Dark frame library capture ignores "don't rescale 10/12/14 bit setting"

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ehea
Posts: 4
Joined: Sat Jun 01, 2019 1:27 pm

Dark frame library capture ignores "don't rescale 10/12/14 bit setting"

Post by ehea » Sat Jun 01, 2019 2:11 pm

Using Version 3.2.5986.0
  • I have a ZWO ASI 178mm camera (14 bit ADC)
  • I have this setting checked: General->Saving->"Save 10/12/14 bit images in FITS files without scaling to 16 bit [...]"
The setting works as advertized when I capture frames via "Quick capture" or "Start Capture", in the sense that the max ADU value I'll get is 2^14 this way, but when I use "Capture->Capture Dark...->Dark-Library checked , the resulting dark frame that is stored in the dark frsme library directory structure is scaled to 16 bit. This is perhaps a bu, at least it is unexpected and counter-intuitive. It requires an extra step of dividing the drak frame by 4 (in my case) before subtracting this from light frames in external software.

Is this intended or a bug?

CS
HBE

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Re: Dark frame library capture ignores "don't rescale 10/12/14 bit setting"

Post by admin » Sun Jun 02, 2019 2:41 pm

Hi,

This is intentional behaviour, because you are creating a master dark image that is an average of multiples individual dark frames. Under some situations (ranges of camera settings) the noise may be low enough that keeping the master dark frame at the cameras native bit depth might become the biggest source of errors in the image. For instance a particular pixel might have values (unscaled) of 1001, 1002, 1002, 1001, 1001, 1002 etc in the individual dark frames. The correct average value for the master dark here would obviously be 1001.5, but that couldn't be represented correctly if the master dark was only written at the bit depth of the individual dark frames.

Hope this helps explain it, Robin

ehea
Posts: 4
Joined: Sat Jun 01, 2019 1:27 pm

Re: Dark frame library capture ignores "don't rescale 10/12/14 bit setting"

Post by ehea » Mon Jun 03, 2019 10:15 pm

Oh I see.

However I was expecting that the master dark frames would be stored with floating point format FITS files anyway. That way it would comply to the native camera ADU range and not lose any information. I still think it's adding extra troubles when you keep the light frames (intentionally) at 14 bit but the master dark gets scaled to 16bit (e.g. if you just drop the stuff into, say, Deep Sky Stacker).

Thanks
HBE

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