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Single vs. Multiple Exposures: Is Stacking Worth it?

Stacking_vs_single_feature

Is it worth it to stack and process multiples vs. a single exposure?

I think so.

It’s more work, it takes more effort, but in the end the image quality you get from stacking multiple exposures can drastically improve your final product in multiple ways.

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Reduce noise with reality

Stacking multiple exposures reduces noise by increasing the signal:noise ratio using reality. I like that.

But, what is signal and what is noise? It’s pretty simple — signal is the stuff (light) we want, noise is the stuff (camera sensor errata) we don’t want. One of the best benefits about stacking multiple exposures is the dramatic increase in the image quality, noise removal, by increasing your signal:noise ratio.

When you stack, you reduce the differences in the digital representation of the light that hits and excites the camera sensor. Each time you shoot an image, the electrical characteristics of the sensor cause it to do its best at representing the photons it “sees.” However, from shot to shot, there are slight brightness and color variations on each pixel for the exact same image. Image stacking produces an intelligent average of each pixel of all exposures, detail for detail, instead of trusting just one exposure and hoping it’s accurate. Sounds like a good idea to me.

single_zoom

This is a 200% zoom of a single 180-second Milky Way image, note how many stars are lost compared to the stack (right).

stack_zoom

This is a 200% zoom of five stacked 180-second Milky Way images (which include the single image on the left and four others.)

Along with decreasing noise — you are also able to saturate your images to get more accurate color because you are boosting  signal instead of boosting the noise.

I’ll let you compare for yourself. Below you can see an example of a single 180-second (tracked) Milky Way image, followed by a stack of 5 different exposures (180-seconds each). Both were post-processed a little differently from the same raw images, but after stacking, there was lot more data to work with.

Milky Way 180 seconds single
A single 180-second exposure of the Milky Way. Photo: Cory Schmitz
Milky Way Image Stack 5x180s
Image stack of five 180-second Milky Way exposures. Photo: Cory Schmitz

How you do it

There are a few good software packages out there. Some are free, some are not.

I’d highly recommend PixInsight, it’s what I’d consider the gold standard — but isn’t free and it has a big learning curve. It’s worth it, though!

So, if you’re just starting out — give Deep Sky Stacker a try. It’s free and easy to use, and works pretty darn well, too. Once you have a TIFF image from your stacking in DSS, you can edit it with your favorite image processing application and tweak to your heart’s content. And the best part is, you know that what you’re working with is more accurate data than a single exposure.

Try it out!

Have I piqued your curiosity? I’ve got some homework for you, then!

Go download some of our data to play with and stack (or not stack) yourself, and share the results with me!

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About the author

Cory Schmitz

Co-founder of PhotographingSpace.com, co-owner of several telescopes and mounts, too many cameras, and not enough hard drives, Cory is an American expat living in South Africa with his wife, Tanja Schmitz.

An avid astrophotographer for timelapse, deep-space imaging, lunar, planetary, and star trail imagery, he is an all-around jack-of-most-trades for night-sky photography.

He is also an internationally published and commissioned astrophotographer, where his photos have been used in multiple online and print publications.

19 Comments

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    • Technically you can stack in Photoshop, but it is a very repetitive, labour-intensive process, and the more images you’re stacking, the longer it’s going to take. Better to use one of the tools Cory mentioned to do all the heavy lifting for you. Beyond learning how stacking actually works, there isn’t really any value in trying to do it by hand because the stacking process follows mathematical algorithms. It’s not something you can tweak or adjust, or try to improve upon.
      On my old slow laptop, stacking 20 images might take a minute (using the standard simple no-frills stacking options), but doing it by hand might take half an hour, if I don’t make any mistakes. It’s pretty boring, even as a learning experience!

    • Hi Elizabeth,

      Yes — PhotoShop can be used to stack, in theory…but it’s not really worth your time.

      On Mac, you can use a software package called Nebulosity (of which I believe there is a free trial). But other than that and PixInsight, there is a lack of Mac-native software.

      I run a Mac as well!

      Cheers,
      Cory

    • this is obvious – but – what I did was to get parallels, buy windows. This is about $300 total, I think, then get Deep sky Stacker (free). You are in business. Yeah, it costs a bit, but you will use the virtual windows machine for other things. And Parallels is fast, and pretty bomb proof. Also kind of fun to prank your Windows friends, by starting a windows computer on one screen, moving to your mac screen and while they are watching, just swipe the screen, and voila, it is a windows machine.

  • Hi, great article.
    I’m making single-shots, just starting.
    If you take multiple shots, I guess you need a following system, otherwise the stars will have moved, is that correct?

    • They are probably going to cover this in another post, but if you’re shooting wide-field images, you can actually get away with a static mount. Stacking tools have a feature called “Registration”, which analyses each shot and shifts it a bit so that the stars within them all line up exactly – you always do this before stacking because even the best tracking mounts have *some* mechanical give in them, so you can’t trust that the alignment will be perfect.
      So what I’ve done in the past is to not try to track at all, and then abuse the registration tool to bring it all back in line.

    • If you decide to buy a star tracking mount I am getting good results with the Ioptron Star Tracker….its $299…. I can get 2 minute exposures at 75mm with zero drift…I havent tried longer exposures yet

    • Hi Robert,

      Allen is right, for wide-field work you do NOT need to track if you shoot short-enough exposures (see http://photographingspace.com/perfect-stars-rule-of-500/). And if you remove the landscape from the equation, you can stack these short exposures to obtain a better result.

      Stay tuned to our website and social media in the coming days, because I actually shot a couple of sets of static-tripod Milky Way images to share with everyone to play around with this method on my data. I’ll share links to download them soon!

      Cheers,
      Cory

  • Yeah 100%, tried this method and its shocking the amount of detail you can get. I only had an APS-C sensor which isnt great with noise but using the stack method allowed me to get better shots than those who had full frame. Now that i have full frame i STILL will use stacking!

    Great article Cory!

  • Brilliant article!! NICE!!!!
    I use another free method:
    Copy the file over multiple times (+-10 Copies of the same file)
    Open the 10 files in StarStax and run it (same way as which you generate star trails).
    I figure it doesn’t get the same results as PixInsight, but hey beggars cannot be choosers
    What is your opinion around it?

    • Thank Johan,

      Good idea — however, in practice copying the same file wouldn’t decrease noise, because the pixels in each file are all the same, to there is nothing to “average” out. Also, StarStax doesn’t do an average of the pixels, it just chooses the lightest pixels from each exposure for their blending. Since they all images input are the same, the output will be the same as the input.

      Instead, for free — I’d highly recommend giving DSS a try! (unless you’re on a Mac…)

      Cheers,
      Cory

  • How do you stack astrophotography pictures tho? If by using a 30 second shutter you can already tell some star tracks. By taking one 20 second picture, and another 20 second picture the second one would be slightly shifted, the stars would’nt match the same spot on the image. Is there a trick that you didn’t mention, or does DSS do this fix for you?

    • Hi Helder

      DSS and most other stacking packages will fix this for you, through a process they call “Registration”. They look at each individual shot and find the brightest stars, then shift the entire image in whatever direction is needed to make them line up exactly.

    • Hello Helder,

      Allen is correct. The specialized astrophotography software, like DSS, automatically analyzes and aligns each image for you before it averages out the pixels. Most often called “star alignment” or “registration.”

      Cheers,
      Cory

  • Great articles! Thanks for all the info – so far I think I’ve bookmarked about 4 or 5 pages from here to read up on. Just starting out on night and start photography and have learnt a WEALTH of info in a few short reads already – especailly ‘The 500’ rule – thanks again…really appreciate the info!

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