Potential MidIR Imager Impacts on the TMT Design

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Potential Mid-IR Imager Impacts on the TMT Design
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Assumptions

• In most areas the mid-IR imager is intended to be
  performance limited by the site, not the
  telescope
• As an IR optimized telescope, many of the design
  features of Gemini are applicable to TMT

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Telescope Optical System

• Up-looking Cassegrain configuration preferred to
  minimize thermal signature of the telescope
• Expecting relatively small instrument
• Do not expect large TMT secondary to be capable
  of chopping fast enough (few Hz)
• Off load this to cold chopping assembly in the
  instrument?
• Instrument also handles tip/tilt compensation?
• Higher order compensation needed?
• Adaptive secondary capable of chopping gt30
  arcsec?
• Diffraction limited imaging performance at 10 µm
  up to TBD wind speed
• Dominant PSF degradation due to wind shake
• Expect imager to be useful under most wind loads
  - must model servo performance of telescope mount

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Telescope Optical System

• Baffles need to be designed so they cannot be
  seen from imager perspective
• Change between optical/IR configurations
• Narrow secondary support structure to minimize
  diffraction component in background
• Undersized secondary, preferably with central
  hole so imager views cold sky
• Secondary acts as entrance pupil in optical system

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Coatings

• Low emissivity telescope coatings required
  (telescope ? 3?)
• Protected silver on the primary and secondary?
• Aggressive and regular mirror cleaning procedure
• CO2 or laser cleaning?
• Presumably use interchangeable window system on
  imager to prevent windows from limiting background

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Telescope Enclosure Mount

• For thermal control reasons, need telescope
  enclosure and structure to have low emissivity to
  rapidly equilibrate with surroundings (Lowmit
  paint)
• 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 chop/nod
  cycles
• Very important to maximize open shutter
  efficiency of system

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Control System

• Likely need 2 WFSs for dual-beam chopping,
  o/wise will only get good image quality 50 of
  the open shutter time
• Likely need queue based system to exploit best
  mid-IR conditions (20 µm)

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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