MTHR ModeratetoHigh Resolution Spectrometer

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MTHRModerate-to-High Resolution Spectrometer

• Summary presented to the TMT-IWG
• 3/17/04
• S. Vogt, UCO/Lick

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Lessons learned from Keck

• Telescopelarge Spectrometerhires Sciencegood
• Primordial D/H ? ?b
• QSO spectroscopy Ly Alpha forest, galaxy halo
  kinematics, etc.
• CMBT at high z
• Dark matter in dwarf spheroidal galaxies
• Light element abundance work (Be, Li)
• Extrasolar planets
• many more...

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The Science Case for TMT MTHRS

• 2nd light priority TMT SAC SRD
• Extrasolar planet studies
• Galactic and extragalactic chemical evolution
  studies

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More science

• High-z QSO spectroscopy Ly-alpha forest, DLAs
• Dark matter in dSphs
• Globular cluster kinematics
• Astroseismology
• Doppler Imaging of PMS, Ap, RS CVn stars
• Cosmic abundances ratios D/H, Li, C, Be, B

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MTHR design challenges

• Seeing-limited in the optical no AO to the
  rescue...
• Throughput RS W m ? / a Dtel 2 Dcoll tan ?
  / Dtel
• Image slicing is not a magic solution
• sacrifices wavelength, object, sky coverage for
  slit throughput
• presents practical difficulties with sky
  subtraction
• hinders multi-aperture work in crowded fields
• hard to do with multiple-fiber feeds
• cools down image hurts S/N due to readout noise
  contribs.

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The Price of Inadequate Throughput

• through a long slit in 0.8 fwhm seeing
• 
  RS
• 
  Resolution

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Scaling Existing Instruments?

• Spectrometer RS RStmt Scale
  factor
• HIRES 39,000 13,000
  3.0
• VLT/UVES 39,000 10,660
  3.7
• HET/HRS 36,800 11,040
  3.3
• Gemini/HROS 28,500 7,600
  3.8
• Fundamental limitation for scale-up lens glass
  lt 18?
• Scaled up UVES camera lenses 30
• Scaled up HIRES camera lenses 90

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MTHR Design Study Goals

• Can one design a high-res spectrometer for TMT
  which
• achieves RS gt 40,000 arcseconds?
• does not require AO for seeing compensation?
• captures at least 3000A in a single observation?
• is compatible with f/15 Nasmyth focus
  constraints?
• can be fed with fibers for multi-object/IFU
  moderate resolution spectroscopy from 0.38 1.1
  um?
• What constraints does the instrument place on
  telescope/dome design?

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MTHR Concept

• Dual white pupil configuration to limit pupil
  sizes at echelle, CD, and camera
• Steeper blaze angle than HIRES (R-4 vs. R-2.7)
• Use of scaled-up HIRES-style camera (48 diameter
  fused silica lenses)
• Multi-fiber feeds ( TBD) for both red and blue
  sides
• Situated at f/15 Nasmyth focus needs large
  platform!
• Folded to minimize Nasmyth platform size
  requirements

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Shack-Hartmann pickoff pellicle?
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MTHR Principal Components

• 4 Collimators each sections of a 3.0m f/2.76
  parabola
• 2 Echelles 5 x 8 mosaic (1.0m x 3.5m)
• 41.59 gr/mm for blue side
• 31.6 gr/mm for red side
• 6 Cross-dispersers 2 x 3 mosaics (1m x 0.8m)
• 400 gr/mm for blue side
• 250 gr/mm for red side
• 2 Cameras f/1.3 HIRES-style, with 1.28-m lenses
  2.0-m mirrors
• CCDs Two 8k by 8k mosaic of 15um pixels
• Fiber feeds TBD fibers from DIFUs,
  multi-probes, etc.

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Dual white pupil
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20 arcsecs
7 arcsecs
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100 um box ? R106,000 and 0.41
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7 arcsecs
20 arcsecs
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100 um box ? R106,000 and 0.41
R480,000
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Feeding MTHR with fibers
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Fiber transmission (?lt 1.8?m)
10m
40m
Keep the fiber run short if you want the UV
From Keith Taylor, 9/01
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2,612 fibers (87 um)
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Fiber mode spots (300 gr/mm grating)
Rnyquist 6,000
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MTHR Fiber mode examples
0.75
Scale at CCD 21 pixels/arcsec or
0.047/15um-pixel
Maximum length of fiber stack 332mm
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MTHR-lite
(two-camera version)
Dichroic
cd
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Advantages of Dual-arm

• Doubles wavelength coverage (spectral pixels)
  per exposure
• Allows optimization of coatings (reflectances,
  AR)
• Allows optimization of echelle format match to
  CCDs
• Maybe takes advantage of unused portions of
  parent collimator
• Packages well on Nasmyth platform - flexible
  folding geometries
• Allows convenient descoping possibilities
  and/or future upgrade paths

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MTHR Summary Characteristics

• Wavelength span 0.3 to 1.1 um
• Wavelength coverage per exposure 0.3 - 0.75 um
  (typical)
• Field of view 15 arcsecs (along a slit)
  multi-fiber feeds (thousands of fibers possible)
• RS 49,000 arcsecs (high resolution echelle
  mode)
• Maximum resolution 390,000 (2 pixels)
• Typical limiting resolution 229,000
• Image scale at CCD
• 0.063 arcsecs/pixel (echelle dispersion)
• 0.060 arcsecs/pixel (along slit)
• Nasmyth platform footprint 12m x 16m

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Technical Challenges

• Large grating mosaics (1m x 3.5m)
• Large off-axis parabola collimator mirrors
• Large fused silica lenses (1.2m-1.5m diam)
• Large dichroic single arm dual camera option?

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Issues for further study

• Collimator fabrication 4.0m parent vs.
  stress-mirror
• Cost and availability of 50 diameter fused
  silica camera and fiber collimator lenses
• Grating mosaics passive vs. an ACS approach
• Faster cameras? (f/0.79 instead of f/1.3)
• Single/dual Arm and single/dual Camera options
• Large (1.4m) dichroic options
• Optimal strategies for fiber feeds (OzPoz
  multi-probes, IFUs, dIFUs, AO-fed IFUs,
  SPIRALs, etc.)
• How best to share the platform with other
  instruments

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Conclusions

• Can build a moderate-high resolution spectrometer
  (with present-day technology) for TMT which
• Achieves RS 49,000 arcsecs
• Achieves Rlim gt 200,000 (easily)
• Captures gt 0.3 to 0.8um in one shot
• Covers 15 fov along a slit
• Accepts high-multiplex fiber inputs (for mod-res)
• Accepts the nominal f/15 TMT beam
• Fits on the nominal Nasmyth platform
• Speed gain over HIRES/Keck 1.2 9 2.5 27

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Impact on Telescope Design

• Needs 12m x 16m space with fixed gravity
• Can be either R-C or Gregorian with any
  reasonable final f/ratio (f/15 nominal)
• 0.30um to at least 1.1 um spectral reach
• ADC and image rotation internal to instrument
• Locate MTHR near fiber head (if fiber-fed)
• Mass lt 69 tons (single-armlt45 tons)
• Cost (Full) 51M
• Cost (1 arm 1 camera 1 fiber) 29M