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Deep Sky Stacker - how to use different flats for different nights?

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I just realised, I don't know what I'm doing. It's a strangely frequent experience...

So, in Deep Sky Stacker, which I've been using for quite a while now, I always put my flats and dark flats into the Main Group, because my understanding is that everything in the Main Group gets used across all groups. Then I bring my lights and darks into Group 1, 2, 3 etc for each temperature, seeing as I'm using a non-cooled DSLR. That way, my lights match my darks in each group, and the light and dark flats are used across them all because they're in the Main Group. I'm pretty sure that's how it should be done, so if I'm already wrong, let me know!

I've just tried however, to bring together subs from two separate sessions, in which I took two different sets of flats. So, now I don't want the flats to be used across all the groups, so I can't put them in the Main Group. However, I do still want the dark flats to be in the Main Group, because they haven't changed. 

So, I thought the way to do this would be to have dark flats in the Main Group, then the lights and darks in each group according to temperature, as before, but also including the flats from that session in each group too. This would mean I'd be duplicating flats in groups, but I didn't think that would be a problem.

As I was merrily going along compiling all of this, I realised that DSS wasn't registering the flats in all the groups, just the first one I imported them into. So, for some reason, DSS will only let me bring in one set of flats, into one group, not use those flats again in other groups.

So it seems to me the only way around this is go back to square one: have all the flats and dark flats in the Main Group, and all the lights and darks in subsequent groups, and stack them for each shoot, just using the one set of flats and dark flats. Then, in a second round of DSS, stack the stacked output from each shoot, but not using any calibration files at all.

Does this make sense? Am I right about how DSS behaves? And would this be the best workaround? Or does none of this make any sense whatsoever?

Edited by BrendanC
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I haven’t used DSS as main stacking software since I moved to APP but I do recall it only lets you use the files once.

Can you try to rename the flats - copy and paste all the flats that you want to use in several groups into separate folders named group 1, group 2 etc.  In group 1 folder highlight all flat files using control + A and right click to rename the first file. Change it to ‘group 1’.  This will then name all the files ‘group 1’ followed by a number in brackets eg ‘group 1(1)’ Repeat for group 2 and so on.

Now try loading these into each group in DSS


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  • 2 weeks later...

Right, so, I've been using this method recently and one thing I'm noticing is that it's a right old pain in the Bottom. It works, but it's hard work.

The only files common to all the sessions are the dark flats - in fact, that's just one file, because it's the master.

Everything else - flats, darks and lights - have to be very carefully arranged and copied according to temperature and, in the case of darks, exposure time.

It takes ages and creates a lot of duplication which is always to be avoided for many, many reasons.

For example, I'm trying to bring together three nights' worth of work on the Rosette Nebula. I had to work around some cloud and moon issues, which means I have 60s, 180s and 240s exposures. As a result, right now, I'm stacking them all in a DSS session that comprises 16 groups, with flats and dark flats copied many times throughout. It's a nightmare, and very wasteful on disc space (I know I could remove the duplicated calibration files and just keep the masters but even so...)

So I'm wondering whether, instead of throwing ALL the files into the mix in one go, it's as effective to do them in batches, so I stack for example all my 60 second exposures, then 180 second, then 240 second - and then, in the next step, stack the resulting three images? Rather than stacking absolutely everything in one go?

Before I spend ages confirming this either way, does anyone happen to know? Shout out to @tooth_dr who seems to like answering my questions...! :)

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You do not have to have any lights in the main group other than a reference frame, that does not have to be ticked. So can use the groups as required for the various calibration files and lights to fit your need.

If the nights were very different and darks are not cross session I would likely use a fresh stack for that and see how that went.


Though if using a DSLR have you tested with your camera if darks help or if you might be able to use bias files and create a dark from the master bias, then it might all go in one stack ok. It's another approach to dark use with a DSLR but your mileage may vary and it is not exactly orthodox.




Edited by happy-kat
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  • 9 months later...

I did sound out the DSS forum and one person tried to replicate the issue and confirmed it. It's a bit weird, you'd have thought just using the same flats in different groups wouldn't be a problem.

The only way I found was to duplicate the master flat for each group. So, you can't have the flats in the main group, you can only have your dark flats and/or bias in there, to work across all groups because temperature isn't important for them. Then for each subsequent group, you need to have the master flat duplicated so that it's being used properly (alongside your darks for that temperature).

It's slightly inconvenient because you're duplicating files, but you only need to duplicate the master flat, not the subs. You can even leave a text file in the folder as a note to remind yourself that's what you've done, and why.

Hope this helps. :) 

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