mountain profile Institute for Astronomy University of Hawaii

Adaptive Optics

Maintained by W-W


Seeing

NeptuneStarlight is strongly affected by temperature fluctuations in the Earth's atmosphere. Winds blow these fluctuations across the telescope causing stellar images appear to shift in position and to break up into speckles. Over time, the image blurs, ending up many times larger than it would have been in the absence of the atmosphere. As a result, the resolution and sensitivity of ground based telescopes are significantly degraded from ideal values. Seeing, as astronomers refer to these degradations, varies with the site, the season and the time of day, but it is always there, and always a problem.

The Race to Space

One way of avoiding atmospheric seeing problems is to place a telescope in orbit above Earth's atmosphere. The Hubble Space Telescope (HST), for example, can achieve images well below 0.1 arc second in width whereas seeing limited images on Mauna Kea are at best three times larger. Even small telescopes in space are very expensive, however, and it is hard to launch large aperture telescopes. Consequently, space doesn't offer the large collecting area needed for high sensitivity. One, Earthbound eight meter telescope has more than ten times the collecting area of the Hubble Space Telescope.

So we have high resolution telescopes in space, but with low sensitivity and large aperture telescopes on the ground, but with seriously degraded performance due to seeing.

Adaptive Optics

Adaptive Optics (AO) is a growing technology area that tries to restore the performance inherent in large ground based telescopes by partially correcting the effects of the atmosphere. The simplest adaptive optics system is one that corrects for the seeing induced tip-tilt excursions stellar image. The position of the position of the star is constantly measured and then corrected by steering the light beam with a fast controllable mirror. Since image tilt changes with a characteristic time scale of order 0.1 seconds, the correction system must operate about ten times faster in order for the corrections to be valid for a reasonable fraction of the time. Tip-tilt systems are in use on many telescopes and can narrow the image width by about a factor of two. The next step is to correct more that just the wandering of the image by trying to correct higher order errors like focus created by the atmosphere. If enough of the starlight can be restored to its state prior to entering the atmosphere then the image core is close to its ideal value and we can have large, high sensitivity telescopes that also offer high resolution

AO at UH

The key components of any AO system are a Wave Front Sensor (WFS) to measure the incoming light and a Deformable Mirror (DM) to correct it. Most AO systems in the world measure the slope of the incoming wave front and then use push-pull actuators behind a thin mirror to correct the wave front. In 1988 Francois Roddier, working at the IfA, discovered a new way to measure the starlight and showed that when coupled to a different type of correcting element, a very powerful, very simple AO system could be built. These curvature adaptive systems were developed at the IfA and have slowly grown in use around the world. In June1999, the eight meter Gemini telescope was dedicated on Mauna Kea using a 36 element curvature system built at the IfA. Current research at the IfA is focused on extending the application of curvature adaptive systems by increased understanding of these systems, developing a ready source of components and demonstrating high performance systems. The IfA's Adaptive Optics Group currently consists of Christ Ftaclas, Mark Chun, Peter Onaka, and Doug Toomey.

Adaptive Optics Research at the University of Hawaii is supported by the National Science Foundation, The Gemini Observatories and the Institute for Astronomy.