration • All-aluminum (6061-T6) construction allowed warm testing of optical system • Bench tests indicate all 71 mirrors (69 w/power) aligned within tolerances on 1st assembly – no adjustment needed • Telecentricity close to, but not quite at, goal • Integration and cold tests in April/May 2004

  18. FISICA Integration: Telecentricity • Telecentricity goal of <0.005-radians • Not quite there – some (few %) vignetting at FLAMINGOS pupil stop for some field positions

  19. FISICA Integration

  20. FISICA Works!  • First light on KPNO 4-m telescope in July 2004 • Image reconstruction  ~0.9-arcsec FWHM in J-band, limited by seeing (hurray !!); in May 05 had ~0.7” FWHM • Note that large, rectangular field allows AB-nod “on-chip” for targets as large as ~15-arcsec

  21. Early Science: NGC 1569 • HeI 1.083m (blue); Pa (red); continuum (green) • Raw sky-subtracted image reconstruction (not flatfielded yet) HST - visible FLAMINGOS - Ks FISICA – Oct04 • Starburst dwarf galaxy with 3 Super Star Clusters (SSCs) • FISICA reconstructed image shows young windy massive stars near the SSCs, but mostly OUTSIDE them

  22. Children of FISICA: FRIDA • Adaptive Optics-fed IFS/imager for Gran Telescopio Canarias 10.4-meter • Operates at the diffraction limit of the telescope (resolutions of ~20 mas or ~100 nanoradians

  23. FRIDA/FISICA Similarities • Fundamental similarities: • Monolithic approach to mirror arrays • Similar structural approach – all 6061-T6 aluminum structures • Same basic team/expertise • Maximizes utilization of “lessons learned”

  24. FRIDA/FISICA Differences • Slightly different format for IFU • approach same as FISICA • overall size/scale of mirrors mechanically very similar • Geometric aberration requirements tighter (high Strehl): • 2-mirror anastigmat relay approach • but, direct heritage from FISICA  easy to fab/align • Surface roughness requirements tighter (low scatter): • FISICA dominated by SiO2 inclusions in 6061-T6 • FISICA roughness OK, but not great for FRIDA • Investigate different material/coating for FRIDA

  25. FRIDA Materials Test Conclusions • Electroless Nickel with Al substrate is a “standard” diamond-turned material with excellent roughness ( 3nm RMS) • As expected, this material DOES experience measurable cryo-deformation from bimetallic stresses, seen as edge rollup • However, the amplitude is small (P-V ~0.07 HeNe) • All FRIDA mirrors/arrays will/can be slightly oversized to avoid edge effect  P-V ~0.016 HeNe • Thus, Ni/Al mirrors will meet all FRIDA performance requirements

  26. FRIDA IFU Mechanical Design • Bench-mounted Nasmyth environment (fixed gravity vector) • Much easier than FISICA (flexure, and thermal too)

  27. FRIDA IFU Mirrors

  28. Back to HSI:Imaging vs. Spectroscopy • If you need relatively few spectral channels (i.e. 1, up to ~4-5) and large areal field of view, can use narrowband filters and multiple detectors with dichroics • But, if you need MANY spectral channels (i.e. 5 to >1000), best use of detector area is probably dispersed spectroscopy

  29. IFS vs. Long-slit Spectroscopy - I • If your target is large compared to the angular length of a typical slit (i.e. linear FOV ~1000 times the angular resolution element), can use a simple long-slit spectrograph and “push-broom” across the image • But, if your region of interest is large in area but small in linear extent, IFS can cover it more efficiently (by factors up to ~30 or more in scan time)!

  30. IFS vs. Long-slit Spectroscopy - II • If your target is steady in flux/position/etc. over the 2-D scan time, can use a simple long-slit spectrograph and “push-broom” across the image • But, if your target is time-variable or moves on the scan timescale, IFS “freezes” the motions/variations and captures a 2D spectrum instantaneously!

  31. IFS vs. Long-slit Spectroscopy - III • If your detector format/geometry matches your needs for combining FOV with wavelength coverage, then can use a simple long-slit spectrograph and “push-broom” across the image • But, if your FOV*bandpass needs differ, IFS can allow “optical flexibility” in slit placement/geometry on the detector, and may allow different combinations of FOV and bandpass than available for longslit

  32. Slicers vs. Fibers for IFS • Optical fibers have reasonable transmission at optical wavelengths out to ~1.5µm or so • Most fibers do NOT transmit well at wavelengths >2µm, and the ones that transmit at all are delicate and expensive rare-earth-based fibers • Slicers work well down to wavelengths of ~500nm (well into the optical bandpass), and work very well out to wavelengths of 100µm and beyond • Mirrors are the ultimate “achromatic” optic • Slicers can be VERY robust (solid aluminum construction and/or combine Al mirrors with carbon fiber structures for lighter weight) and VERY compact

  33. Ultra-compact Slicer IFS • New concept developed by SSE at UF for astrophysics (smallsat) and remote sensing (space or UAV) applications • Full size ~10x10x10cm for COMBINED slicer and spectrograph, with mass ~1 kg • Can provide FOV from ~1 sq. arcmin to >10 sq. degree, with resolutions from ~1-arcsec to ~0.1-deg, depending on input optics • Spectral resolutions (R   / ()) ranging from ~100 to >20,000 • Can operate over wavelength ranges from ~0.5 µm out to >100µm

  34. Conclusions • Image-slicing integral field spectroscopy is a maturing approach to HIS, particularly relevant for 2D fields of view with high spectral multiplexing requirements • Monolithic diamond-turned mirror technology produces compact, mechanically robust, no-alignment-needed slicer units • Existing slicers operate from visible light to far-infrared bandpasses • Slicers can provide significant advantages over competing technologies (i.e. long-slit “push broom” spectrographs or fiber-fed integral fields)

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