Technology Nugget—Progress towards Prototyping the Tip/Tilt Correcting Stage for the TMT Adaptive Optics
  Brent Ellerbroek

Last month, the TMT Adaptive Optics group met (virtually) with our supplier CILAS to review their progress towards performing a prototype demonstration of the tip/tilt stage for the TMT adaptive optics system NFIRAOS. This stage will play the critical roll of providing real-time image motion compensation to correct for the effects of telescope vibration and the image wander introduced by the atmosphere itself. In order to obtain the sharpest possible images that are possible with a thirty-meter aperture, NFIRAOS must correct this random image motion to a residual level approximately one-two-hundredth the diameter of an uncorrected, blurry image of a star. This is about one-ten-thousandth of the distance that the star appears to move across the sky during an interval of just one second!

The NFIRAOS tip/tilt stage will face some significant implementation challenges in meeting these requirements. As illustrated below, the tip/tilt stage will serve as the mount for one of the NFIRAOS deformable mirrors, with a diameter of 30 cm, a mass of approximately 40 kg, and electrical wiring to nearly 3000 piezostack actuators (please see the earlier technology nuggets in April 2007 and April 2006). The stage must accurately and dynamically adjust the pointing of the deformable mirror at frequencies up to 20 Hz, thereby allowing the limited stroke of the mirror’s piezostack actuators to be reserved for the correction of image blurring and the very highest frequency image wander. Deformable mirrors have been successfully mounted on tip/tilt stages in this fashion before, but only in the case of much smaller and lighter mirrors. Although analysis predicts that the tip/tilt stage will meet its 20 Hz requirement, its performance is considered sufficiently important that TMT is funding CILAS to fabricate and test a prototype now to validate the design.

The design of the prototype was completed at CILAS earlier this year, and the procurement of parts is now underway. The project is holding to schedule and expects to be able to begin testing the prototype during the third quarter of this year. Formal acceptance testing at CILAS will take place in December or January, and the tip/tilt stage will then be transferred to the Herzberg Institute of Astrophysics (HIA) for further tests (HIA is the TMT partner responsible for NFIRAOS). The stage will then be returned to CILAS for final assembly with the NFIRAOS deformable mirror, with eventual deployment to TMT and testing on the sky with NFIRAOS shortly following telescope first light.