Quote:
Originally Posted by gregbradley
When I first used the 694 in CCDsoft the first thing that was obvious was FPN. I could see some manufacturing cut marks in some shots (short focus shots) and also a grid like flyscreen so the chip definitely has FPN.
But I like the idea of using PI to correct you mentioned. The idea of noise filtering on the flats I have not heard before and it'd be worth trying as is Martin's approach of multiple level flats and average combine them.
What tool did you use in PI to correct the uneven illumination?
Perhaps a test of the various methods with photos posted showing the results could be useful.
Greg.
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thanks Greg.
Under what conditions do you see the grid structure? - my flats and bias images are very smooth at f4. I have seen fixed noise structure in narrow band images, but that is due to thermal noise and cannot be handled with flats (flats can deal with variability of pixel sensitivity, but most cameras seem to have negligible pixel gain variation anyway).
I think that flat smoothing is widely used - for example Nebulosity applies a smoothing filter by default to remove hot pixels. This is different to averaging multiple flats, because filtering blends individual pixel values and removes the ability to do FPN correction on noise. However, it looks to me like FP noise is a non-event anyway with most cameras and certainly on targets with texture, so why not smooth the flats and reduce their noise contribution to the final image.
Martin's method of averaging flats with different ADU would probably help to deal with non-linearity in the CCD - flat fielding falls in a heap if the flats or lights stray into a non-linear region.
The PI dynamic background extraction process will clean up gradients pretty well, including minor vignetting - but not dust.
would be nice to be able to post some comparative images - for the past 6 weeks though, that would have been almost exclusively the undersides of clouds and of limited interest. Maybe tonight if the wind drops below 20kts
Cheers Ray