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Old 13-08-2015, 10:24 AM
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SkyViking (Rolf)
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Quote:
Originally Posted by Paul Haese View Post
In that case, am I to assume that reported values for my wide field system are inaccurate too? It might be prudent to obtain one of these. Just don't fall off the chair when you see the price.
Hi Paul, I'd say yes FWHM as a measurement of seeing only makes sense when images are properly oversampled. If the stars are less than a couple of pixels wide then measurement is impossible.
To give an example, when I look at my oversampled raw frames I can see stars typically covering 5-6 pixels across, with 2-3 significantly brighter pixels at the very centre. By definition these 2-3 pixels make up the FWHM. Hence Maxim's reported FWHM of ~2 arcseconds makes perfect sense given this equals 2-3 pixels at my image scale of 0.86"/px.
However, if I inspect a 2x2 binned raw frame taken at the same time in the same seeing conditions I get nonsensical FWHM values from Maxim because the stars are now undersampled and so the tool cannot measure properly.

You mentioned using a DIMM to measure seeing, but as I understand it this is done at high frequency and therefore not applicable to deep sky imaging? It seems more akin to planetary imaging where the details that can be teased out with lucky imaging are of course much finer.

For deep sky purposes I think the only reliable measurement is actual FWHM of stars in subframes with integration times of tens of seconds or more.
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