There are ways of reliably and confidently determining the wavefront aberration entering the telescope. We are not simply relying on guesswork about what the best image "should" look like.

A common method of determining how the wavefront (or rays) coming from an astronomical source are distorted is to use a Shack-Hartmann (S-H) lenslet array. A S-H is a 2-D array of little lenses. If you focus the telescope pupil (an image of the telescope objective) onto the S-H, each lens in the array images what that portion of the telescope objective is seeing. The result is a 2-D array of sub-aperture images – looking kind of like a flies-eye arrangement. Each sub-aperture, however, is imaging exactly the same image, just moved about due to atmospheric "seeing". Measuring the offset of the image in each subaperture allows you to map out the contour of the incoming wavefront at each moment in time. The deformable mirror (DM) is then set to have the opposite contour, canceling out the distortion.

The correction is not, however, perfect as seeing is constantly changing and all DMs have a limited number of actuators. So, you want your "wavefront sensor" to update as quickly as possible (something like a 1000 times/second) and to have a DM with as many actuators as possible. Cost normally turns out to be the limiting factor for both of these parameters as fast cameras & large DMs are both expensive!

Cheers,
Claude

The primary difference between Active and Adaptive Optics is in their response times. Active Optics are slow, responding in something like the second range. They are meant to correct for aberrations induced within telescope due to deformations in the telescope optics. This is becoming increasingly important as telescope mirrors become ever lighter, thinner and "floppier". As the telescope moves around the sky, these thin mirrors tend to deform under their own weight. In Active Optics, actuators are typically mounted directly to the backs of the mirrors to "actively" support them in the correct figure. Telescopes move rather slowly so these actuators can respond equally slowly.

Adaptive Optics have fast response times closer to the kilohertz range (1/1000 second) to respond to the ever changing deformations in the incoming wavefront due to the effects of our turbulent atmosphere. It is normally not possible to warp the telescope's large primary mirror quickly enough so a smaller deformable mirror (DM) is commonly used. In some cases the secondary mirror in a cassegrain is used with an array of high-speed actuators (often piezoelectric pistons). In other cases, an image of the telescope objective is formed onto a much smaller, lighter, less expensive DM. Keep in mind that what ever is done to the real image of a telescope objective is equivalent to doing that to the objective itself. The actuators on the back of a small DM at the image of the objective have much less mass to push around and can respond many times quicker than actuators on the back of the telescope primary.

Please don't let these few comments insinuate that I don't have the greatest respect for the show and look foreword to each installment!

Thanks so much,
Claude

With best regards.
M.W.
[louth-observatory.org -- soon!]

With best regards.
M.W.
[louth-observatory.org -- soon!]