Astrophotography stacking for the best images

I’ve been stacking astrophotography images for a while now and one thing I’m finding is that my images require many hours of imaging. Recently I took photos of the Iris Nebula and so far I’ve taken about 5 hours’ worth of 30 seconds images. Ideally, for an amazing picture of this stunning nebula, I would need about 10 hours or more.

When I photographed a nebula recently I used my alt-az mount and Celestron 130slt telescope. I took 5 hours of 30-second shots is a total of 600 separate photos! The last time I stacked about 700 or so images that I had taken it took well over an hour to stack them in Sequator (the software I usually use for my astrophotography stacking). I’m wondering if there might be a quicker way to do this by using fewer resources on the computer to achieve the same quality in the final image.

So I’ve decided to run a couple of experiments, the first of which will be testing different astrophotography stacking processes to achieve the final image. By comparing the final result I may be able to figure out if there is a quicker way.

The first experiment: Group Stacking

For my target, the Needle Galaxy, I’m going to choose 40 images and I’m going to divide them into 2 groups of 20 each. I’m going to stack these 2 groups separately so that I have two stacked images.

Then, I’m going to combine the two stacked images by again stacking them in Sequator. Learn more about Sequator and other software I use for image stacking.

For the second image, I am going to do the standard astrophotography stacking method where I will stack ALL 40 images in one go.

Finally, we’ll compare the differences between these two processes.

After we have a look at the results it may be necessary to run further experiments to answer this question:

Can I divide a large number of images into separate stacks and then combine them together to produce the same or better quality then stack all the images together in one go which may take a long time?

This should show whether it is possible to separately stack smaller groups of images and combine them later rather than all images in one go. The reason I think this could save time is that if I image a deep sky object over several nights I could stack it after each session and then combine them later rather than waiting until I have finished all my photography on one target and then stack the images altogether. It doesn’t really save time but seems more convenient and would allow me to check my progress as I went along. But, what will the result be?

Comparing two groups stacked separately then combined vs one stack of all images

divided into two groups
Direct comparison between two images of the Needle Galaxy, (left: stacked in two groups of 20 then combined, right: one stack of 40 images)

Side by side overall view

40 image stack in one go
40 images stacked in one go all together
divided into two groups
40 images were divided into two groups of 20 stacked and then the stacked images combined

As can be seen the image on the left is much noisier! The images have not been post-processed in photoshop as this is a direct comparison.

It would appear from this test that we shouldn’t combine images into groups and then combine them. This idea occurred to me because sometimes I might take 1000 images and one stack will take a long time to complete but if I broke it down to stack each night’s images separately I could then stack them together at the end, saving time. It appears from the first test, that this is not going to be such a good idea!


Test Two: stacking with many more images

Let’s this time experiment by using the same process as above and stacking 700 images of a different target, (the Iris Nebula), in two different ways. Process A will stack two groups, one of 482 images, the second of 218 images, and then combine the two stacked images together. Process B will be a stack of all 700 images done in one go.

Will we verify the result we just got above?

A: Combination of two stacks (482 and 218 images)
B: Stacked all 700 images in one go

I think the difference in image quality can be seen. The second image seems less noisy although not such a big difference this time. Let’s look at a close up of each image and see how the details compare when inspecting more carefully:

A. Separately stacked and combined image
B. One stack of all images

What do you think this result shows? Does it confirm the first experiment? Should we do partial stacks and then combine them to save time or should we do a complete stack of all the images to get the best result?

I think the first experiment and the second both show that the latter is the best course of action. We can’t save time if we want the best image. We need to stack ALL images each time we compose a stacked image of a target to do it justice.

I think the noise improvement is still obvious and the detail is improved in the nebula stacked in B (all images stacked together). The color contrast is also better and the stars also have better color. But it is close!

I think this settles the argument for me and I won’t be doing any partial stacks again. This agrees exactly with what was claimed in several posts I read online and especially on the Cloudy Nights Forum.

It always is best to test these things for yourself though or you are at the mercy of what others say. I hope my tests have helped you figure out what to do and perhaps you might even run a quick test of your own to verify my results with your equipment.

Check out some of my other astrophotography images.

I should mention that these images were taken with the following equipment:

  • Celestron 130slt reflector telescope with AZ-Alt mount
  • DSLR Canon 600D astro-modified
  • No filter was used
  • Bortle 5 sky
  • Generally good seeing conditions with some high clouds at times
  • Exposures were 30 seconds at ISO 800

One particular feature of the Iris Nebula is that it has a dark nebula and dust which needs time to become clear in the images. The single larger stack does seem to bring out the darker dust and nebula more and I have no doubt that with more time on this target it will improve still further.

I plan to do at least another five hours on this target and will post the final image when it is done. Stay tuned for that!

If you have any questions for me or want to tell me what you think about this test. Or…if you have any interesting tests you have done similar to this please drop a comment below and let’s get a discussion going here.

You can check out this image of Markarian’s Chain, a number of different galaxies in one shot, that uses the power of stacking to create an image that is not visible in the eyepiece of a small telescope.

Until next time, thanks for taking the time to drop by and learn about astrophotography with me Karl Perera! See you again soon.

Enjoy the journey and the stars above!