Bias Frames

Somewhere to ask questions about the best way to use SharpCap
Post Reply
Paul
Posts: 6
Joined: Mon Feb 04, 2019 3:56 am

Bias Frames

Post by Paul » Fri May 17, 2019 5:26 am

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

User avatar
admin
Site Admin
Posts: 2737
Joined: Sat Feb 11, 2017 3:52 pm
Location: Vale of the White Horse, UK
Contact:

Re: Bias Frames

Post by admin » Fri May 17, 2019 5:02 pm

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

vichman
Posts: 7
Joined: Mon Jun 18, 2018 7:05 am

Re: Bias Frames

Post by vichman » Sat May 18, 2019 4:39 am

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

cuivenion
Posts: 182
Joined: Wed Feb 15, 2017 7:37 pm

Re: Bias Frames

Post by cuivenion » Sun May 19, 2019 2:52 pm

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.

vichman
Posts: 7
Joined: Mon Jun 18, 2018 7:05 am

Re: Bias Frames

Post by vichman » Sun May 19, 2019 3:02 pm

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

cuivenion
Posts: 182
Joined: Wed Feb 15, 2017 7:37 pm

Re: Bias Frames

Post by cuivenion » Sun May 19, 2019 3:42 pm

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

vichman
Posts: 7
Joined: Mon Jun 18, 2018 7:05 am

Re: Bias Frames

Post by vichman » Sun May 19, 2019 4:08 pm

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

cuivenion
Posts: 182
Joined: Wed Feb 15, 2017 7:37 pm

Re: Bias Frames

Post by cuivenion » Sun May 19, 2019 4:23 pm

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.

Post Reply

Who is online

Users browsing this forum: No registered users and 1 guest