Quote:
Originally Posted by LightningNZ
Depends how you make your flat. If you image a twilight sky then you may pick up some stars. If you image a t-shirt then there may be varying density of threads or whatever across it. If you image an LCD screen like me then they are pixel defects or backlight inconsistencies.
So I move my scope around in front of the screen to "dither". It has the effect of averaging out the inconsistencies.
Cheers,
Cam
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Yep that's normal dithering of the incoming light source. But since here we're dealing with ideally a uniform incoming illumination dithering (as in moving the telescope) should not be relevant. For sky flats then yes, it is relevant to dither as much as possible to remove stars, but that is another issue.
As I understand it, Ray simply manipulated the resulting flat frames by shifting the central portion of the recorded signal around using Photoshop. But this also shifts the noise, which would be the opposite of what happens during normal dithering. So just wondering how practical that is? Effectively it simply blurs the noise component of the integrated flat, so how about instead applying a Gaussian blur to the integrated master flat frame and thereby removing all noise contribution from it? If the blur is at a sufficiently small scale then any pattern noise would still be present, which is the really nasty noise component that only flats can remove properly.
I did some S/N analysis while integrating data for my new antennae image and found that a stack of images calibrated with darks and flats resulted in a combined image with around 10-15% lower S/N. So not using calibration at all would have yielded a superior final image. My problem is that the KAF-8300 chip does exhibit some large scale horizontal and vertical pattern noise which I can only get rid of by using flats.
I'm thinking that ditching darks altogether and using a Gaussian blurred master flat frame should give a better result. I'm not worried about the dark noise as both that and hot pixels are dealt with very efficiently by statistical rejection i.e. PixInsight Windsorized sigma clipping etc. so darks frames are really not necessary. I need to experiemt more with this, but the above is my current thinking.