WFOS closing report

1
WFOS closing report

• Keith Taylor (Caltech)
• May04

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WFOS study

• Purpose
• UV/optical MOS seen as critical 1st gen.
  capability
• Significant challenge which MAY impact telescope
  design/config.
• Strategy
• Develop a range of solutions whose sensitivity to
  telescope design can be tested
• Select an optimal design which can be advanced to
  a concept design phase.

3
WFOS costs to date (4/23/04)

• UCSC no significant charges to project
• Epps design for free (discussion document
  available)
• Both HIA and CIT moving more slowly than
  anticipated
• CIT ME and modeling effort deferred until optical
  design complete.
• Staff costs based on 60/hr

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WFOS Requirements Goals

• Wavelength range
• 0.31-1.0?m (requirement) 0.3-1.3?m (goal)
• Simultaneous is a goal, but not currently a
  requirement
• Resolution
• Rlt5000 w/0.75 slit
• This is serious not achieving it will compromise
  much of the science currently envisioned.
• FoV
• gt75 arcmin2 (requirement) 300 arcmin2 (goal)
• Not necessarily contiguous
• Throughput
• gt30 (0.31-1.0?m)
• Image quality
• lt0.2 (FWHM) including ADC
• Spatial sampling (per pixel)
• lt0.15 (requirement) lt0.1 (goal)
• NB f/1 gives 0.1/15?m pixel goal and
  requirement met automatically !

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Why a Large Field? (Steidel)

• For many (if not most) scientific applications of
  faint object spectrographs, at a given level of
  sensitivity, science is directly proportional to
  solid angle.
• Importance of 20 Mpc co-moving scales for
  placing any high-redshift observation into
  context of large-scale structure
• 20 Mpcgt 8 arcmin _at_z6
• 10 arcmin _at_z4
• 13 arcmin _at_z2
• 20 arcmin _at_z1
• No accident that the sizes of survey fields for
  Chandra, GOODS, etc. are in this neighborhood
• Large field is essential for even considering
  projects involving large scale structure at high
  redshifts (e.g., baryonic structure survey),
  which will require degree scale survey fields.

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Why the UV? (Steidel)

• Very low terrestrial background
• 22.5 AB/arcsec2 (cf. 15 AB/arcsec2 _at_2microns)
• Unlike the near-IR, AO is not necessary for high
  spectral sensitivity
• Large amount of accessible astrophysics,
  particularly for galaxies during the epoch of
  galaxy formation, z1-3
• Needed for complementarity with AO-fed IFU
  spectroscopy of rest-optical spectral diagnostics
• Far-UV resonance lines kinematics, chemistry,
  energetics
• Far-UV stellar features IMF, chemistry, stellar
  populations
• Complementarity to ALMA UV spectroscopy of
  far-IR/submm sources (z2)
• Strategic there will be no UV spectrograph in
  space, for the forseeable future, after the
  demise of HST.
• Essential for high priority 30m telescope science
  case Baryonic structure in the high redshift
  universe.

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WFOS studies

• 3 independent studies
• HIA (Bev Oke, Denis Laurin, Ian Powell)
• Caltech (Keith Taylor, Damien Jones)
• UCSC (Harlan Epps, Joe Miller)
• 3 completely different solutions
• UCSC uses raw Nasmyth focus to deliver a
  single, contiguous field, spectrograph
• Caltech uses a massive focal reducer to feed a
  single, contiguous field, spectrograph
• HIA uses a 2-by-2 mosaic of off-axis focal
  reducers feeding 4 independent spectrographs
• All come close to, or exceed, the FoV
  requirements
• UCSCs ELVIS falls short in FoV but is likely to
  be the most efficient
• HIAs system meets the FoV requirement and is
  almost certainly practical
• Caltechs system possesses a much larger FoV but
  has very significant risk

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Common threads

• Many similar problems
• Size of CaF2 refractive elements?
• need large beam to get required resolution
• Size of beamsplitters/filters
• Efficiency of AR and reflective coatings?
• 0.31-1.1?m
• Efficiency and size of VPH gratings?
• may need VPH gratings to get spectra resolution
• Almost certainly need to mosaic gratings and
  filters
• Very different volumes
• is size and/or mass a real issue?
• HIA design is dual beam, simultaneous blue and
  red spectra
• UCSC offers selection of UV, broad-band or NIR
  spectra
• Caltech design offers biggest contiguous field

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Design options system overview
Telescope
R/C, Fprimary, Feff
Greg, Fprimary, Feff
Initial point design
In favor by others
WFOS
Fore-optics
Cameras
Collimator
Reflective fore-optics
Refractive
Reflective re-imaging
Refractive
Catadioptric
Lens re-imaging
Catadioptric
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Telescope

• Telescope Models (30 m, 20 field)

Data from excel file data on telescope parameters
with equations. Zemax models created for all.
from Sys Eng Work Group.

• Effect of type curvature of image surface.
  R_curv. depends on F/primary and secondary.
• Effect of F/eff scaling of relay optics, less
  effect on rest of optics.

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WFOS _at_ UCSC summary

• Accepts raw f/15 Nasmyth focus
• Requires Gregorian configuration
• Trombone ADC ahead of image surface
• Fully refractive collimator/cameras
• Collimator forms 250mm beam
• 3 selectable wavelength ranges
• 2 simultaneous with suitable dichroic
• FoV 8 arcmin. dia (50 arcmin2)
• Imaging and spectroscopy
• Single contiguous field
• Uses VPH transmission gratings
• Articulated cameras
• Could be built now low risk
• Uses large (but not too large) CaF2

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ELVIS layout (Epps)
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ELVIS (in person)
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ELVIS (in situ)
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ELVIS (detector layout)
234mm
150 Mega-pixel array
142mm
0.04/pixel (17 pixels/slit)
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WFOS _at_ HIA summary

• Relays raw f/15 Nasmyth to an intermediate f/5
  focus
• Prefers R-C configuration but can work with
  Gregorian
• Masks at intermediate focus ADC (tbd) but
  practical
• Catadioptric collimators cameras
• Collimator forms 1m beam
• R0.75 more than adequate
• Why not smaller beam ? (internal vignetting)
• All wavelengths ranges (0.31?m 1.0?m) supported
• Dual beam option
• FoV 4.5 -by- 4.5 arcmin. square field
• (4-shooter gives 81 arcmin2)
• non-contiguous
• Access to center of field for other capabilities
• Requires mosaic gratings
• VPH/Articulated cameras may not be practical
• No obvious show stoppers

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Relay optics with 3-mirror design

• Relay optics - 3 mirror anastigmat

Square Telescope image surface
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HIA WFOS reflective optics
Whole spectrograph optics. Two possible
arrangements.
Telescope image surface
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f/2.2 camera (0.75 15 pixels)
assume 15?m pixels
HIA detector layout
60 Mega-pixel array (4)
0.05/pixel (15 pixels/slit)
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Multi-instrument Options

• 4-instrument layout possibility (fore optics
  only)

Square Telescope image surface
1
2
3
Square field limits (1.5,1.5) to (6,6) each.
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Platform

• WFOS on TMT (CELT)

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WFOS Caltech summary

• Relays raw f/15 Nasmyth to an intermediate f/5
  focus
• Prefers Gregorian configuration but can work with
  R-C
• Masks at intermediate focus ADC design verified
• Refractive collimator Catadioptric f/0.8 camera
• Needs gt600mm CaF2
• Needs curved CCDs
• Collimator forms 500mm beam
• All wavelengths ranges (0.31?m 1.0?m) supported
• R0.75 more than adequate
• FoV 13 arcmin square field (175 arcmin2)
• Contiguous field Scale 0.12/pixel
• Requires mosaic gratings
• VPH/Articulated cameras are OK
• High risk / high gain option

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3MfR close-up

• M2 is at the telescope pupil
• ? 0.5m
• ie adaptive correction ?
• Fast guide / tip-tilt
• Wind buffeting
• Beam switching
• Correct boundary layer turbulence ?
• F/5 image surface allows
• Much reduced size for
• ADCs
• Slit masks
• More favorable field curvature and pupil imaging
  for collimator
• Ideal MOS fiber or d-IFU feed

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Collimator Disperser

• 3MfR can deliver focal surface which is
  pupil-centric and concave to the spectrograph
• No field lens
• Collimator group 700mm dia
• Room for a beam splitter before collimator ?
• Need separate blue and red collimator ?
• Still tbd but 3MfR definitely helps

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Spectrograph configuration
Camera
Collimator
First attempt failed to meet imaging
specs Images 0.35 FWHM
Descope camera speed to f/1 ? Collimator become
easier / camera FoV reduced Requires significant
trade study
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f/0.8 camera (0.75 6 pixels)
Caltech detector layout
38 Mega-pixel array
9090mm image
0.13/pixel (6 pixels/slit)
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Conclusions

• 3MfR offers possibility of
• FoV approaching that inscribed within the 20 TMT
  field
• Contiguous field of view
• Single spectrograph
• Much smaller pixel count (cosmic rays)
• Low order adaptive correction in optical
• Highly desirable
• Vertical optical axis

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WFOS comparison
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WFOS comparison, cont.
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The way ahead for ELVIS ?

• Refine design
• Optical design already fairly mature
• Mechanical design ?
• Articulated cameras stability/flexure issues
• Develop top-down cost estimate
• Risk analysis relatively small risks
• Prospects for larger FoV ?
• Is 75 armin2 requirement negotiable ?
• Mosaicing ELVIS will be difficult/impossible
• NB if ELVIS then Gregorian
• Gregorian f/1 preferred
• R-C excluded

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The way ahead for HIA/WFOS ?

• Refine design
• Optical design trades required
• Faster cameras ?
• Dispersion optics / Grating mosaics ?
• ADCs ?
• Mechanical design trades
• Vertical optical axis ?
• Develop top-down cost estimate
• Risk analysis relatively small risks
• NB if HIA/WFOS then R-C preferred
• Gregorian f/1 accommodated

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The way ahead for CIT/WFOS ?

• Validate optical design for 3MfR FR
• Differential distortion
• Image quality (IQ)
• Back off on FoV (trade study with IQ)
• Slower camera (to f/1 ?)
• Smaller collimator field
• Mechanical design issues for 3MfR FR
• Optical fabrication
• Mounting
• Active M4 ? S/N modeling (IMWG activity)
• Develop top-down cost estimate
• Risk analysis high risk option
• NB if CIT/WFOS then Gregorian (f/1) preferred
• R-C accommodated (but need all we can get !)

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Issues related to TMT instrument procurement

• See Greenbook WBS for example
• LoI (2 months ?)
• Proposal phase (4 months ?)
• Proposal down-select leading to
• Concept design phase
• What groups can bid ?
• Is proposal phase funded ?
• Are generic tech.dev. studies supported ?