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Old 23-11-2013, 08:05 AM
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Satchmo (Mark)
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Originally Posted by Peter Ward View Post
If on the other hand, the optical system is better than "diffraction limited"
(yes... this is possible) it can suck up, for example, a 1/6th wave of atmospheric distortion before *total aberrations* take it into the 1/4 wave "diffraction limit"

Hope that makes sense
Peter ,

I doubt that there is a single deep sky imager on this forum who is sampling the airy pattern with enough pixels or has near diffraction limited seeing with a mid size instrument to even put that theory to the test .

I don't think it is sufficient to point to experienced amateurs with high quality gear and say the differences are down to their having 1/10 wave rather than 1/4 wave optics , or that this extra quality is `money in the bank ' when it comes to all the different quality scopes trying to image a star that is say a real 1 arc second fuzzball in the sky .

Its worth noting that generally large observatory mirrors are specified by how much light they are expected to focus into X sub arc second spot, not by Strehl ratio.

The situation is made that more complex that smaller scale errors like 1/6 wave may be acroos 6 or 8" apertures but much greater over largere ones taking into account the tilt components between the cells .

My gut feeling is that fast optics will bloat more easily via the defocus component of the seeing error, but fast systems are the most likely to be well undersampled compared to the potential resolution of the telescope

The type of seeing is all important to . I'm sure there are many scientific papers around on the topic which are probably beyond our grasp .

There is some Hartmenn testing software around by SBig ( probably free if you ask nicely ) - it would be really interesting to see someone do some systematic measures at the beginning of their imaging session to see how often the real errors in their optical train are visible and compare to the FWHM they get with the actual images. I suspect most of the time you would be getting snapshots of the seeing .

Any takers? The question of `how good is good enough ' when it comes to amateur gear is one really worth looking into.
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