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Old 18-05-2016, 12:18 PM
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Shiraz (Ray)
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Join Date: Apr 2010
Location: ardrossan south australia
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Quote:
Originally Posted by multiweb View Post
Hi Ray, I've noticed that many times as well. It makes sense to me. Taking mount performance out of the equation and assuming you have an hypothetical mount that tracks perfectly the seeing comes and goes in small intervals of time usually tens of minutes. It is very obvious when guiding let's say a series of 5min subs. This was very obvious at the SPSP. The seeing being very good and the temperature stable it was very easy to pick up small variations. So you might end up with very sharp subs and some others not so sharp. That's in a perfect world. Add the mount PE and wind buffeting and other factors on top and you invariably will end up with bigger star profiles the longer you expose. There is also the well depth of your camera. More exposure time, more photons and the star is usually bigger.
Thanks for the confirmation Marc. I tried to take varying seeing out of the equation by interlacing the subs - ie if a good bit of seeing came along, they all had a look at it. Similarly if there was wind buffeting, it was as likely to affect a 1 second sub as a 2 minute sub - no sorting was done to remove bad short subs.
I didn't measure star size, just FWHM, which is nearly independent of brightness for unsaturated stars. I manually compared similar stars in a few subs to ensure that star saturation did not affect the result.

If it is this easy to decrease the effective seeing from 2.6 to 2.1 arcseconds, why haven't I been doing it??
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