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Bias Frames

Posted: Fri May 17, 2019 5:26 am
by Paul
Apologies if this is a silly question: Why does SharpCap have options for capturing Dark and Flat frames, but not Bias frames?

I am using a ZWO 1600 MM Pro camera, connected to a Celestron Edge HD 8.

Should I conclude that Bias frames are not needed at all? OR is SharpCap "assuming" that Bias frames have been captured manually off-line already?

Regards: Paul

Re: Bias Frames

Posted: Fri May 17, 2019 5:02 pm
by admin
Hi,

My understanding has always been that bias frames are only necessary if your dark frame have different exposures to the light frames you are trying to subtract them from. In theory the combination of the dark frame and the bias frame allows you to calculate what a dark frame would look like for a different length exposure. In practice I don't think this works as well as people would like particularly due to hot pixel is not really responding in a linear manner to exposure length.

So, in summary, as long as you make sure your dark frames match the exposure and other settings for your light frames then there is no need to use bias frames.

Cheers, Robin

Re: Bias Frames

Posted: Sat May 18, 2019 4:39 am
by vichman
Hi Robin,

I have the same understanding concerning subtracting bias frames from the lights: Typically you do not need bias frames as they are already included in the dark frames.
But: Don't you need them (or flat darks) to calibrate your flats? IMHO they are needed there, otherwise your flat correction will be wrong.

CS
Stefan

Re: Bias Frames

Posted: Sun May 19, 2019 2:52 pm
by cuivenion
Bias frames can be used to calibrate flats, but only very short exposures (milliseconds). If your flats are 1/4 second or longer I would use flat darks. To be honest I always use flat darks anyway, it doesn't take long to set the exposure to the same length as your flats.

Re: Bias Frames

Posted: Sun May 19, 2019 3:02 pm
by vichman
Hello cuivenion,

yes, that is exactly what I do when I user other imaging SW.

I am only wondering why Sharpcap does not apply Bias frames or Flat Dark frames to Flats when shooting Flat frames inside of Sharpcap (and applying them automatically while doing live stacking).
Or does Sharpcarp apply them, and I missed that so far?

Thanks
Stefan

Re: Bias Frames

Posted: Sun May 19, 2019 3:42 pm
by cuivenion
Hi, there is the option to capture bias when using Sharpcaps flat capture facility:

http://docs.sharpcap.co.uk/3.2/26_Captu ... Frames.htm

Re: Bias Frames

Posted: Sun May 19, 2019 4:08 pm
by vichman
Ah, missed that, thanks!
But there is no option to shoot Flat Darks, right?.
With my CMOS OSC camera I need to do that, as Bias frames are not recommended by the manufacturer.

CS
Stefan

Re: Bias Frames

Posted: Sun May 19, 2019 4:23 pm
by cuivenion
It looks like you can't do that at the moment, but to be fair the flats capture facility is meant for live stacking, which usually means higher gain, so your flats should be quite short exposure anyway. In this case bias frames should work fine.

The option to choose the exposure time and do flat darks would be preferable though.