Technology Nugget—TMT's Adaptive Optics Program Enters a New "Stage"

The unprecedented size of TMT's primary mirror will provide an opportunity to obtain astronomical images far sharper and clearer than any that can be produced using today’s telescopes—if the distorting effects of the earth's atmosphere can be almost completely corrected. The technology of sensing and correcting these distortions before they ruin an image is known as adaptive optics, or AO. Although AO has now been used by many astronomers for nearly a generation, TMT will require new concepts and components that are larger, more capable, and/or more sophisticated than any system developed to date. This includes the deformable mirror and tip/tilt stage, two devices that may be considered the business end of adaptive optics.

Atmospheric distortions can be divided into the twin categories of image motion and image blur. Both types of aberrations change on time scales of less than one-hundredth of a second, so that a dancing, speckled image of a star rapidly averages out into a featureless blob. But the two different types of errors are best corrected using different types of hardware. The deformable mirror contains a very large number of actuators which push and pull the surface of the glass into a distorted shape to precisely cancel the blurring effects of the atmosphere. Image motion is corrected by tipping and tilting the overall surface of the mirror in a compensating direction. Roughly 80 per cent of the atmospheric distortion consists of image motion, so the range of correction required for tip/tilt is considerably larger than the small adjustments applied to the surface of the deformable mirror.

Because of the large diameter of TMT, the deformable mirror will require over twice as many actuators as any other mirror attempted to date. The actuators' range of correction will also need to be 40-50% larger than typical existing actuators, but their size must be minimized so that the deformable mirror does not become unacceptably large. The actuators must also work well at temperatures well below zero, a new requirement which will reduce the thermal radiation emitted from the surface of the mirror that would otherwise add background noise to images of stars and distant galaxies taken using infra-red light.

All of these new demands on the deformable mirror actuators have been demonstrated in a small prototype mirror produced last year by the French company CILAS. The mirror (see figure 1) consists of 69 actuators arranged in a small 9x9 array with the corners clipped, all of which worked properly and met all of the above requirements (see the April 2006 Project Manager's Corner or further details). TMT will use two much larger deformable mirrors based upon 63x63 and 75x75 arrays of these actuators. This is a large step up, but CILAS is now in the final stages of fabricating a mirror with 41x41 similar actuators for the European Southern Observatory.

TMT's requirements for image motion correction are similarly stressing. The objective is to stabilize images so that their residual wander is no more than about one-ten-thousandth of the distance a star appears to move across the sky during an interval of just one second! Before correction, the disturbances caused by atmospheric disturbances and the vibrations of the telescope itself are expected to be about 100 times as large, and the position of the images will oscillate many times each second.

Figure 2: CILAS 63x63 Actuator Deformable Mirror
Mounted on a Tip/Tilt Stage

CILAS proposes to correct these errors by mounting one of the TMT deformable mirrors on a tip/tilt stage (see figure 2), which will pivot the entire surface of the deformable mirror as needed to cancel the image motion. This approach helps to reduce the total number of mirrors and the total size of the TMT AO system (no separate tip/tilt mirror is required), but the TMT deformable mirrors are much larger and heavier than other mirrors which have been mounted on tip/tilt stages in this fashion. The objective is to correct that fraction of the image motion which oscillates no faster than about 20 cycles per second; this appears to be feasible on the basis of mechanical modeling and the measured performance of the smaller existing tip/tilt stages. Much smaller (but still significant) oscillations at still higher frequencies can be corrected by the deformable mirror actuators themselves.

Image motion compensation at this level is clearly very challenging and very important to TMT. Because of this, CILAS will build and test their tip/tilt stage as the next phase of their work on the TMT adaptive optics. The formal "kickoff meeting" for this activity was held recently on April 12th; a 20-month effort is anticipated, with test results available to confirm the performance of the stage in late 2008.

The tip/tilt stage will then be transferred to the Herzberg Institute of Astrophysics (HIA) for additional testing. HIA is the TMT partner responsible for NFIRAOS, the AO system which will use the CILAS deformable mirrors and tip/tilt stage. The stage will then be returned to CILAS for final assembly with the 63x63 actuator deformable mirror, with eventual deployment to TMT and testing on the sky with NFIRAOS shortly following telescope first light.