Transcript Quantum Well Electron Gain Structures and Infrared Detector Arrays

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?
• 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
Integral Field Spectroscopy: fibers
IFS: Slicers
IFS: slicers
IFS: slicers
FISICA: IFS Slicer Example
• FISICA is a fully-cryogenic large-format, seeinglimited 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)
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-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 diamondturning fabrication
Mechanical Layout
FISICA Fabrication
•Slicer mirror
•22 slices
•0.4x19-mm each
FISICA Fabrication
•Pupil mirror
•2x11array
•~9-mm dia. each
•Integrated with
fold flat
FISICA Fabrication
•Field mirror
•22x1 array; non-constant radii
of curvature (by design!)
•~9-mm dia. each
FISICA Fabrication
FISICA Integration
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
FISICA Integration: Telecentricity
Telecentricity at -x field posn
0.02
0.02
0.015
0.015
0.01
0.01
Y (radians)
Y (radians)
Telecentricity at +x field posn
0.005
0
-0.005
0.005
0
-0.005
-0.01
-0.01
-0.015
-0.015
-0.02
-0.02
-0.01
0
0.01
0.02
-0.015 -0.005
0.005
0.015
-0.02
-0.02
-0.01
0
0.01
0.02
-0.015 -0.005
0.005
0.015
X (radians)
X (radians)
•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
•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
Children of FISICA: FRIDA
•Adaptive Optics-fed
IFS/imager for Gran
Telescopio Canarias 10.4meter
•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-T6
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 SiO2 inclusions in 6061-T6
• 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
( 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
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 2D 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
“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
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
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
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, 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)