HyperSpectral Imaging with Image Slicers Prof Stephen Eikenberry

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 2 D 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. 2 Kx 2 K) – IFUs have ~1000 spatial elements with 1000 spectral elements each, simultaneously – Narrowband imagers have ~1 M spatial elements and 1 spectral element

Integral Field Spectroscopy: fibers

IFS: Slicers

IFS: slicers

IFS: slicers

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 15 x 32 arcsec (KPNO 4 -m f/15), or 5 x 11 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)

FISICA Concept • FLAMINGOS is a fullycryogenic 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

Optical Design Layout

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 -T 6 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 diamondturning fabrication

Mechanical Layout

FISICA Fabrication • Slicer mirror • 22 slices • 0. 4 x 19 -mm each

FISICA Fabrication • Pupil mirror • 2 x 11 array • ~9 -mm dia. each • Integrated with fold flat

FISICA Fabrication • Field mirror • 22 x 1 array; non-constant radii of curvature (by design!) • ~9 -mm dia. each

FISICA Fabrication

FISICA Integration

FISICA Integration • All-aluminum (6061 -T 6) construction allowed warm testing of optical system • Bench tests indicate all 71 mirrors (69 w/power) aligned within tolerances on 1 st assembly – no adjustment needed • Telecentricity close to, but not quite at, goal • Integration and cold tests in April/May 2004

FISICA Integration: Telecentricity • Telecentricity goal of <0. 005 -radians • Not quite there – some (few %) vignetting at FLAMINGOS pupil stop for some field positions

FISICA Integration

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

Early Science: NGC 1569 • He. I 1. 083 m (blue); Pa (red); continuum (green) • Raw sky-subtracted image reconstruction (not flatfielded yet) HST - visible FLAMINGOS - Ks FISICA – Oct 04 • Starburst dwarf galaxy with 3 Super Star Clusters (SSCs) • FISICA reconstructed image shows young windy massive stars near the SSCs, but mostly OUTSIDE them

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

FRIDA/FISICA Similarities • Fundamental similarities: • Monolithic approach to mirror arrays • Similar structural approach – all 6061 -T 6 aluminum structures • Same basic team/expertise • Maximizes utilization of “lessons learned”

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 Si. O 2 inclusions in 6061 -T 6 • FISICA roughness OK, but not great for FRIDA • Investigate different material/coating for FRIDA

FRIDA Materials Test Conclusions • Electroless Nickel with Al substrate is a “standard” diamond-turned material with excellent roughness ( 3 nm 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 He. Ne) • All FRIDA mirrors/arrays will/can be slightly oversized to avoid edge effect P-V ~0. 016 He. Ne • Thus, Ni/Al mirrors will meet all FRIDA performance requirements

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

FRIDA IFU Mirrors

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

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)!

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 2 D spectrum instantaneously!

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 “pushbroom” 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

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 ~500 nm (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

Ultra-compact Slicer IFS • New concept developed by SSE at UF for astrophysics (smallsat) and remote sensing (space or UAV) applications • Full size ~10 x 10 cm 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

Conclusions • Image-slicing integral field spectroscopy is a maturing approach to HIS, particularly relevant for 2 D fields of view with high spectral multiplexing requirements • Monolithic diamond-turned mirror technology produces compact, mechanically robust, noalignment-needed slicer units • Existing slicers operate from visible light to farinfrared bandpasses • Slicers can provide significant advantages over competing technologies (i. e. long-slit “push broom” spectrographs or fiber-fed integral fields)