Quote:
Originally Posted by Octane
Cheers, Lee. I knew about the first bit. I'm just not sure what it objectively measures. That's what I'm having issues getting my head around.
I just ran a bunch of hydrogen alpha frames through SubFrameSelector:
1.339 - 2.39 with a median of 1.706.
I take it this isn't too bad? 
H |
if I may also intrude...
FWHM is a measure of star shape. It is a useful measure because all unsaturated stars (whatever brightness) in an unstretched image should have similar FWHM. I think that this characteristic is used by CCDinspector - if you have consistent FWHM, your field graph will be flat.
If you don't get consistent FWHM, you may be trying to measure on stretched data or your optics has problems. Also, some automated software will happily treat hot pixels as though they were stars and will also combine touching stars - the results are approximate at best. Better to use software that allows you to isolate one or more stars so that you can make sure that the test star(s) is not saturated and is not near a hot pixel or another star.
As Lee says, at 3.5 arcsec, you are heavily undersampled, so FWHM is probably not a very useful measure, because the star FWHMs will be similar to or smaller than the pixel size and what is reported will depend on where the stars fall on the pixel boundaries.