Quote:
Originally Posted by sjastro
Since flat fields are a source of Gaussian noise (in particular when poor flats are used), the subject of how professional astronomers perform flat fields cropped up.
Olivier Hainaut wrote.
Typically we use twilight sky flats - that's the standard.
Dome flat are used only when TwiFF are not available or as part of a more advance FF technique.
The best is to combine Dome FF, TwiFF and night sky FF (ie using deep images on ~empty fields) -
the Dome FF have ~infinite signal-to-noise ratio, but very poor representation of the illumination; the TwifFF have decent S/N; the night FF have the best representation of the illumination, but have poor S/N. Using some techniques like wavelet filtering, one can extract the S/N from the DoFF, and combine it with the illumination from the TwiFF and the NiFF. Using that, you can flatfield at the 99.99% level. Of course, one needs that level only for specific cases; in most situation, 99% is more than enough.
Cheers
oli
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Wow! That's incredibly useful info. Worthy of a thread of its own
Makes perfect sense when you think about it. Probably also emphasises the increasing difficult in obtaining good flat fields with increasing focal length most people encounter.