Flats/Bias or Flats/Darks for Flats with CMOS Cameras?

[tomato]

Firstly, apologies if this has been answered already within this forum, I suspect it probably has.

With a CMOS camera (specifically the ASI 178 mono) is it better to calibrate with Flats and Bias frames or with Flats and Darks taken with the same exposure time as the flats? Does it depend on the exposure time of the Flats? I find this can vary from 1-2 secs for the Lum filter to 10 secs for the Red filter if I keep the illumination intensity of the light panel constant. I suppose this begs another question, should you vary the illumination of the light panel for each filter, rather than the exposure time?

[Laurin Dave]

Not sure about the 178 but with my ASI1600 I use matched darks and matched flat darks ie. time, temperature, gain and offset for lights and flats..  I try and keep the exposure lengths similar by adding or removing sheets of paper..  I’ve recently switched to an EL panel and have found that flat exposure lengths are more similar (red intensity from the led panel I was using was much less than  G and B ) and that with my CCD the lum flat now works properly..  I also put a delay of 10 seconds between each flat or flat dark as I read somewhere that it allows the electronics to stabilise between exposures..  

Edited February 25, 2020 by Laurin Dave

[vlaiv]

Proper calibration is with flat darks rather than bias subs.

In principle, you can get away with using bias instead of flat darks - if your flat exposure is rather short / dark current very low and your bias is stable.

 

[tomato]

Thanks for the replies, I’ll be amending my calibration routine accordingly.

[ollypenrice]

Bias as dark for flats is fine with CCD but proper 'matched' darks for flats is the best with CMOS.

Olly