Stacking astro images with Siril

As described in a previous section Siril is the stacking software I currently use as its Linux based. If you want to use Siril in windows you will need to run it in Virtual Box or better yet use a Live USB operating system.  Its handy piece of software which makes running cross platform software easier than dual booting a PC if you dont have the resources.

A good alternative for Windows users could well be Sequator I trialled this software using the unedited RAFs from this tutorial  and the results were very good. It doesnt replace some of the more advanced features of packages such as Siril, but for straightforward stacking this could be just what you need. For me though there wont be any need to change from Siril.

Start Siril and load your images. Note for best results your images should be in 16 bit Tiff format for best detail. Siril only works with Fits, so it is necessary to load and convert the images. Single images may be loaded or select in groups.

Ensure you change the directory to reflect where the images are to be found and stored. You should now have a window that looks like this after starting Sirilsiril-loading-screen

Select the + icon and navigate to your folder ( if you haven’t changed the directory below ) where your edited files are located. Add the files until all are loaded. Your windows should show your loaded files. And the File Conversion Tab should be highlighted.

siril loading screen2.png

As we have already batch edited our files we will not need to use the built in image processing section. If your are doing images specifically in colour channels R G B & Luminance you will have the option of processing each channel of an image to best suit the final output. There are Tutorials supplied on Sirils’s website and I will include the links at the end of this section. For now we will use the basic stacking tools.

Once all the images you require are included in the list, you need to move to the Destination section of the window and enter a sequence name for the conversion processsiril loading screen3.png

In this instance  I have named it Southern Cross. Leave everything else as seen in the image above. Click the Convert  button and Siril will do the rest.Conversion log.pngOnce done you will get 3 windows, the conversion log which tells you if all your images were converted and the image processing and RGB output screens. We will take a quick look at the Image processing screen.Minimise the log  window.


Click the List button ( Highlighted ) in the left hand window to bring up a list of images which can be manipulated to bring up detail. This can be done on an image by image basis, and in three colour channels. It is instructive to spend a little time trying these controls as there may be times where boosting colour channels is necessary for further colour enhancement.

For now though we will leave this and continue with our stacking.

Image Sequences…

Now that we have converted and named our image sequence we select the Image Sequence Tab. If this is your first sequence you will see it in the drop-down menu under Sequence Selection as shown below.

sequence screen.png

If the sequence isn’t visible then select it from the drop down menu list.Leave all the other controls as they are and move down to Sequence export and give the export sequence a name. Select include all and select Yes.

sequence screen2.png

Once you have created the Export Sequence , its time to move to the registration section.

Note the Export sequence step can be omitted if stacking is to be done in Siril, or you can use the export sequence in another stacking program or simply as a back up to your main sequence if there is file corruption.

Pre-Processing & Dark Files …. For now this section can be skipped.

Register the images…

With your current sequence loaded as seen in the Image Sequence window its time to register all the images for stacking. Select the Registration Tab to view the window seen below.registration tab.png

Ensure that register all images is selected and Global Star Alignment is also selected from the drop down menu. Registration is automatically set to green colour channel and for our purposes wont be changed. Note that the registered sequence is prefixed by r_

This makes it easy to re-stack an image if you have difficulty with post processing.

Once the settings have been checked simply click the Go Register button and Siril will start the registration process and output a log file as it does this so you can follow the process. When complete it will return you to the control window.


Once the registering process is complete, select the Stacking tab. You should see the active window as shown here

stacking sequence.png

If the controls aren’t automatically loaded as above you will need to change them to reflect those seen in the above image, from the drop down menus. Once the settings are as above click the Start Stacking button. Once stacking is complete the window will change to output logs and you will see a report of the final process.


Note that there is a full image file exported as a fit file that can be edited as a single image in Siril and saved. I dont ordinarily use this process as its convenient to save the image as a Jpeg or Tiff depending upon final usage.

Save the Image...

Select File from the main menu and select Save As, this will bring up a selection menu from which five file types are available. Choose what you need and select  any settings required and save your image.You should see a save box as below as you do this.


And thats it. Your file is stacked and saved.

Some final thoughts… 

You will note that I have chosen to pre process the images before stacking them. This is simply a personal preference as it gives me the option to ensure that all the frames are edited identically. The normalise option within Siril could be used to do a similar job and you could then simply post process the final saved image. I prefer the former method as it gives greater control of how the final image will look when stacked, and if done correctly there should be no necessity to do final post processing.

The use of Dark, Flat or bias files is entirely over to you. My personal feeling is that for images taken with the XA2 dark files aren’t all that necessary, whereas images shot at prime focus using a telescope could well benefit by their inclusion.

The Siril Tutorial can be found here and should help you understand at least some of the processed involved.

Siril Tutorial Pages

The final stacked image after the above stacking method.

The Southern Cross sets to its lowest point in the sky

7 thoughts on “Stacking astro images with Siril”

  1. Excelente tutorial. It helped me a lot!!!
    Now I can align LRGB FITS, adjust intensities, stack layers, etc. The RGB image preview looks ok, but when I right click on the window and export to Tiff, PNG, etc, the final image is almost completely black. I am usinf FITS files from Slooh and iTelescope.
    Do you have a hint? What step am I skiping? (using Mac High Sierra 10.13.5 and Siril 0.9.9)


    1. I had to same problem and it seems that Siril doesn’t like fit files that it hasn’t generated itself. I also found that Deepsky Stacker did the same thing.

      When you fire up Siril and start with the conversion section it tells you what files types it supports and I think you will find that while it sees your files when outputting the final file the problem is a file type mismatch.

      You may need to change the file type you are using as your source. Thats about all I can think of that would be causing this.
      If you can snap off some images with a dslr or a camera that will do RAW files, convert them and see if you get a good result, this will tell you it probably a file type error.

      You could also look through the website and contact the people who supply Siril to see what may be going wrong.
      Let me know how you get on and if you find an answer.


  2. Save As does not seem to work on siril v0.9.12 for OSX. No matter what format I select, it appends a .fit extension and the file does not seem to be the format I selected. The whole process seems to be exactly what I need, except it’s useless without a final image.


  3. As I’m about to go to work I dont have time to check it out fully. I will do so over the weekend for you and report back.
    Thanks for the heads up on that. I dont use a Mac so haven’t tried Siril on one but it should be the same process as I use.


  4. I have the same problem on my Mac. I imported a series of ten images and created a .fit file. Using the default settings, I think I stacked them, for I can see an image of much better contrast that any individual image. I exported that to .png format and it looks entirely black. I opened it in Pixelmator and, completely blowing out the gamma, was able to see the image.

    Is there a simple getting-started tutorial somewhere that just does a basic stacking of some images and produces a decent result? No cropping, no color adjustments, no denoising, no zero-level adjustment … just the basic process?


    1. Due to a major workload I haven’t had time to investigate the problems further regarding saving images in Mac OS. However if you are able to save or export an image, please email it to me so I can take a look. I was contemplating doing a tutorial on the processing side of things for Siril users in a basic form.
      Its sounds like you did successfully save your stacked image it just needs further process. If you are willing I can do a step by step processing guide using images you or other users send me. Preferably in tiff format but png and dng files should be okay as well.


  5. From a “Johnny-come-lately” here: Just getting started with Astrophotography. Signed up for “observations” from, which results in downloadable FITS files. But, all the online instructable type videos focus on denoise, and stretching histograms, etc. I can’t get the 24 LRGB FITS files registered, stacked and ready for all the finessing! I need a simple step by step. I’ll be trying the process outlined above, with the Feb. 2021 update to Siril.
    ( Also tried Sequator, because reviews suggested it was a simpler way to stack. But Sequator always chokes on saving the output, and I end up with a truncated file displaying only the top 30% or so of the original.) seems to have a deal with PixInsight to not promote / even discuss any other software!


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