Flat Frame Over-calibrating

[PHannah]

In image calibration of light frames I am finding with a recent set of H-alpha images that the master flat is being applied too strongly, so that I have light-coloured donuts visible in the calibrated frames.
However if I don't user a master flat, I have vignetting in the corners.
Is there any way to apply a strength factor to control how strongly the master flat is applied?

Thanks
Peter.

[pfile]

this is usually caused by bad calibration of the flat subexposures. how are you calibrating your flat subs?

rob

[PHannah]

Hi Rob

I'm using a Superbias made from 50 bias frames and a master dark made from 30 1800-second subs taken at the same temperature. Then using PI's Image Calibration. No calibrate on the bias, calibrate+optimize on the master dark.

Peter.

[pfile]

try skipping the dark entirely, the flat exposures are pretty short, right? canon DSLRs do all kinds of crazy dark signal suppression internal to the camera which may contribute to this problem.

also perhaps try just a master bias instead of the superbias.

rob

[PHannah]

Just wanted to post the resolution to this issue.

The problem was...my mistake! In making my own processing notes I incorrectly wrote down that Calibrate should be checked under Master Dark, even though I had already bias-subtracted the master dark when I created it.

The result was a set of calibrated flats using a double-subtracted master dark. With the problem corrected the images look fine.

Thanks to all for your helpful suggestions.

[pfile]

thanks for following up, glad to hear it's sorted.

rob