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Hi All,
Very interesting discussion. Some parts of Ray's hypothesis I would tend to agree with, some parts not.
I don't reject a sub unless it has an obvious fault, i.e. visibly poor star shapes. In most cases I get to use all my subs, (ROR jobs, PMX, off-axis guiding etc) even small defects are rejected by the data processing. So, depending on the brightness of the object I vary the exposure time. For NB imaging, 10 -15 minute subs bring out the detail in the DSO, shorter subs generally don't allow the signal to be collected in the dimmer regions of extended objects.
For LRGB, 5 minute subs are all I can do, LP from Melbourne kills the subs after that.
I'm also a fan of collecting lots of subs, but not so you can reject them based on a FWHM measurement, but so the data rejection algorithms are more accurate. So I tend to let the software decide which parts of the sub to retain and which parts to discard. After this a combined image can be sharpened heavily without showing artefacts, which in essence is removing the blur induced by seeing.
Cheers
Stuart
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