CFOS for TMT

1
CFOS for TMT

• An attempt to satisfy the WFOS requirements for
  the TMT
• Damien Jones
• Keith Taylor

2
SACs requirements for WFOS

• Wide-field UV/optical imaging spectrograph
• 310 lt ? lt 700nm 500 lt ? lt 1300nm
• Independently optimized ?
• Wavelength ranges need not be simultaneous
• 300 lt R0.75 lt 5000
• FoV Requirement 10 Goal 15 (cf R-C field
  ? 20)
• Spatial sampling 0.1 - 0.2
• Possible d-IFU (sampling tbd)

3
CFOS guiding principles

• Maximize FoV
• Maximally utilize TMTs ? 20 field
• Science ? FoV (to 0th order)
• Contiguous field preferred over segmented
• Maximize efficiency
• Trade aberration performance with S/N through
  full system modeling
• If possible, use low-order adaptive (fast
  tip-tilt) correction to minimize slit-coupling
  losses
• Preserve R0.75 lt 5000 as sacrosanct
• Facilitate d-IFU (or fiber) feeds
• Satisfy physical, rather than commercial,
  constraints (for now !)

4
3MfR global view

• Based on R-C f/1.5 TMT
• 4 extra reflections
• Fold mirror M1,M2,M3
• All atoroidal
• with relaxed surface and positional tolerancing
• Final image surface
• FoV ? 20
• Manageable distortion (?)
• F/5 output to sgraph
• Flat focal surface
• but can take any curvature by design
• Telecentric by design
• pupil-centric for curved focal surfaces

5
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

6
Disadvantages of 3MfR

• ? 92 (UV - 1?m) assumes best LLNL coatings
• With adaptive M2, slit coupling gains could be
  factor of 2
• Increased aberrations
• Optimization required
• Optical path length
• Telescope design parameters
• Complexity of mirror surfaces
• Manufacturability ? - challenging but not
  impossible
• Support structure
• Adaptive M2 mitigates Nasmyth vibrations and 3MfR
  mechanical instabilities
• Space constraints
• 3MfR (15 meters high)
• Single rotating spectrograph
• Cost
• Need trade study w.r.t. directly mounted sgraph
  (eg ELVIS)

7
Advantages of 3MfR

• 3MfR tailored to give concave, pupil-centric,
  image surface for the collimating optics
  downstream.
• Need for large spectrograph field lens is thereby
  eliminated so that bulk of collimating optics
  will be stacked close to the much smaller pupil.
• Achromtic of course !
• Adaptive correction at intermediate telescope
  pupil
• Gain of 2 (GSMT white book)
• Practical deployment of d-IFUs and fibers
• (cf MTHR) Is the red spectrograph fiber based ?
• Full field ADC
• Almost certainly need to tessellate materials
• Single sensibly sized MOS masks
• Avoid flexure problem with differential motion
  between independent fields
• Can support a single spectrograph utilizing full
  R-C field (20) contiguously
• Delivers a vertical optical axis for spectrograph

8
Show stopper for 3MfR

• Distortion mapping changes with field rotation
  (unless you are very careful !)
• Requirement
• Differential distortion vectors lt 0.05
• 3MfR is a non axi-symmetric system
• Suffers from intrinsic non-radial distortion
• Work, in-progress (Feb/Mar04) has focused on
  minimizing distortion while controlling/optimizing
  aberrations

9
ADC for 3MfR

• Direct Vision Double Prisms
• Uses CaF2 FS
• Size ? 900mm
• Tessellated construction necessitated by size
• Consequently thinner/cheaper with less chromatic
  damage

10
3MfR Spectrograph

• What does the ideal TMT spectrograph look like ?
• Camera pixel scale requirement
• Pixel sampling 0.1 0.2/pixel (SAC)
• Assume 15?m pixels
• Camera speeds faster than f/1
• Best achieved with catadioptric systems
• Internal detector
• Large beam
• Minimizes vignetting
• Maximizes resolution
• Mitigates VPHG blaze shift effect
• Curved CCDs ? no field-flattener
• Minimizes vignetting
• Maximizes speed

11
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
• VPHG issues
• Blaze width in UV ?
• Need very high dn
• Blaze shift as function of off-axis field angle
• VPHGs are good for fiber systems but bad for
  multi-slits

12
Spectrograph configuration
3MfR pupil-centric feed
f/1 camera
Collimator group
13
Conclusions

• 3MfR offers possibility of
• FoV equal (or larger) than 20 TMT field
• Contiguous field of view
• Single spectrograph
• Low order adaptive correction in optical
• Mitigation of nasty VPHG effects
• Camera allows for classical gratings
• Beam size reduces VPHG blaze shift by factor of
  2 w.r.t. ELVIS
• Vertical optical axis

14
Impact on Telescope ?

• Favours Gregorian over R-C
• Now confirmed (but not quantified)
• Favours Gregorian f/1 over f/1.5 ?
• tbc
• Mitigates need for adaptive secondary
• Requires massive Nasmyth platform
• 3MfR is a facility in its own rights with the
  possibility of feeding
• UV/optical spectrograph
• d-IFU system
• Fiber system (for MTHR)

All these instruments will be required to share
the same Nasmyth platform