Stacking images from different sessions

[mickw]

When you have the same subject taken on different sessions that you want to combine would you -

Process each session's results to the max (best possible image) before stacking.

No processing, just stack the raw stacks from each session.

[JohnP]

Mick - I would stack them all from start again or if flats etc were totally different between sets I would stack each of the two calibrated sets individually and then combine the two images. I would then process the result. In either case I would not do any processing untill images had been stacked.

Hope that makes sense.

John

[mickw]

Yep, thanks John

[MarkS]

Mick,

I'm struggling with this very problem with my Markarian Chain data taken over 3 nights - mainly because I always like to do a sigma stack.  The problem I have is that the background gradients were different enough on each night to confuse the sigma stacking algorithm enough to cause background artefacts and also to not fully remove satellite trails, cosmic ray hits and warm pixels.  So I intend to sigma stack each night's data separately and then combine those 3 substacks into a single stack.  This has to be done in a weighted manner e.g. if I shot 15, 25, 20 frames on succerssive nights then the substacks have to be weighted in the proportion 15:25:20 when combining them into the final stack.  After this I'll continue with the remainder of the processing sequence as normal.

Another reason for doing it this way is that you may also need to apply a different dark for each night depending on ambient temperature.

Mark

[mickw]

I think I understand the need for weighting - SNR is higher in the larger stack/session and worse in the shorter stack and not weighting would degrade the quality of the good stack ?

I haven't noticed a weighting option in DSS, have I missed it or is it done some other way ?

[Carole]

Hi Mick,

Don't know about weighting issues, but there is the option to stack several sessions in DSS.  At the bottom there is a tab that says Main group, then once you start loading subs another tab will appear Group 1 and then once you start loading subs into group 1 a further tab will appear Group 2, and so on......
It allows you to load separate darks and flats with each set of subs.

Carole

[mickw]

Yes I'd seen that, it might be in there somewhere - I think a nose around is in order 

[The Thing]

There's a feature in DSS just for doing this - Groups.