Capture bias and dark flat?

[abmscopes]

I'm sure there's a really obvious answer to this but I can't see it! In the 'Capture Flat frame' dialogue there are tick boxes for "Capture and subtract bias frames..." and also "Capture and subtract dark flat frames", but it's an either/ or choice. Why can't both be captured in sequence after taking the flat frames ?

[Borodog]

Because it is either/or. You don't want to use both, as you would be double correcting the readout noise, since it is captured in both the bias frame and the dark flat.

[admin]

Hi,

yes, basically the dark or bias frames captured here are used to calibrate the flat frame being created and you only want one of them. If your flat frames are short exposue (<1s) then bias is quite good enough. If they are longer then dark frames may well be better.

If you want dark frames to be used when imaging, then they need to be captured at the same exposure/gain settings as your light frames, so the ones captured when taking a flat frame are no use.

My understanding is that using bias frames in addition to darks and flats does not give any advantage unless it happens that your dark frames are a different exposure time to your light frames (which you should avoid anyway). Unfortunately, because most processing applications allow you to use bias frames if you want to, many people think incorrectly think they are always useful.

cheers,

Robin

[soulearth]

Hello,
Does the substract bias frame option remove the bias from the master flat or is it only applied to the screen?
I want to re-stack with SIRIL but I don't know if I should remove the bias with SIRIL. If it is already done by sharpcap it must be unnecessary.

Best regards,

[admin]

Hi,

it removes the bias from the master flat, so no need to correct that frame again later.

cheers,

Robin