I am not sure that is correct Rich. Doesn't a 16 bit CCD do 65535 ADU? Ins't that a measure of how many shades of grey it can
differentiate?
I'll have to check but I think well depth is a different thing. It measures how many electrons each well can hold. You could be right here though. Does that mean a camera with a well depth of less than 65535 cannot differentiate all shades of grey? (mind you your typical image only sits in a fraction of those 65535 levels of grey).
But what I was saying was go 80% of 65535 = 52,428.
I may well be the one here who has it wrong so let me check that question out first.
The other possibility with a light ring or other things is uneven illumination.
Also you didn't move the camera orientation after the flats did you? It has to be the same - probably was though if the dust donuts went.
Did you use median combine?
I have had odd flats at times that did not work. I put it down to the fact they seemed too bright. Also perhaps a wrong conclusion.
I get good results doing 3 flats median combined shooting for 20,000 adu or thereabouts with at least 2 or 3 second exposures (my cameras have physical shutters that show in less than 2 or 3 secs) using a white Tshirt at dusk and median combine with bias subtraction (not that important).
Can you post one of your flats?
Greg.
Quote:
Originally Posted by richardo
Thanks for this Greg.
I've been using a light box.
I set my flats in all channels to around 29-30,000 ADU which is about 70-75% the full well depth of the KAI4021m chip.
While my Lum flats divided well after bias subtraction on flats and lights, I saw over correction in the colour channels to the corners where my vignetting is. Dust motes were corrected nicely though.
Has me baffled.
I've tried lower ADU levels with no success, I might give the 80% a go and see what happens.
Maybe I'll try the 'T' shirt flats with twilight and see how the colour gets on. Seems strange that the colour flats correct the motes but not correctly do the vignetting. 
I'm a bit over it now as I've tried and tried..
As I've mentioned, once I get the 2" filters, vignetting will no longer be an issue. I still will be flat fielding the Lum however.
But hate it when things don't do as they should.
Cheers
Rich |