TMT IWG

1
TMT IWG

• Members

2
IWG Charge (L. Stepp, Oct 30, 2003)

• Develop plans for all TMT instrument development
  activities including initial plans for interim
  activities from November 2003 through March 2004,
  based on the guidelines listed in Attachment 1
• Assess impact of instrument requirements and
  designs on the telescope and adaptive optics
  systems
• Coordinate all TMT Instrument development
  activities
• Work closely with the Adaptive Optics Working
  Group
• Advise the Science Advisory Committee regarding
  instrumentation issues
• Provide periodic progress reports to the ISG, as
  requested

3
IWG Meetings

• Telecons every 1-2 weeks since Nov
• Two physical meetings
• Tucson, Dec 16, 2003
• Santa Cruz, Mar 17, 2004
• Joint meetings with SAC
• Tucson, Dec 17, 2003 (AO issues)
• UCLA, Jan 13, 2004 (MIR issues)
• UCSC, Mar 18, 2004 (WFOS HROS)
• Vancouver, Apr 26, 2004 (MCAO MOAO)

4
Global Comments

• WFOS
• Larger f/ratios ( f/15) tend to limit WFOS field
  size
• Largest field (all-reflective) WFOS instruments
  prefer RC
• But an attractive (albeit smaller field)
  all-refractive design prefers Gregorian
• Some other instruments may be just as problematic
  (or be impacted by telescope design) as WFOS,
  e.g.,
• MIR
• ExAO instrument
• MOAO
• Some simple, non AO, instruments should be built
  for first light
• to produce science in poor seeing and full moon
  conditions
• Need to consider detector procurement early
• potentially huge gains to be realized

5
MIR - Why Its So Hard

• The atmosphere, even at the best ground based
  sites, presents major challenges
• Telescope emissivity dominates in the
  scientifically fruitful 10 µm window
• The 20 µm window is mostly closed
• Imaging requires chopping
• not considered further
• not in SRD
• see Simons Sprayberry slides from Jan 13

Telescope emissivity ? 10 ? 3
Emission
Wavelength (µm)
Transmission
Wavelength (µm)
Figure courtesy Univ. of Florida
6
MIR Telescope Optical System Low emissivity
essential

• Cass preferred
• Slow beam preferred
• Baffles need to be designed so they cannot be
  seen from instruments perspective
• Change baffles between optical/IR configurations?
• Narrow secondary support structure to minimize
  diffraction component in background
• M2 supports and drive electronics are hidden
  behind mirror and cooled via water/power lines
  that fit behind vanes
• Undersized secondary, preferably with central
  hole so imager views cold sky
• Secondary acts as entrance pupil in the optical
  system

Old NIRI/Gemini Pupil Image
7
MIR Coatings Cleaning

• Low emissivity telescope coatings required
• Currently 4 emissivity on G-S with silver M2
  and Al M1 coatings
• Protected silver on the primary and secondary?
• Aggressive and regular mirror cleaning procedure
• CO2 or laser cleaning?

8
MIR Telescope Enclosure Mount

• For thermal control reasons, need telescope
  enclosure and structure to have low emissivity to
  rapidly equilibrate with surroundings (Lowmit
  paint)
• Also desirable to have -
• Low thermal mass of telescope structure
• Low wind cross-section of telescope mount to
  minimize wind shake and maximize wind flushing
• Capable of relatively fast mount offsets over
  30-60 arcsec to support efficient nod cycles
• Very important to maximize open shutter
  efficiency of system

9
MIR Telescope Motions

• Chopping not required for MIR spectroscopy
• due to array detectors
• Nodding is required
• 5-10sec timescales
• distance 0.25 along slit
• distance
• accuracy
• May be implications for wavefront sensors
• Scan mode required
• create data cube by stepping telescope
• small (0.025) steps while guiding
• Non-sidereal rates required
• to track asteroids for calibration
• Likely need queue based system to exploit best
  mid-IR conditions (20 µm particularly)

10
MIR Facilities

• Extensive grade-5 helium distribution system
  needed
• Facility instrument handling equipment, lab
  space, vacuum systems, etc.
• Large capacity air conditioning system to preset
  air temperature within the dome for predicted
  evening value

11
MIR Summary

• Mid-infrared instrumentation has a broad range of
  potential observatory impacts
• Baffling
• Dynamic performance of telescope mount
• Mirror coatings cleaning procedures
• Wavefront sensor control systems
• Etc. (even the paint)

For details on MIR see Simons and Richters
slides from Jan
12
HROS (MTHR)

• RC and Gregorian telescope design OK
• not sensitive to f/ratio (f/15 nominal)
• ADC and image rotation internal to instrument
• Huge, requires tennis court sized Nasmyth
• 12m 16m area with fixed gravity
• separate level of Nasmyth platform?
• Mass
• Fibre mode desirable?
• If yes, then should be located close to minimize
  losses
• No new technologies
• Good candidate for first light instrument
  (natural seeing, any conditions)
• Could build minimalist version first
• Descope options exist

For details see Vogt slides from Mar
13
HROS (MTHR Concept)
14
NIR Deployable IFU

• Deployable 1-5 micron instrument.
• Up to 5 arcmin diameter field correction in
  each element.
• Coarse sampling (0.05).
• R5000 over broad band.
• 20 deployable units of 20x20 elements.
• 2K detector each
• Telemetry may be sufficient for PSF.
• Issues
• must demonstrate that MOAO works in real
  conditions
• open-loop, off-null operation
• space requirements?
• wavefront sensing schemes?
• What will delivered PSF be?
• Knowledge of PSF available from AO system?
• Wavefront error budgets?
• how partitioned between AO and instrument?

15
MCAO On-axis IMAGER

• Monolithic 1-5 or 1-2.5 micron wavelength.
• Fine Sampling 0.005
• Small single field of view 10
• High stability precision relative astrometry
• Few moving parts.
• On-board active alignment and focus.
• Requires very early sampling of the focal plane
  to preserve high Strehl.
• Could also serve as part of extreme-AO instrument
• Need parallel imaging capability for PSF.
• Will probably be detector limited.
• 30 field of view is reasonable.
• 30 at 0.005 is only 6000x6000 pixels.
• Also, given the plate scale and typical pixel
  size (18 microns), the camera focal ratio is
  reasonable (F/25).
• 4096x4096 arrays with 9 micron pixels may be
  possible soon
• 2 or 3 side buttable.
• What will delivered PSF be?
• Knowledge of PSF available from AO system?
• Wavefront error budgets?
• how partitioned between AO and instrument?

16
MCAO On-axis IFS

• Requirements
• 0.8-2.5 mm (goal of 5 mm)
• R4000 (OH lines, kinematics)
• R10,000 for UV lines from z6-10 Echelle Mode
  (20,000-100,000)
• Broad band coverage in each spectrum (2000-4000
  spectral elements)
• Spatial sampling 0.005 for Nyquist at J

• Problem pixel math
• 10 at 0.005 sampling 2000x2000 field
  elements
• R4000 for broad band with 2 pixels per element
  2000 pixels per spectrum
• Efficiency factor 1.1 to 2 pixels
• Total 9-16 billion pixels!
• 4000 of todays largest detectors!

Compromises are needed with MIFS field of
view/configuration. Most science can be preserved
with smaller field of view (0.5)
17
MIFS Example configuration

• Central 100x100 lenslets feeding 9 modules with
  R4000 (0.5x0.5).
• Adjacent 5x20 lenslets feeding echelle
  (0.025x0.100).
• Other lenslets feed imager (10x10)
• Imager used for active alignment
• Other configurations could have
• coarser scale in outer region or
• deployable regions for astrometry.

18
MCAO IFS ISSUES

• Compromises are needed with MIFS field of
  view/configuration.
• Wavelengths 2.5micron bring penalty (below)
• Detector Performance and Cost
• Detector dark current Background in 0.005,
  dispersed to R5000 is two orders of magnitude
  below existing detector dark currents between OH
  lines.
• 1-5 micron devices may have significantly higher
  dark current and read noise over 1-2.5 micron
  devices.
• Lose significant sensitivity in 1-2.5 microns
  region.
• Caveat latest Rockwell 1-5 micron detectors
  claim good performance.
• Same questions re PSF, error budgets from AO

19
WFOS concepts

• Many similar problems with all designs
• Size of CaF2 refractive elements?
• need large beam to get resolution requested
• Size of beamsplitters and filters?
• Efficiency of AR and reflective coatings?
• 0.31-1.1microns
• Efficiency and size of VPH gratings?
• may need VPH gratings to get spectra resolution
• Incorporate ability to nod shuffle
• Nods 30sec
• ideally with
• Must synchronize nods, WFS, CCD controller
• Require large ADC
• Require large mask maker, mask storage

20
Future Work

• WFOS
• Continue exploring design space to establish what
  is really feasible in a single concept
• VPH grating development
• Size of refractive elements and filters (for HROS
  too)
• MOAO
• conceptual design(s) of complete MOAO dIFU
• partition WFE with AO system
• MCAO IFS
• reduce field or examine alternate configurations
  for IFS
• conceptual design
• partition WFE with AO system
• ExAO
• explore impacts and requirements on telescope
• add ExAO aficionado, e.g., James Graham