How to stack DSLR data using Siril software and boost detail in your astrophotography

Easily combine multiple frames to boost detail in your astro photos.

By Iain Todd

Published: Wednesday, 14 August 2024 at 09:35 AM

We’re going to run through how to stack data in Siril. Stacking is referred to as ‘pre-processing’ within Siril and occurs right at the start of the processing workflow. 

If you’re new to stacking, don’t be put off: the process is quite simple, when you know how.

Creating a detailed astrophotography image typically involves these steps:

It’s quite common to use multiple software programs to carry out the above steps – for example, stacking software followed by specialist processing software to do the rest.

One of the initial 250 light frames of the Sadr region in Cygnus, awash with noise and low on detail, before we put Siril to work

Why use Siril?

The free astrophotography processing software Siril is capable of supporting the entire workflow, from stacking data to managing background gradients and colour saturations afterwards.

It comes pre-programmed with various scripts designed to automate many functions and even interfaces with other tools, including StarNet and Pixel Math, to perform additional processing steps.

We started with 250 individual light frames of the Sadr region in Cygnus, each of 60 seconds, taken with a Canon 6D DSLR.

We’d also taken 25 frames each of bias, dark and flat calibration images to stack with these light frames to reduce thermal noise, electronic noise and vignetting in our final picture.

After Siril has stacked and aligned all the light and calibration frames and cut out noise and vignetting, ready for final processing
After Siril has stacked and aligned all the light and calibration frames and cut out noise and vignetting, ready for final processing

Using Siril

We opened Siril and set our working directory (the location where our image data would be stored) to ensure that when we ran the stacking script, Siril knew where to collect the data from.

To do this, we clicked on the home icon (see image below), navigated to the image folder on our computer and clicked ‘Open’.

We then had to ensure that our image folders were stored and named in a way that Siril’s stacking script could recognise each set of light and calibration images.

The image below shows the correct layout of this folder system: the main target images are stored in a ‘lights’ folder, bias frames are in a folder called ‘biases’, dark frames in ‘darks’ and flat frames in ‘flats’.

Once you’ve set your working directory, put all your frames into folders. These must be named ‘lights’, ‘darks’, ‘biases’ and ‘flats’ for the program to work properly
Once you’ve set your working directory, put all your frames into folders. These must be named ‘lights’, ‘darks’, ‘biases’ and ‘flats’ for the program to work properly. Click image to expand.

These folders must be named exactly this way and should be the only folders present in the working directory, or the script won’t be able to execute.

Once done, we ran Siril’s stacking script by clicking Scripts > OSC_Preprocessing (see image below).

You’ll see the script start to run in the Console on the right-hand side of the screen.

Siril first creates Master files for each of the calibration frames before aligning and then stacking with the light frames by subtracting the noise and vignetting.

The time taken will depend on how many light frames and stars there are. For my star-heavy widefield image, it took over 24 minutes.

Once complete, the Console read ‘Script execution finished successfully’.

Run Siril’s stacking script. In our star-heavy example, it took some 24 minutes to create Master files for each calibration frame, and then align and stack them
Run Siril’s stacking script. In our star-heavy example, it took some 24 minutes to create Master files for each calibration frame, and then align and stack them. Click image to expand.

Stacking complete, now for the final adjustments

We then opened the stacked file in our working directory, titled ‘result_[number]s.fit’, where [number] refers to the time in seconds taken to complete the stacking script.

The stacked image will look dark and flat; it now needs stretching.

We did this initially by clicking on the ‘linear’ dropdown box and selecting ‘AutoStretch’ (highlighted, image below).

Siril performed a preliminary histogram stretch, allowing us to gauge how much signal versus noise was present within our stacked image.

Open the stacked file and select ‘AutoStretch’ to perform a preliminary histogram stretch before cropping. After this step, go ahead and process as you wish
Open the stacked file and select ‘AutoStretch’ to perform a preliminary histogram stretch before cropping. After this step, go ahead and process as you wish. Click image to expand.

This stretch helped guide our processing steps going forward, but also highlighted stacking artefacts on the edges of our image caused by camera rotation over the period of multiple imaging sessions.

To crop these out, we right-clicked on the image, selected ‘Crop’, selected the area of the image we wanted to keep and hit Enter to perform the crop.

Our final image (shown at the top of this article) was then ready for further processing using Siril or another photo-editing software of our choice.

3 quick tips

  1. Before running a script, ensure you have backed up your data to avoid file corruption.
  2. Ensure there are only raw files in the working directory, as random JPEGs or other files can cause errors.
  3. Avoid making adjustments to colour or saturation until after cropping the stacked image and removing artefacts.

Are you an astrophotographer? Whether beginner or seasoned, don’t forget to send us your images and they could appear in an upcoming issue of BBC Sky at Night Magazine.