Best way to combine stacked images (no PI)? - Forums

If you have two or more complete, stacked and calibrated images, what is the best way to combine them that would give the same result in terms of S/N as just stacking all the images together? I know you can simply add the images in pixel math but I suspect that’s not the best way.

Average them in pixelmath, not add them.

Why can’t you just register and integrate them both the same tool/method you used to generate each one?

Craig Towell · Jan 21, 2026, 07:20 PM
Why can’t you just register and integrate them both the same tool/method you used to generate each one?

I certainly cloud, I’m just looking for a quicker way to do it.

Hi Tony,

I use FastIntegration. This method uses weights, so in theory you are not degrading your best stack by simply averaging the different substacks. If stacks come from the same scope, I use weights based on PSF; if stacks come from scopes with different FL, I use weights based on Resolution.

I know you said no PI but this is a very quick method!

I did this yesterday. Added two sets of calibrated lights (from different months) to the “Lights” tab in WBPP.
No calibration files. Default settings.
WBPP ran image registration, weighing, and integration and produced a masterlight of all of the calibrated lights.

Álvaro Méndez · Jan 21, 2026, 07:49 PM
Hi Tony,
I use FastIntegration. This method uses weights, so in theory you are not degrading your best stack by simply averaging the different substacks. If stacks come from the same scope, I use weights based on PSF; if stacks come from scopes with different FL, I use weights based on Resolution.
I know you said no PI but this is a very quick method!

It could be quick as boiled asparagus but useless to me….:-)

You should add them scaled by the relative weight of each one's SNR.

andrea tasselli · Jan 21, 2026, 09:12 PM
You should add them scaled by the relative weight of each one's SNR.

So in Siril, the quickest way to do that just be to import-register and stack.

Tony Gondola:
So in Siril, the quickest way to do that just be to import-register and stack.

I guess... Although I jave no idea how to evaluate the "signal" part of the SNR in Siril. I also suppose normalization of one with respect to the other helps.

Yup, normalization is part of the stacking process.

I have used APP to do this in a very straightforward way. Th default combining two stacks seems to be weighting them based upon the total exposure in each stack. Please correct me if I am wrong!

Yes, that’s the right way to do it.

Averaging in Pixel math, or any other process that simply averages the images, is what you want done provided they have been previously aligned.

No, I was just looking for a quick way to equalize and combine the results of multiple stacked images. In Siril at least, the quickest way is just to stack them without calibration, just import, align and stack. That’s working well for me.

I assume the images are captured with same magnification and are aligned.

I suggest to assess ( whatever works method,eg evaluate the sigma of background in a “desert” area) ) the quality of each image and weights the sum in pixelmath accordingly.

But the only optimized approach would be to generate a complete stack from scratch.

alpheratz06 · Jan 25, 2026, 04:26 PM
I assume the images are captured with same magnification and are aligned.
I suggest to assess ( whatever works method,eg evaluate the sigma of background in a “desert” area) ) the quality of each image and weights the sum in pixelmath accordingly.
But the only optimized approach would be to generate a complete stack from scratch.

Same image scale yes. The images are also very close in density. Normalization automatically happens as the last step before the actual stacking in Siril. That seems to do the job nicely.