Dark and Bias?

[sammy1075]

hi can anyone give me a brief lesson on what Dark and Bias shots are? i have just ordered "Making Every Photon Count" which i assume will go in to detail about this, but any info in the meantime will be greatly received

cheers sam

[jgs001]

Sam, a dark exposure is an identical exposure to your light exposures, same ISO, duration etc. A bias frame, is the shortest exposure you can manage at the same ISO as the lights (on my dSLR, the shortest exposure is 1/4000s). The Deep Sky Stacker help is pretty good for this sort of thing.

[bigbox]

Hi Sam,

A Dark frame is a picture take at the same ISO & Exposure as the light frames, except this time the telescope cover or lens cap is put back on so all that is being photographed is a black image...

A Bias frame is a simple group of pictures to take, these only need to be taken once. for each of the cameras ISO settings and can then be reused on subsequent objects that are photographed.

these frames can be taken at any time. Remove the lens from the camera and place dust cap on to the camera to stop any light from entering. set camera to ISO 1600 & exposure to fastest possible value and take a set of picture. save these RAW pictures somewhere safe and mark as ISO1600 & repeat the sequence of pictures again but this time at ISO 800 and so on for the other iso settings. The idea behind bias frames is to cancel out any electrical noise made in the camera...

Hope this is helpful to you

[springbok]

Hi Sam,

Have a look a this thread

http://stargazerslounge.com/topic/94102-darks-flats-bias-frames-explained/

I have a text file which explains the basics, it'll help until yiou receive "The book"

[sammy1075]

Great response guys, its making a lot more sense already, cant wait till my book arrives.

Thanks again

Sam

[jgs001]

Sam sorry, I got distracted, I meant to say both should be taken with the lens/scope covered... so no light can get in.

[sammy1075]

no problem john,

cheers

[ollypenrice]

It's as well to think why you are taking these exposures. A dark frame with the lens capped off records the noise your camera produces, so although each pixel should record zero light, they won't. Some will record a certain amount of signal. The dark frame allows the stacking software to subtract this false signal from those pixels in which it is recorded, so correcting (calibrating) the brightness values recorded in your picture. In other words the noise is removed.

Now bias is rather more complicated. In principle they are not needed at all since the bias signal, essentially the read noise of your camrera, is contained within the dark frame you have already captured and subtracted. However, if your camera does not have set point cooling then the bias can be used by the software to 'scale' the dark frames which may not have been taken at the same precise temperature. I'll admit to being a bit vague on this since I use set point cooled CCD cameras for which I never need bias frames.

When you come to look into flat frames (frames which record what should be a perfectly featuresless white image if your system were perfect) you'll be able to use bias frames to remove the noise from them. Bias frames are, in effect, perfect dark frames for the very short exposures needed for flats.

Darks, bias and flats are subject to random variation so should always be created by averaging a fairly large number of individula exposures. How many is a matter of debate but if you take about 30 and combine them you'll doubtless be happy enough.

Confused? Welcome to the dark side. You ain't seen nothin' yet!

Olly

http://ollypenrice.smugmug.com/Other/Best-of-Les-Granges/22435624_WLMPTM#!i=2277139556&k=FGgG233

[sammy1075]

Thanks for that Olly, kind of understand what you say! The book arrived today so will have a good read, then maybe i will be a little more knowledgeable on the subject.

Thanks again for all your replies