Bias Frames?

[adamsp123]

Here are 2 images of M1 in - Ha 59 x 2 min subs binned 2x2 from Atik 383L+ camera on MN190.

Stacked in DSS with darks and flats, one with no bias frames the other with bias frames added.

No other processing done to the stacks

See the difference note - noisy banded image has the bias frames added... what gives?? I thought bias frames helped

[RobH]

Hello Pete.

If you're subtracting darks, then the bias is already there in the dark frame. If you subtract both darks and bias you are actually adding bias noise.

Either dak or bias subtract your lights, not both, but you must bias subtract your flats (or subtract flat darks....again, not both.)

Cheers

Rob

[daz]

Have you bias-subtracted the flats and/or darks Pete?

If so, don't use bias for the lights as well.

[daz]

like what he said!

[jgs001]

Pete... That's what I got... although I also got a pain load of green... I think I worked a way around it with DSS...

load 1 light, the flats and bias into DSS. Stack em... throw away the list. Now load all the lights, the darks and the master flat that the previous stack created. The master flat (if I've understood it correctly) will be debiased (??) the darks will remove the bias from the lights, and it should all be good. I think...

[SteveP]

Pete

I agree with Rob and Daz! Have a look at how DSS deals with darks, and indeed flats, to determine whether these are automatically adjusted for bias

Remember bias frames captures the dark current present in the image just by 'clicking' which is why you normally create bias frames on the lowest exposure time your camera/software can produce. Normally, because the bias current is already included in the darks, I rarely use bias frames. They really come into their own if you have some darks which don't quite match the exposure times of the lights and the processing software needs to scale the darks to the correct exposure length. At that point, the bias frame can be deducted from the dark so that the bias component isn't also being scaled

HTH

Steve

[martin_h]

Pete... That's what I got... although I also got a pain load of green... I think I worked a way around it with DSS...

load 1 light, the flats and bias into DSS. Stack em... throw away the list. Now load all the lights, the darks and the master flat that the previous stack created. The master flat (if I've understood it correctly) will be debiased (??) the darks will remove the bias from the lights, and it should all be good. I think...

Thats what I do too!

[adamsp123]

Thanks very much for all the replies, I will stack either without the bias frames or try the one light stack method a la John/Martin and see the difference.

[Earl]

Pete... That's what I got... although I also got a pain load of green... I think I worked a way around it with DSS...

load 1 light, the flats and bias into DSS. Stack em... throw away the list. Now load all the lights, the darks and the master flat that the previous stack created. The master flat (if I've understood it correctly) will be debiased (??) the darks will remove the bias from the lights, and it should all be good. I think...

Ill give that a try.

[Tim]

Hang on a minute!

The thrust of this thread is that the method used by DSS is incorrect?

The stacking process is described in detail here, as regards treatment of offset files in relation to flats, darks and lights.

DeepSkyStacker - Free

The advice given for DSS is bung them all in and let DSS do the magic, and I have to say, that works just dandy for me.

But the advice above deviates from that as far as I can tell ?

So come on, let's have the answer

[Tim]

Question Pete:

How many darks, and how many bias did you use?

Nice pic Btw

[adamsp123]

Question Pete:

How many darks, and how many bias did you use?

Nice pic Btw

Bias 2x2 binned = 36

Darks = 18

cheers Pete

[Tim]

I think thats your answer Pete. Dont use more bias than darks, and definitely dont bin them, I would think that would give a completely false read noise output.

You can try, just take some more bias unbinned and see how it looks. I'd be most interested to see

The whole point of DSS is that you can just load up all your files and forget them, more or less, I'd hate to think you had to add an extra step in there. Besides, if it IS doing it wrong, then we can likely ask the author to change it.

[adamsp123]

I think thats your answer Pete. Dont use more bias than darks, and definitely dont bin them, I would think that would give a completely false read noise output.

You can try, just take some more bias unbinned and see how it looks. I'd be most interested to see

The whole point of DSS is that you can just load up all your files and forget them, more or less, I'd hate to think you had to add an extra step in there. Besides, if it IS doing it wrong, then we can likely ask the author to change it.

Thanks Tim I will give that a go.

The point you make about DSS - load and forget is why I like the program, simple to use and rarely get problems. Registar is another nice to use piece of software.

[Tim]

Waiting with bated breath Pete

[adamsp123]

Waiting with bated breath Pete

Ok I tried unbinned bias frames, DSS threw a wobbler as the images did not match the size of the binned subs/darks/flats.

I then tried using less bias than darks, I still got the horrible noisy result as before so.... more playing around when I have time.

[Tim]

Ah right, I didn't spot that your lights and everything else was binned too. oops.

How did you take the bias frames? Can you do a 0 second exposure with the 383?

How do the bias frames look individually Pete? I'd expect them to show some evidence of that banding when you stretch them??

[adamsp123]

Can't see any banding Tim, the Atik can go down to 0.001 sec.

[George]

I always take equal amounts of darks & bias frames namely 31 of each, apparently DSS prefers odd number amounts.....don't ask why but I read that from Luk on the DSS Yahoo group.

[ollypenrice]

I am with Rob on this. Just ditch the bias altogether for the lights. It is already in the dark. I never use bias for the lights.

Following Harry Page I use a master bias as the dark frame for my flats to save doing separate darks-for-flats. Works a treat for me.

Olly

[RobH]

I do exactly the same as Olly, and it works fine.

Rob

[Tim]

But what that means, is that DSS isn't doing it right?