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Stack together or separately is the question

astrophotography beginner
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#1 MSJK1171

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Posted 17 May 2019 - 10:52 PM

Hi everyone, I’m still new to Astrophotography and was wondering. If you take sub frames of the same deep sky object in different iso’s should you stack all of the different exposures together as one stack or should you stack each iso sets separately and then blend them together in post processing? I’m still fuzzy about this one for deep sky stacker. Thanks in advance for any help.


Edited by MSJK1171, 17 May 2019 - 10:53 PM.


#2 james7ca

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Posted 17 May 2019 - 11:31 PM

It would depend somewhat on the difference in the exposures. If you're talking about a 2X or greater difference then I'd stack them separately. However, if they are pretty close in exposure (say 1 minute versus 50 seconds) then you may want to stack them all together. You might also want to stack each separately and then combine them with a scale value to match each total integration time. So, if you had one hour total with one sequence and only 30 minutes on the other you could combine then in a ratio of two to one (two parts for the one hour sequence and one part for the 30 minute sequence).

 

Blends like the above are easy to do with PixInsight using that software's PixelMath. Also, many (most?) integration or stacking routines scale each sub based upon some form of weighting, which can be something like signal to noise or even exposure time. Thus, knowing how the integration combines the subs will help in determining whether you can stack the sequences all together or separately.

 

If you are going for some kind of high dynamic range result (HDR) then it's probably better to stack the sequences separately and then blend the results (to maintain detail in both the highlights and shadows).


Edited by james7ca, 17 May 2019 - 11:34 PM.

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#3 MSJK1171

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Posted 18 May 2019 - 11:23 AM

It would depend somewhat on the difference in the exposures. If you're talking about a 2X or greater difference then I'd stack them separately. However, if they are pretty close in exposure (say 1 minute versus 50 seconds) then you may want to stack them all together. You might also want to stack each separately and then combine them with a scale value to match each total integration time. So, if you had one hour total with one sequence and only 30 minutes on the other you could combine then in a ratio of two to one (two parts for the one hour sequence and one part for the 30 minute sequence).

 

Blends like the above are easy to do with PixInsight using that software's PixelMath. Also, many (most?) integration or stacking routines scale each sub based upon some form of weighting, which can be something like signal to noise or even exposure time. Thus, knowing how the integration combines the subs will help in determining whether you can stack the sequences all together or separately.

 

If you are going for some kind of high dynamic range result (HDR) then it's probably better to stack the sequences separately and then blend the results (to maintain detail in both the highlights and shadows).

Thanks for the reply james7ca. yeah that's exactly what I'm talking about. All the exposures are the same length of time but at different iso's. So if there all the same length of time then it's ok to stack them together. I'm using Deep Sky Stacker V4.1.1 for my stacking process. But I like the idea of the HDR might think about that.



#4 Stelios

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Posted 18 May 2019 - 11:37 AM

You won't like my suggestion--but what I would do is stack separately, process the two stacks separately, see which is better--and then add more integration time for that object with *that* ISO and re-stack. And in the future stick to a single ISO  (400--but not with Canons--800 and/or 1600, *no* higher) and exposure length per target.

 

I have a strong feeling that what you want to do will result in an image that will lose the best features of both.



#5 MSJK1171

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Posted 18 May 2019 - 11:51 AM

You won't like my suggestion--but what I would do is stack separately, process the two stacks separately, see which is better--and then add more integration time for that object with *that* ISO and re-stack. And in the future stick to a single ISO  (400--but not with Canons--800 and/or 1600, *no* higher) and exposure length per target.

 

I have a strong feeling that what you want to do will result in an image that will lose the best features of both.

 

 

Most of the time I do stick to one iso, but was trying to figure out if I do different iso's is it ok to do so. And if their is any benefit to doing that like more dynamic range or what not, thanks for your suggestion. I'm using a Nikon D7000. Usually use 800, 1000, 1600, or 2000 iso depending on the surface brightness.



#6 james7ca

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Posted 18 May 2019 - 06:06 PM

Differs ISOs, I overlooked that aspect. Probably similar to using different exposures since your signal to noise ratios will be different. However, you are going to have to use different calibration files (DF and bias, if you use the latter). I’d probably still go with my original suggestions, but you may want to experiment with various techniques and as Stelio’s suggested you may just want to collect more subs (which is always better).

 

If the stacking software scales the subs on signal to noise then stacking all together may work fairly well (if each set is calibrated separately).


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#7 the Elf

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Posted 19 May 2019 - 05:45 AM

https://www.youtube....h?v=3RH93UvP358

Watch this. It is not exactly about ISO but when you understand the theory behind it you know that there is no point in using different ISOs.  As Stelios says: stay with one ISO. Being a PI user I can't tell for sure but DSS should scale the individual subs by multiplication and offset as PI does. Stacking them toghether will not ruin the image but taking them all with the same settings is better. Ask Nikon users what ISO is best for your camera.


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#8 MSJK1171

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Posted 19 May 2019 - 02:14 PM

Differs ISOs, I overlooked that aspect. Probably similar to using different exposures since your signal to noise ratios will be different. However, you are going to have to use different calibration files (DF and bias, if you use the latter). I’d probably still go with my original suggestions, but you may want to experiment with various techniques and as Stelio’s suggested you may just want to collect more subs (which is always better).

 

If the stacking software scales the subs on signal to noise then stacking all together may work fairly well (if each set is calibrated separately).

Yeah I did one of my photos like that were I used to different iso's with calibration frames for them acttualy it's my M33 picture and I agree too more subs is always better. I usually stick with one iso, just trying to see if their is any benefit to stacking different iso's, but sounds like it's not a good idea. Thanks for your reply.


Edited by MSJK1171, 19 May 2019 - 02:18 PM.


#9 MSJK1171

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Posted 19 May 2019 - 02:30 PM

https://www.youtube....h?v=3RH93UvP358

Watch this. It is not exactly about ISO but when you understand the theory behind it you know that there is no point in using different ISOs.  As Stelios says: stay with one ISO. Being a PI user I can't tell for sure but DSS should scale the individual subs by multiplication and offset as PI does. Stacking them toghether will not ruin the image but taking them all with the same settings is better. Ask Nikon users what ISO is best for your camera.

 

I will watch this probably tonight got something to do right now. I'm interested in using PI, but how hard is it to learn. I just started with DSS because it's free and I follow Astrobackyard on YouTube and that's what he uses so naturally that's what I started out with. I just started doing astrophotography last year and there's another thread on cloudy nights that tells me the best ISO for my camera(it's 400iso) which is kinda low for my area. I live in a bortle 5 area. Basically it sounds like it's better to stick with one ISO rather then different ISO's together. That's what I wanted to find out and thank you for your reply good info.



#10 MSJK1171

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Posted 19 May 2019 - 02:35 PM

Just want to thank everyone for their reply's it's been most helpful and glad to see everybody reaches out to everyone here on cloudy nights love being a member of this community. Good info from everyone.


Edited by MSJK1171, 19 May 2019 - 02:37 PM.



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