Flats, bias and DSS

[namreg]

Hi guys

Are there any DSS users out there that know how to subtract bias from flats only? I know there is a facility to load bias into DSS, but I believe the software will subtract these bias frames from the lights. I don't want to do this as the darks already contain bias data.

Thanks

[ollypenrice]

You could just make a master by loading the flats into the images box and the bias into the darks. I always work from master darks, bias and flats anyway. I like to put a slight Gaussian blur on my master flat before applying it.

Olly

[namreg]

You mean loading flats with the lights as if they were lights and the same with bias and darks? Would that work??

[Paxo]

Take a look at "The Calibration Process" at

DeepSkyStacker - Free

It goes through the process DSS uses to calibrate images.

Steve

[namreg]

Thanks Steve. I've seen that before - maybe I should read it again and take more notice!

[martin_h]

Just chuck the lot into DSS and it sorts it out for you.

[namreg]

Cheers Martin!

[Tim]

I have to say, that although it is often purported that the method is wrong, just bunging the lot into DSS works a treat for me! Maybe we should do a side by side comparison one day

[ollypenrice]

I'm sure that your (and DSS's) method works absolutely fine, Tim, it is just that shooting separate darks-for-flats for each colour (say Ha, O111 R, G, B, ) is pretty time consuming and a total waste of time when a single master bias will do perfectly well instead. I don't know if you saw narrowbandpaul's explanation on another thread? I feel I spend long enough on calibration as it is!! I am pretty certain that the results will be indistinguishable since the proponants of both methods get good images. It is just down to time.

(Well, no, the master bias and master flat method does confer one small advantage; you can put a light blur on the master flat to reduce noise further. I have even thought of running my master flats through NeatImage before applying them. In theory it might be a good idea but in all honesty I think we are tilting at windmills here. The other thing I like to do is see my master flat before it is applied because sometines they can be a bit suspicious looking.)

Olly

[martin_h]

From DSS help files

[namreg]

Those diagrams explain the whole process pretty clearly Martin thanks. I'd seen them before, but that was before I knew what they all meant!

[Tim]

Not sure if we are at cross purposes Olly? I take Lights, Darks, Flats & Bias and just chuck the lot into DSS as they are. DSS then does what it says in the diagrams above. I dont do separate flats for each filter, I keep my filters clean, but the dust motes on the CCD are dealt with very nicely by the flats.

Not sure how you could make calibration any quicker than that? If I do different filters on one night I suppose I could choose and use the master flat/dark/bias from the original stack, but I tend to just swap over the light frames, as DSS is so quick at producing the masters each time it doesn't seem to add a lot of time to the stacking process.

I know what you mean about the flats looking dodgy though, they are the one frame that I tend to check as they roll in. Nebulosity 2 makes that a doddle.

[Tim]

Going back to a previously mentioned query, doesn't the diagram above demonstrate that the Bias is subtracted from all of the calibration frames as well as from the lights?

Which would mean that not doing bias frames because the bias was already in the darks is irrelevant, because the bias is subtracted from the dark BEFORE the dark is subtracted from the lights?