Re: Adaptive optics for a small telescope

From: Chris L Peterson (clp_at_alumni.caltech.edu)
Date: 03/20/05 

Date: Sun, 20 Mar 2005 21:27:28 GMT

On Sun, 20 Mar 2005 20:38:17 GMT, Don Bruns <dbruns@stelalrproducts.com>
wrote:

>It turns out that large apertures are not necessarily better. The largest
>preferred subaperture is about 20 cm...

That's what I meant by "huge" (at least in terms of amateur optics,
which is presumably what we are discussing here). That is, for a 20cm
subaperture, you need a large primary aperture just for a few zones of
correction- at least 1/2 meter. Cross that requirement with amateur
scope sizes, and you don't have much market for a high order AO device.

>The main advantage of laser guide stars is that they might be able to
>generate lots of photons for this aperture.

I'd add that it also allows for a reference object near (or coincident
with) the object being imaged.

>Low order AO (image motion only) can correct over a 60arcsec field of view,
>but as soon as higher orders are added, the field of view shrinks to a few
>arcesc, so that even Jupiter or Saturn are not improved. For the same
>reason, post-processing is mainly effective for global tip-tilt. When two
>"high-quality" short-exposure images from a 30cm aperture telescope are
>compared, bits and pieces of the image might be clearer, but also might be
>shifted within the image. Unfortunately, the noise in each image is likely
>to be large enough to hide this subtle shift. The probability that the
>seeing is instantaneously good over an entire planet, for a telescope
>larger than about 30 cm, is infintesimally small, while small areas can be
>pretty good.

I wouldn't agree entirely with this. When I collect several thousand
frames of a planet, there are obviously some frames that have a small
amount of internal displacement, and many more with large amounts of
internal distortion. I typically use about 5% of the frames I collect,
and it is clear that I am doing more than simple tip/tilt correction in
my sorting process. Seeing operates on a continuum- the probability of
near perfect seeing over a planet may be small, but the probability of
significantly better seeing is high enough to make post processing
effective.

_________________________________________________

Chris L Peterson
Cloudbait Observatory
http://www.cloudbait.com

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