MidIR Spectroscopy

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Mid-IR Spectroscopy

• Matt Richter UC Davis
• with help from
• James Graham UC Berkeley
• John Lacy UT Austin

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Outline

• Current mid-IR instruments and data examples
• Detectors
• Observatory impact
• Resolution, resolution, resolution
• Innovations
• If I were king..

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Current Mid-IR high resolution instruments

• Michelle (UKIRT 2001 Gemini imaging 2003)
• COMICS (Subaru 2000)
• VISIR (VLT 2004)
• TEXES (IRTF 2000)
• EXES (SOFIA 2007)

COMICS on Subaru
TEXES in IRTF prep room
VISIR in lab
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Current Mid-IR high resolution instruments

• Michelle, COMICS, VISIR
• Multimode with imaging, R1000, R20,000
• Switchable capacitance detectors (high/medium
  flux)
• Echelle (R2-R4) for highest resolution
• Single order with limited spectral coverage
• VISIR can be cross-dispersed
• Get four 0.03 mm bits of orders separated by 1
  mm (in N band)
• Do-everything instruments with broadband emphasis

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Current Mid-IR high Resolution Instruments

• TEXES
• Multimode with slit positioning camera, R3000,
  R15,000, R105
• Low background 2562 detector
• Diamond-machined echelon (R10) for highest
  resolution (36 long, 0.3 groove spacing)
• Echelle (R4) for cross-dispersion and medium res
  mode
• Cross-dispersed in high resolution mode
• Get 5-20 0.67 cm-1 orders
• Continuous spectra out to 11.5 mm
• Single order in lower resolution modes
• Primarily a high resolution spectrograph

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Michelle spectrum of b Peg (courtesy of A. Glasse)
Hot H2O
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TEXES observations of b Peg
Over same wavelength range
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Current Mid-IR high resolution instruments

• EXES
• Builds on TEXES design
• 10 longer echelon (10 better resolving power)
• Fewer bounces in low and medium resolution modes
• Low background 256x256 detector
• Room for 1024 x 1024
• Cross-dispersed in high resolution mode
• Get 5-20 0.67 cm-1 orders
• Continuous spectra out to 10 mm
• Single order in lower resolution modes

Forward
Echelon
EXES Mounting plate
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Mid-IR Detectors

• Well depth read noise linked to capacitance
• Ground-based imagers need large well depth
• Can tolerate high read noise and high dark
  current
• Space and high resolution spectrographs need low
  read noise and low dark current and can tolerate
  low well depth
• Switchable capacitance can help give broader
  range (x 3)
• Multiple sampling can help overcome read noise

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Mid-IR Detectors

• Current
• High background and switchable 320 x 240 pixels
  and 256 x 256 pixels
• well depth 1-3 x 107 e-
• read noise 1000 3000 e-
• dark current 4000 e- / s
• Low background 512 x 412 pixels and 256 x 256
  pixels
• well depth 1 x 105 e-
• read noise 30 e-
• dark current 1 e- / s
• Future improvements (on order 3-5 years)
• Commercially available 1024 by 1024 pixels
• Larger High flux / low flux range

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Mid-IR Detector materials

• SiAs
• good from 5 to 28 mm
• increases cooling/shielding requirements
• fairly mature
• builds on JWST detector development

• HgCdTe
• good to 12 mm
• runs at higher temperature
• SiGa
• good to 18 mm

1.0
0.5
QE Gain
0.1
5 mm
10 mm
20 mm
30 mm
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Mid-IR spectroscopy requirements on the telescope
motions

• Instrument command telescope to nod to remove
  background
• With array detector, MIR grating spectrographs
  don't need to chop.
• Timescale
• 5-10 seconds depending on conditions
• Havent systematically explored upper limit in
  good conditions
• Distance
• 0.25 for nodding along slit
• Accuracy of
• Efficiency
• Speed and settling set deadtime for observations.
  
• If 1 second for nod to stabilize, then maximum
  efficiency is 83-91

Aside TEXES frame rates are 0.5 to 2 seconds
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Mid-IR spectroscopy requirements on the telescope
motions

• Scanning
• Create data cube by stepping telescope
• Highly efficient since sky taken from off source
• Many objects will be resolved by TMT, so maps
  important
• Instrument command small (0.025) steps while
  guiding
• Track asteroids for atmospheric standards at high
  spectral resolution
• Non-sidereal rates

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Mid-IR Spectroscopy Requirements on Telescope
Emissivity

• Emissivity should be as low as possible.
• Two mirror system preferred
• Keck at best has 6.5 emissivity for 2 mirrors
• Three mirror system harder to meet 10 overall
  emissivity
• Atmosphere gives good wavelength reference so
  stability of Nasmyth not as important
• Nasmyth allows bigger instrument volume easier
  to package
• Slow beam preferred
• Pupil at secondary, hole in secondary viewing sky
• Spider should be made low emissivity
• AO emissivity should be minimized
• Deformable secondary with off axis NIR wavefront
  sensor
• Tip-tilt with cold NIR sensor in instrument

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Mid-IR spectroscopy requirements on the site

• Want site high, dry, and cold
• Mauna Kea, Atacama peaks, South Pole (daylight
  doesn't really matter)
• With TEXES, we dont observe H2 projects with
  PWV 2 mm unless Doppler shift 25-30 km/s
• Want clear skies
• Can do MIR spectroscopy with cirrus clouds at
  some wavelengths, but spectroscopic nights have
  different meaning from optical observers.
• TEXES can work on bright objects at some
  wavelengths with
• Spectral regions with transmission vary much with clouds

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TMT Mid-IR Spectroscopy Imperative

• Do high resolution well
• High resolution improves access through
  atmosphere
• D4 gain in observing speed

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TMT Mid-IR Spectroscopy Imperative

• Do high resolution well.
• Complement ALMA
• Probe warmer gas with similar velocity bin
• often get multiple lines in single setting
• Access to symmetric molecules (CH4, CH3, C2H2,
  C6H6) for studying chemical evolution
• Consistent beam size for absorption experiments
• Complement JWST
• 30 to 100 times the spectral resolution
• 5 times spatial resolution

ISO vs TEXES H2 S(1) importance of R
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TMT Mid-IR Resolving Power

• Delivered resolving power should be a minimum of
  R 50,000 (R100,000 desired)
• Best sensitivity to weak lines achieved by
  matching line width.
• For background limited performance, can smooth
  resolution after data are taken for broader
  lines.
• To study line profiles, want several resolution
  elements across profile.
• TEXES H2 so far with FWHM
• R 50,000 gives 6 km/s
• Safe to assume Rmax (effective grating length)
  / l
• Theory predicts Rmax (optical path difference)
  / l
• Large gratings (not FTS, FP, or Heterodyne)

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Innovations

• Immersion Echelles
• Diffraction in material
• Gain effective size in proportion to material
  index.
• Smaller beam, grating, and cryostat
• Materials for full mid-IR band pose challenges
• Technology requires further refinement
• Heterodyne
• R1,000,000
• Improving spectral range, coverage, and spatial
  multiplexing
• Sensitivity always factor of 100 worse than
  grating
• Interesting science for very limited number of
  targets

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Technological Retreat?

• Large diamond machined gratings
• TEXES (36, 0.3 groove spacing, R10), EXES (40,
  0.3 groove spacing, R10), AIRES (42, R4, 1mm
  groove spacing) all used Hyperfine, Inc.
• Diamond turning ruling engine had 48 travel
• Made of simple 6061 Aluminium with stress-relief
  anneal
• Steady improvement shown (no interferometric
  control)
• Is Hyperfine still in business?
• At this moment, I dont think TEXES grating could
  be reproduced
• Expect someone has/will have capability, but
  design study would have to confirm

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Future Mid-IR instrument

• R100,000 to 150,000 with grating and 1.6 l / D
  slit width
• Requires 1 to 1.5 m grating
• 1024 x 1024 detector(s) with low read noise
  dark current (switchable desired)
• Design beam for 2048 in focal plane
• Background limited performance
• Cross dispersed
• Continuous wavelength coverage (no inter-order
  gaps) to at least 14 mm (C2H2 Q-branch)
• Cross disperser for low resolution mode,
  especially if switchable detector
• Two cross dispersers desirable echelle 10th
  order R4 and 2nd order grating
• At least 1 coverage with slit long enough for
  nodding, scanning (5)
• Get 4 C2H2 high R branch lines at single shot
• Nodding source on slit cancels sky noise

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Future Mid-IR instrument

• Pipeline reduction software for evaluation during
  observations
• All reflective optics, cold K-mirror, window
  rotator for protecting salts
• Fast, cold NIR camera for slit positioning,
  guiding, tip-tilt corrections, science imaging?

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The End
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Mid-infrared (here) 7 to 28 mm

• Atmosphere has good regions throughout
• For TMT, this does not include H2 J2-0

Transmission from Mauna Kea
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Mid-infrared (here) 7 to 28 mm

• SiAs detector arrays cover entire region
• can go to 5 mm

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Current Mid-IR Echelle Spectrographs
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MICHELLE

• Multi-mode camera, low resolution, echelle
  instrument
• R 15,000 (18,600 at 17 microns for 2 pixel
  slit Sherat et al. 2003)
• Wavelength coverage 0.185 mm _at_ 12.27 mm and 0.05
  mm _at_ 17 mm
• Detector
• 320 by 240 Raytheon IBC with switchable
  capacitance (2500 e- and 1000 e- read noise)

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COMICS

• Multi-mode camera, low resolution, echelle
  instrument
• R 10,000 (8200 demonstrated) at 12.8 um 5000
  (17 um)
• Wavelength coverage 0.18 microns _at_ 12.8 microns
  NeII
• Detectors
• 6 320 by 240 Raytheon IBC switchable capacitance
• 1 for imaging/slit viewing, 5 for spectrograph
• Only one used for echelle because no order
  sorting filters.
• Size 2m by 2m by 2m and 2 tons

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VISIR

• Multi-mode camera, low resolution, echelle
  instrument
• R 25,000 at N band 12,500 Q band (diffraction
  limited performance)
• Wavelength coverage
• Without cross dispersion with selected narrow
  band filters
• With cross dispersion
• Parts of 4 orders on detector at a time.
• 4.5 slit
• N band 0.03 um (order spacing 1 um)
• Q band 0.06 um (order spacing 2 um)
• Detector
• Two 256 by 256 DRS BIBs (switchable capacitance)
• 400 e- read noise, dark current around 4000 e-
  per second
• 1 for imaging and 1 for spectra
• Size 1.2m diameter and 0.7m high

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TEXES

• Multi-mode echelon (high), echelle (medium)
  spectrograph, low resolution, slit positioning
  camera
• R 100,000 at 10 microns 60,000 at 20 microns
  (delivered performance with echelon)
• Diffraction-limited performance is 180,000 at 10
  microns and 90,000 at 20 microns
• R15,000 for echelle
• Wavelength coverage 0.7
• Cross-dispersed by echelle
• 5-12 orders depending on wavelength. Continuous
  coverage for lambda
• Slit length 5-15 depending on wavelength (on 3
  meter)
• Detector
• 256 by 256 Raytheon low-background (SIRTF) array
• 30 e- read noise, dark current 75 e- per second
• Size 0.4m diameter and 1.7m high

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Innovations

• Immersion Echelles
• diffraction in material
• gain effective size in proportion to material
  index.
• smaller beam, grating, and cryostat
• Materials for full mid-IR band pose challenges
• Salts (CsI, CsBr, KBr) hygroscopic relatively
  low index
• KRS5 and CdTe n2.5 (40 cm for EXES echelon),
  transmission 75 at best
• ZnSe, ZnS, GaAs, Ge N band only
• ZnSe does transmit at 17 microns, but
  significantly down
• Technology requires further refinement

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Innovations

• Heterodyne
• R1,000,000
• New LOs give access to all frequencies
• Improved back ends give increased coverage
• Spatial multiplexing?
• Sensitivity always factor of 25 or more below
  grating
• Interesting science possibilities but science
  limited

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Other features of future Mid-IR instrument

• K mirror for slit positioning
• Decker wheel for continuous slit length options
• Closed-cycle cooler

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NDR v Stare (60kel/pix/s, 1x10s exposure)
1 x 10s stare. (1 reset)
500 x 20ms interval NDR. (0.1s reset delay, 1
reset)
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TMT Mid-IR Spectroscopy

• Potential Mid-IR high resolution instruments
• FTS
• Samples Fourier components of entire spectrum at
  one time
• takes noise from entire spectrum
• low sensitivity
• Difficult to observe near atmospheric lines
• Requires long path length for high resolution
• With array detector, can spatially multiplex
• Best for spectral scan of clean atmospheric
  regions
• Heterodyne
• R1,000,000
• Low sensitivity
• Spatial multiplex?
• Limited bandwidth (100 km/s?)
• Best for detailed line profile measurements of
  planets and extremely bright sources

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TMT Mid-IR Spectroscopy

• Potential Mid-IR high resolution instruments
• Fabry-Perot
• Scanning monochrometer
• 2D spatial information at single point
• Subject to noise from background variations
• Requires multiple cryogenic FPs for high
  resolution
• Best for small spectral coverage over extended
  area
• Grating
• Dispersive instrument records spectrum along slit
• Always monitoring sky
• Requires large grating for high resolution
• Best for extended spectral coverage of point
  sources
• In theory, Fabry-Perot and grating take same time
  for datacube, but grating less suceptible to sky
  noise.