Combining images from separate imaging sessions

[broadsword]

A quick and hopefully not too daft question. Having suffered a flat camera battery half way through an image run last night I'm back out again tonight taking the second half. I've tried to get the camera in as near as possible the same orientation, which I assume is all you can do, but I'm wondering how to eventually combine the two sets. I assume they can't all be stacked together as the flats won't match. So should I stack them separately and then somehow combine the two stacked images?

I know the serious imagers take many hours over a number of sessions, but I haven't seen any mention of how to put them all together.

[Bizibilder]

If you are using DSS to stack then its easy. Load all of your first session (lights, darks flats etc) and then look at the bottom of the screen to find the "main group" tab, next to it will be a "group 1" tab (it only appears when you have loaded the main group), click this and simply load all your second session lights and flats into this. DSS will stack both sets and stack the resulting images into one final image for you automatically.

If you take a few more next year - just do the same and add "group 2" as well!!

[broadsword]

Thanks Roger - very simple answer to a long question! Looking at the tab in DSS as I speak...

[broadsword]

Roger, anyone, help!

I tried the two tab method but without success. The first couple of times DSS simply refused to stack the second lot. When I reloaded and tried a third time it stacked most of the frames,but ended up putting what looked like a blob of dark green ink right in the middle of the image. Haven"t a clue what was going on there.

I've now had to just produce two separate, calibrated, stacked images. So, what is the best way just to combine these? DSS again, or maybe the Stacking function in Pixinsight? Or just add two layers together in GIMP?

Appreciate any help, as it is a nice image of the Whale galaxy and I will share the final result!

[Bizibilder]

What exactly did you try to stsck? In the master stack you shold have:

Lights1, Flats1, Darks (and Bias if you use them).

In group 1 you have:

Lights2, Flats2 only (assuming exposure etc is identical to the first set).

DSS uses the darks (and bias) for both sets of images.

The images must have been taken with the same optical set-up (ie be the same scale or size). If you have used different exposures then you must stack each different exposure as a seperate group - along with the correct darks, flats and bias for each group.

[broadsword]

Hi Roger,

No, I did exactly all of that. The only differences between the two sets were one had more LP from the second night. Also I didn't do a good job on getting the orientation and framing right on the second go, so there was a largish rotational offset.

The two separate stacked images look pretty good, so won't I get a reasonable result just by combining the two? And if so, what is the best way of doing it?

Thanks, as always...

[Bizibilder]

Just put the two images into DSS and stack them. DSS will warn you there are no darks/flats/etc but you can ignore that.

[broadsword]

Hi Roger, guess what, it says it can't stack both images due to too low star detection, yet there's about 500! Guess it is picking up noise as stars, I've seen other posts with this problem. Guess I may need to combine them in post processing. Is there a easy way to do this? Thanks foe any help, again...

[Bizibilder]

When you load your pictures and click on "Register checked pictures" you get a box with two tabs at the top - one of them is "Advanced" and has a slider to set the star detection threshold. Slide it about and click "compute number of detected stars" - you need to set the slider to "find" about 50-70 stars. Each image is different so I can't say what setting to use - you will just have to play until you get it right. Then try stacking - it should work OK.

PS for playing about just stack 5 lights and 5 darks for each group - it takes far less time to "sort things out". when you are happy bung (I love that word) in all your images and go for it!!

[ollypenrice]

If you have Pixinsight and can suss out how to work it (not always easy!) you might be best advised to try it because PI will most likely give you a tighter and probably quieter stacking result. Harry Page did some testing to confirm the power of PI stacking.

In principle, setting DSS aside for a moment and thinking in terms of any programme, you can dark and flat-calibrate all your lights without combining them, so you know each has the right flat. Then you can combine them in a second operation. That way you can be sure the programme is doing what you want it to do.

Olly

[broadsword]

Hi Olly, yes, I plan to try PI next as still no luck with DSS! It seems to find plenty of stars, and checking with the star detection function they look to be true stars, but still it will not stack the two frames...is there a limit where too big an offset throws it out?

I've looked at Harry's PI video for stacking. It seems to cover stacking the lights but not the calibration part. Haven't looked at the intermediate and advanced stuff yet.

As I said above, all I want to do is stack two stacked images!!

[ollypenrice]

I don't use DSS but does it not have some way of controlling the amount of offset it will tolerate? For me the simplest and most logical stacking software is AstroArt 5.0.

Olly

[broadsword]

Roger, Olly,

Finally got a result with Pixinsight, although even then it refused to stack just two images, so I had to con it by loading each twice so it thought it was doing four.

Anyway, thought I would share the result. I had to crop it quite hard due to the bad alignment between the two sessions, but overall glad I presevered as hopefully not too bad from 2 lots of 20 x 2min ISO 1600 frames.

[AstroAndy]

Amongst all this expert advice, here's the little tidbit I have to offer..either restart DSS, or load default settings (make sure you one by one reset it to your needs, then move the star detection slider as you need it (e.g. I use abt. 8%, between 10-15 stars for M51, 50 stars 50% for, e.g.M45)..seems that s/times the settings "get stuck".

[broadsword]

Hi Andy,

That's great advice, I've certainly had the star detection slider seem to be stuck on me.