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

1. 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

2. Status • Study started, perhaps 20% complete • Scheduling/manpower issues resolved last week; now can spend more time

3. 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)?

4. 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

5. 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)

6. Tilting OAP 1 + field lens • Need 6 mm of motion on field lens to steer cone by 0.5 mrad • Not really practical

7. 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

8. 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

9. 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

10. Mirror sizes supported by other vendors • Ball: ~ 2” • Newport: 2” • Axsys: ~2” • OpticsInMotion: 2x3”, up to 4” custom