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I've been imaging for some time with a CMOS ZWO ASI1600MM Cool camera and have been mostly happy with the results.  For calibration I've been using Darks + Bias + Flats registered and stacked in DSS but a while ago saw on SGL and elsewhere comments that with a CMOS camera it's better to use Dark Flats - better than what and why?  I've therefore for the first time taken some dark flats (using previously calculated flats @ speeds for each filter but with the telescope cover on i.e. dark - right?).  The questions I now have are:

  1. Is it good to use dark flats with CMOS cameras?
  2. How do I use the dark flats in the registering and stacking process? 
  3. Do I just add them in together with the darks + bias + flats or do they replace one of these, bias perhaps?
  4. Any other guidance?

Graham 

    

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  • 2 weeks later...

Hi Graham, dark flats (or flat darks i think is the proper term) are basically darks, but for your flats, so exactly the same settings as the flats but taken in the dark.

I don't know the reason why, but it is apparently better to calibrate the flat frames with dark flats rather than bias when using CMOS camera.

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My understanding is that the read noise on CMOS sensors is not as repeatable on very short exposures when compared to CCDs hence the use of flat darks rather than bias frames with CMOS cameras. 
 

However, I confess I’ve never tried bias frames with CMOS cameras so can’t comment on the difference from first hand experience, and the latest generation of CMOS cameras have very little read noise anyway.

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Thanks - I've used bias with my ZWO ASI1600MM-Cool for over 4-years with no apparent probelms but with new filters + weight of advice in favour or using dark flats I thought I'd give it a try. 

Just completed my fist image here and all looks good, though whether bias would also have been OK I'll never know.  It's just that bias was easier and quicker, though with the APT Flats Aid function the extra effort required is limited.

Graham     

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The Panasonic sensor in the ASI1600MM behaves differently for exposures shorter than around 0.1sec.  So if the flats are longer than 0.1 sec (which they usually will be) then flat darks should be used.    However this isn't a general rule for CMOS cameras.

Mark

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