Bias or Flat Darks when Dithering

[bottletopburly]

When  imaging i use Dithering ,now i have been taking bias frames and putting bias frames into the Darks calibration (i'm using regim ) but when dithering is it better to use Bias frames or Flat Darks ? i believe it's either one but not both,  is there a better one of the two  to use for dithering ? 

[vlaiv]

No relation what so ever.

Bias, darks, flats, flat darks are used to remove unwanted signal from image - leaving only light signal.

Dithering is used to remove sensor "artifacts", such as hot/cold pixels, bad columns, FPN / Telegraph noise, etc... For this you need sigma rejection stacking. In addition dithering is spreading around random noise "reintroduced" by calibration process - this works with any sort of stacking.

Bias is useful only when:

1) You don't want / you are unable to do proper / full calibration - like when you don't have set point cooling and your dark current is low.

2) When you want to apply some transformation to dark current signal - like using mismatching exposure length darks, or doing dark optimization. Any method that relies on having dark current signal only, needs bias removed from dark subs.

For all other purposes bias can be just skipped, and in standard calibration its use is null/void operation - bias effectively cancels out with the same bias (first you remove it from darks, then from lights, and then you remove darks from lights - you can actually use any file as bias for this operation - it will not matter - it's added once by virtue of two negative signs and removed once which effectively just cancels it out).  light - bias - (dark - bias) = light - bias + bias - dark = light - dark.

Some cameras even have problems when using bias - like some CMOS sensors that adjust bias internally - it can sometimes be different - and this will cause problems.

[bottletopburly]

I had read on here to use bias when dithering and calibrate as darks ie put in to darks folder when stacking that was all just wondered if dark flats would do the same 

[vlaiv]

It is usually recommended for DSLR - cameras without set point cooling, not to use darks, but rather to use bias and dither.

This is primarily because DSLRs don't have set point cooling and proper dark subtraction is hard to achieve. DSLRs also usually don't have much amp glow and dark current can almost be considered constant offset. Part to dither really does not impact darks vs bias only calibration, but it's good to do anyway.

If you are using set point cooled camera - use darks and flat darks.

If you are using DSLR or non cooled camera - I would suggest to try to use dark optimization, provided your bias is stable (does not change and can be used for that purpose).

I would also recommend you to use flat darks in any case. If there is particular reason to do so - you can use bias instead of flat darks.

All of the above stands for both dithered and non dithered subs. Do dither if you can, it only helps.