Tip/tilt optionsTrade Study Report on Stand-alone

T/T vs. DM on T/T Stage(WBS 3.1.2.2.13)

Brian BaumanDecember 12, 2006

Status

• Study started, perhaps 20% complete

• Scheduling/manpower issues resolved last week; now can spend more time

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)?

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

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 @ 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)

Tilting OAP 1 + field lens

• Need 6 mm of motion on field lens to steer cone by 0.5 mrad

• Not really practical

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

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 200mm*350 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 70μ—consistent with the larger stroke PI actuators (120μ); could be reduced with smaller lever arm

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 ≈(1/2π)√(kT/m)

= (1/2π) √(240N/μ)/30kg=450 Hz (30kg per actuator probably pessimistic); seems OK

– Better moment-of-inertia/angular acceleration calculation pending, but I’m pretty concerned about it so far – depends on temporal tip/tilt power spectrum assumptions

Mirror sizes supported by other vendors

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