Re: Adaptive optics for a small telescope

From: matt (electro_optic_at_bellsouth.net)
Date: 03/23/05 

Date: Wed, 23 Mar 2005 15:24:27 -0500

Gleb wrote in message <4241b2fd$0$167$e4fe514c@dreader8.news.xs4all.nl>...
>Dear Marc
>
>It depends on seeing, place and generally acceptance of what is "the right
>thing". Professionals work at good sites and assume that seeing is similar
>everywhere. This is one of the reasons why they are so sceptical about AO
>for small telescopes. Another reason can be that they just know more ;)).
>Yet another possibility can be that it's a kind of commonly accepted
>mistake.
>
>I never managed to find any mention of adaptive optics with a 10"
telescope,
>although everyone knows that it will be not useful, while assuming R_0=5cm
>we get to a sensible AO system with 25 actuators! Something should be
wrong
>here.
>
>To check for myself, I've done a couple experiments with a 25cm Newton in
>Delft - which is probably the worst place you could find - a lot of light
>pollution from greenhouses and very strong winds from the Norths sea - and
>figured out that AO will help even with a very small telescope. Focusing
>camera to the input pupil reveals strong turbulence with a scale much
>smaller than the mirror size. Star images have clear boiling speckle
>structure.
>
>Of course the number of correection modes must be selectable so if
tip-tilt
>is the only significant term, the system will work on it, but if there is
>more and the light is sufficient, then the system must be able to correct
>more aberrations.
>
>I assume that amateur astronomers are distributed uniformly over places
with
>reasonable to good living standards, and these places may have very bad
>seeing conditions as these conditions are generally not taken into account
>when people choose where to live.
>
>
>
>"Marc Reinig" <nospam@nospam.com> wrote in message
>news:M_f0e.761$FN4.388@newssvr21.news.prodigy.com...
>> For visible light, having subaperatures smaller than 12" makes little
>> difference. So for a 12" aperture, you would spend a lot for very very
>> little. However, a big improvement can be had by simply correcting for
>> tip/tilt at >50Hz.
>>
>> Marc Reinig
>> UCO Lick Observatory
>> Laboratory for Adaptive Optics
>>

Gleb,
here's where the problem is with amateur adaptive optics for small scopes.
As you noted above, amateurs don't have the luxury of surveying a number of
sites and selecting the one with best year round seeing. The majority of
amateurs couldn't even measure seeing directly. They have no DIMM or other
similar setups and it wouldn't matter if they had, because people can't go
and live on mountain tops as a mass phenomenon .

Therefore, while professional observatories are built in places that have
dark skies and good seeing, let's say in the 1 arcsec range, amateurs live
in places where seeing is 2 to 4 arcsecs . That's 2 to 4 times worse, and
R_0 is decreased from the professional scopes size correspondingly, 2 to 4
times. This supports your idea that even smaller scopes would benefit from
AO in places with worse seeing than the typical professional observatories .
In other words, R_0 becomes smaller, amateur scopes are smaller too, and a
small to moderate number of subapertures would still fit such scopes.

And now the impossible to solve problem. Since light gathering for each
subaperture is directly related to subaperture area, a subaperture that is 4
times smaller is going to collect 16 times less light . Do not forge that
the most sensitive and fastest CCD's that are used in professional S-H
sensors cost too much for amateurs, and the next best are still expensive
but with readout noise at least an order of magnitude worse . Now you have a
system with 16 times less photons per subaperture, and 10 times noisier =
160 times worse than professional setups. Such a system will not be able to
find suitably bright guide stars within the isoplanatic patch and therefore
will be useless on all but the very brightest objects , among which the
planets . No general purpose would be possible with such a system due to its
160 times less sensitive wavefront sensor .

best regards,
matt tudor

Relevant Pages

  • Re: Adaptive optics for a small telescope
    ... Professionals work at good sites and assume that seeing is similar ... here's where the problem is with amateur adaptive optics for small scopes. ... amateurs couldn't even measure seeing directly. ... subaperture is directly related to subaperture area, ...
    (sci.astro)
  • Re: Adaptive optics for a small telescope
    ... Professionals work at good sites and assume that seeing is similar ... here's where the problem is with amateur adaptive optics for small scopes. ... amateurs couldn't even measure seeing directly. ... subaperture is directly related to subaperture area, ...
    (sci.astro.amateur)
  • Re: Adaptive optics for a small telescope
    ... > matt wrote: ... >> amateurs couldn't even measure seeing directly. ... This supports your idea that even smaller scopes would benefit ... >> subaperture is directly related to subaperture area, ...
    (sci.astro.amateur)
  • Re: Adaptive optics for a small telescope
    ... > matt wrote: ... >> amateurs couldn't even measure seeing directly. ... This supports your idea that even smaller scopes would benefit ... >> subaperture is directly related to subaperture area, ...
    (sci.optics)
  • Re: Adaptive optics for a small telescope
    ... > matt wrote: ... >> amateurs couldn't even measure seeing directly. ... This supports your idea that even smaller scopes would benefit ... >> subaperture is directly related to subaperture area, ...
    (sci.astro)
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