Peter
So two lessons - clean my optics but more importantly, dont disturb the optical train between taking flats and lights
Thanks
Dan
Quote:
Originally Posted by Peter Ward
Daniel, flats are simply an image of a featureless object that you use to show up, then remove the effects of: dust, optical and ccd imperfections of your entire imaging system.
Twilight flats are best, as you are shooting in the same conditions as you are in the dark (i.e. identical instrument illumination and scattering ) , use a median combine ( with at least three exposures) with small field changes create a master flat to ensure you don't get any stars or cosmic rays in the data.
With SBIG cameras you *do not need* bias frames. Bias information ( which is simply a zero second dark frame) is contained within every dark frame, so all you need to calibrate your data well, is a master flat and a matched exposure dark frame.
Back to flats. For the flat field data to be valid, nothing should be changed between taking flat field and light field data. By nothing, I mean focus, filter position & camera orientation. Unless you have a very elaborate registration pin arrangement, there is no way you can remove, then remount a camera and expect the above to be identical. Change any of these and you will get the results you see now.
Flats also need to be dark frame calibrated before you can use them. Using auto dark darks when taking flats makes this a no-brainer. Calibration order is important. Your light frames need to be dark subtracted before you apply flat field data. Most software does this for you, but If you do this in the reverse order you end up with weird looking images.
Greg's comments regarding keeping everything as clean as possible should be heeded, no dust = no dust doughnuts, but vignetting etc will still need fixing via your flats. Hope that helps.
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