Bias and Dark Flats

[Catanonia]

ok

Bias ... The read error of transfering the image from CCD to computer = noise

Dark Flats... Dark files for the time it takes to produce the flat, in my time = 14 seconds or so = noise

So why do we have bias ????

Doesn't the dark flats have the same noise as the bias flats after all it is the dark + the transfer ?

[RobH]

Hello Cat,

If you apply darks, whether standard darks, or dark flats, the bias information is in them so bias subtraction isn't needed.

Cheers

Rob

[Catanonia]

Hello Cat,

If you apply darks, whether standard darks, or dark flats, the bias information is in them so bias subtraction isn't needed.

Cheers

Rob

Thanks mate, logic told me so, but wanted to check.

but a couple of questions,

Does DSS hold to these rules ?

If i take darks, do i need dark flats or bias ?

[Demonperformer]

Cat,

There is quite an in-depth discussion on this here:

http://stargazerslounge.com/imaging-discussion/129762-calibration-dss-doing-wrong-response-luc-dss.html

[RobH]

As DP says, I started a thread about calibration in DSS specifically to get to the bottom of how things work with that program, as there was a lot of confusion about it.

The end result was, that in DSS, you throw everything into the pot (darks, bias, flats, flat darks) and it works fine.

Cheers

Rob

[roundycat]

Pedantic it maybe but the name everyone except DSS uses is Flat Darks. Darks taken with the same exposure as for the relevant Flat which may be a different time for each filter. In English, Dark Flats implies an underexposed Flat, in French it may be different.

Bias is the initial electrical noise your camera makes when switching on to take an exposure. This noise is recorded before any photon noise appears. Bias also contains an offset of x ADU and that must be removed before any scaling or stretching. So, your comment 'Bias ... The read error of transfering the image from CCD to computer = noise' is not quite right. Bias is unwanted noise but it is not read noise and has nothing to do with it.

Your Flats must be Dark or Bias subtracted before stacking to make the master Flat.

In theory the proper way to calibrate is to subtract Darks from your lights and subtract Flat Darks from Your Flats. In practice, Flats have a very short exposure and thermal noise is not generally a problem so a simple subtraction of the Bias may be all you need. Be sure to check each Flat for hot pixels before stacking.

Dennis

[dph1nm]

Bias is unwanted noise but it is not read noise and has nothing to do with it.

Some confusion over terms here somewhere. The bias frame does contain the read noise - it has, after all, to be read out before you can use it.

Perhaps the term "The read error of transfering the image from CCD to computer" is a little misleading, as it sort of implies it is something to do with the cables!

NigelM