Tip/tilt options Trade Study Report on Stand-alone T/T vs. DM on T/T Stage (WBS 3.1.2.2.13)

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Tip/tilt optionsTrade Study Report on
Stand-alone T/T vs. DM on T/T Stage(WBS
3.1.2.2.13)

• Brian Bauman
• December 12, 2006

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Status

• Study started, perhaps 20 complete
• Scheduling/manpower issues resolved last week
  now can spend more time

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Options considered so far

• Pair of flat mirrors
• Tilting OAP1
• Tilting OAP1 move field lens
• Under-DM platform
• Can we use secondary (future or existing)?

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Pair of tip/tilt mirrors

• Coordinated moves of mirrors to affect pointing,
  but not centering
• Place in the telescope focus space
• Mirrors would be 250-300 mm in diameter (3.6-5.0
  kg)may be in range of PI off-the-shelf stages,
  e.g., PI-518.TCD (more later)
• If separated by 500 mm, then tilts necessary are
  1.5 mrad consistent with PI-518.TCD (need to
  calculate resonant frequency)
• Packaging could be interesting
• May merit follow-up if throughput/emissivity
  penalties are acceptable

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Tilting OAP1

• Accommodating 2 arcsec of tip/tilt slews pupil
  around by about 1.5 (about 0.7-1.0 subaperture)
• Plate scale 1.375 arcsec/mm ?1.45 mm _at_
  telescope focus
• OAP focal length 3 meters
• Beam is steered by 1.45 mm / 3 m 0.5 mrad, cf.
  33 mrad, f/15 cone
• On-axis aberrations generated by tilting OAP 80
  nm rms of astigmatism per arcsec on sky.not too
  bad.
• Correcting on-axis aberrations brings off-axis
  performance approximately back to pre-tilt level
• Even accommodating only 1 arcsec of tip/tilt
  slews the pupil by 0.3-0.5 subaperture
• Results in time-dependent illumination pattern on
  DM/WFS/other pupils DM-WFS registration not
  affected
• Perhaps closed-loop performance penalty?
• Could mitigate by stopping down aperture on both
  inside and outside of the annulus
• For reference, mirror would weigh about 16-25 lbs
  (7.3-11.4 kg), without lightweighting (cf. 5 kg
  limit for PI-518)

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Tilting OAP 1 field lens

• Need 6 mm of motion on field lens to steer cone
  by 0.5 mrad
• Not really practical

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Scaled-down CILAS TMT mirror

• TMT DM specs/features
• 360 mm pupil mirror
• 73x73 actuators
• 41 kg
• Assume DM scales down for NGAO
• 64 actuators across
• 315 mm diameter (within range of DM sizes
  considered during Indian Wells)
• About 31 kg
• Very rough assumptions, but enough to get going

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Kinematic vs. gimballed mount

• Gimballed obviously most desirable, but CILAS
  design of integrated gimbal disheartening
• Is kinematic mount sufficient? Example with 315
  mm DM
• Tilt required on 315mm DM (worst case) is 70
  arcsec 350 µrad for 2 arcsec tilt on sky
• If center of mirror is 200 mm from the axis,
  then Abbe motion translation is 200mm350 µrad
  70 µ, which is small (2) compared to
  interactuator distance of 3.5 mm
• Seems practical but should quantify performance
  penalty
• If lever arm is 200 mm, then stroke required is
  70µconsistent with the larger stroke PI
  actuators (120µ) could be reduced with smaller
  lever arm

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PI stages under DM

• Discussions with PI have indicated that the
  question is not whether it could be done, but how
  much it will cost.modulo moment-of-inertia
  concerns below
• Awaiting more information about best
  approaches/using previous designs
• Largest PZT actuators
• can pull 3500 N, push 30,000 N
• About 10K each
• Resonant frequency (1/2p)v(kT/m)
• (1/2p) v(240N/µ)/30kg450 Hz (30kg per
  actuator probably pessimistic) seems OK
• Better moment-of-inertia/angular acceleration
  calculation pending, but Im pretty concerned
  about it so far depends on temporal tip/tilt
  power spectrum assumptions

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Mirror sizes supported by other vendors

• Ball 2
• Newport 2
• Axsys 2
• OpticsInMotion 2x3, up to 4 custom