Combining different length subs

[cventer]

Newb question I know.

I just realised tonight that when i Imaged an object recently over multiple nights I took different Luminance exposure lengths by accident.

About 6 subs are 5 min and 6 x 8 min.

Can these be comined ? is there any benefit in doing so ? Or should I just use the 6 x 8 minute frames ?

[DavidTrap (David)]

You can merge them as a HDR, but they need to be stacked in groups according to length.

DT

[Gem (Grant)]

OK.. newbie questions...

HDR = ??

What do you mean stacked in groups? Why can't you just stack all of them together?

[DavidTrap (David)]

HDR = high dynamic range.

Stacking in groups means stack the 5min subs together and the 8 mins together, the combine (different software routine) them to make an HDR image.

Greatly simplified, if you stack as a median combine, the software picks the "middle value" for each pixel. The idea of using a median combine rather than average is to remove outlying values that will usually be noise. If you mix subs of differing length, calculating the middle value doesn't produce the result you're after.

Remember, stacking images is not adding them together to make a brighter image. It is a statistical process to improve signal to noise of your image. It makes the image background darker because more noise is obvious in the darker regions of the image.

DT

[wasyoungonce (Brendan)]

And.... you can layer mask blend parts of images. For example layer mask in blend shorter subs of M42 core with longer subs so you don't have oversaturated core.

Video tutes on this from Louie, here and Doug here.

Brendan

[mill (Martin)]

It all depends on what object they are from.
If it is not a bright object like M42 etc then just stack them all together without blending, easy peasy.
If it is M42 then stack them separately and mask blend them.

Martin.

[ballaratdragons (Ken)]

If you take images on different nights of different exposure lengths and different image sizes and different rotations and from different cameras and different scopes and different everything else you can stack them all together in a great program called Registar

Registar is a powerful tool for stacking images of the same object you have taken over many years from many sorts of cameras and scopes.

I tried it with about 10 of my M42 images taken over the years. They were all different scales, orientations and FOV's, and it stacked them all into one really nice image

[RobF (Rob)]

Yes, it really does depend on object and software. Most powerful imaging software can correct for different exposure conditions if you give it enough info (e.g. bias, darks, etc to work with to adjust for changing conditions).

Best to at least try it and see what happens. If no good, then have to deal with them separately or use different software.

[RickS (Rick)]

Originally Posted by RobF

Yes, it really does depend on object and software. Most powerful imaging software can correct for different exposure conditions if you give it enough info (e.g. bias, darks, etc to work with to adjust for changing conditions).

I processed a bunch of 15 and 30 minute Ha subs together in CCDStack and found that the normalization process gave approximately double the weighting to the longer subs and gave me the result I was hoping for. A reprocess in PixInsight gave me good results too. It's worth a try!

Cheers,
Rick.

[cventer]

Thanks Everyone

I tired it in CCD stack and as you said Normalising the data gave the shorter subs less weighting and a pretty good end result.