Hands-On Impressions of the Free Software Siril

This article is compiled from my impressions in 2021. Siril has kept updating since, so its features and how it’s rated may already differ from back then — please keep the time frame in mind while reading.

Siril is a free astronomical image processing package. This is my write-up of using it at the time — it’s not the most powerful tool, but as free software it has everything it should, and it’s worth keeping on your shortlist.

Free software, and it has everything it should

Frankly, Siril’s features probably don’t reach even 20% of PixInsight’s, and many of its built-in processing tools have very few options — not exactly great, more in the “serviceable” range. But then again, there’s nothing to complain about with free software, especially when it’s still being actively updated and maintained.

You can see its design philosophy just from the menus: Siril has absorbed the more commonly used astronomical image processing tools from both Photoshop and PixInsight, integrating the most practical batch of them.

Siril’s interface and menus, showing that it integrates commonly used astronomical image processing tools

Look at it with an “amphibious vehicle” mindset

I often use an analogy: Siril is a bit like an amphibious vehicle — on the road it usually can’t outrun a car, and in the water it can’t outrun a boat. Going in with that understanding as your premise is the right mindset for using this free software. It does a bit of everything, but nothing top-tier, and you can’t pit it head-to-head against a dedicated PixInsight or Photoshop.

When Siril is a good fit

So when should you consider it? My advice is blunt: if you haven’t bought PixInsight yet and you’re running into problems stacking with DeepSkyStacker (DSS), then Siril is a great free alternative.

In actual use, also watch the details of the output. For example, with some source material, the integration result will show a small ring of dark area where the center of the frame meets the surroundings; that shot with the FSQ 106 showed this more obviously. Pay extra attention to these edge-and-corner issues when evaluating a stacking result.

A Siril integration result, where a small ring of darker area is visible where the center meets the surroundings

Summary

Siril won’t replace PixInsight, but it’s free, kept updated, and integrates commonly used tools — a solid choice for anyone who hasn’t yet invested in paid software or has hit a wall stacking with DSS. The key to using it is to first set your expectations straight around that “amphibious vehicle.”