How to Process Astrophotos Without Learning PixInsight

“Learn PixInsight” is the standard answer to “how do I process my astrophotos”, and it is not bad advice. It is advice with a price tag attached that rarely gets stated: several months, before you have a photograph.

There is a version of this hobby where that price is obviously worth paying. There is also a much larger group of people who want a good image of the Orion Nebula from the frames sitting on their drive, and for whom three months of learning a signal processing environment is not a reasonable ask. This is written for the second group.

What processing actually is

Strip the software away and the job is eight operations, in this order:

1. Calibrate - subtract what the camera added. Darks remove sensor noise patterns, flats remove dust shadows and vignetting, bias removes the read offset.
2. Grade - find the frames that are ruined. Cloud, satellites, lost focus, a tracking slip, someone’s headlights.
3. Register - align every frame to the same stars, to a fraction of a pixel.
4. Integrate - combine them, rejecting outliers, so the noise falls and the signal does not.
5. Remove the gradient - the sky is brighter on one side than the other, and that has to come out without taking the nebula with it.
6. Calibrate the colour - decide what white is, ideally by matching stars in your frame against a catalogue of stars with known colours.
7. Stretch - the data is linear and the faint parts are invisible. This is where an image appears, and where most beginner images are lost.
8. Sharpen and denoise - carefully, and less than you want to.

Every program does these. What differs is who supplies the numbers.

The part PixInsight is for

Before going further, the case for PixInsight should be stated properly rather than waved at.

Its real advantage is masking. Range masks, luminance masks, star masks and PixelMath let you apply a correction to part of an image - stretch the outer regions without blowing the core, reduce noise in the background only, sharpen the structures that can take it and leave the ones that cannot. That is the difference between an image that is processed and one that is finished, and nothing automatic replaces it.

If you look at award-winning astrophotographs and want to make those, you will end up in PixInsight eventually and you should. What follows is about getting from a folder of frames to a real photograph without that being step one.

Route one: Siril, and let its scripts do the middle

Siril is free, open source, and runs on Windows, macOS and Linux. Its one-shot colour preprocessing scripts will calibrate, register and stack a session without you specifying much, which covers steps 1 to 4.

You then do 5 to 8 by hand in Siril: background extraction, photometric colour calibration, a histogram or generalised hyperbolic stretch, and whatever denoise you want. It is a genuine skill but it is a much smaller one than PixInsight, and Siril’s documentation is written for people rather than at them.

Cost: nothing. What it asks: that you learn six or seven operations and roughly what their parameters do. Where it stops: the scripts are fixed. They apply the same settings to every session, because they never look at the data - so a wide field and a tight crop get the same black point, and a night where the seeing collapsed at midnight gets all its frames stacked anyway.

Route two: Graxpert and Photoshop for the finishing

If you already know Photoshop, a workable route is DeepSkyStacker or Siril for steps 1 to 4, GraXpert for step 5, and Photoshop for 6 to 8 with curves and levels.

Cost: whatever Photoshop costs you. What it asks: existing Photoshop fluency, plus enough astro knowledge to know that a normal photograph’s editing instincts are wrong here - the background is not supposed to be black, and a curve that looks right on a daylight image will destroy faint signal.

Route three: have the measurements made for you

The reason those eight steps take months to learn is not that any one of them is conceptually hard. It is that each has parameters, and knowing what to set them to is pattern recognition acquired over dozens of sessions.

But almost none of those parameters are matters of taste. Which rejection method suits a stack is a function of frame count and variance. How hard to deconvolve is a function of measured star width and signal-to-noise. Where the black point belongs depends on what fraction of the frame is sky. These are measurements, and a program can take them.

That is what Akastroid does - we make it, so weigh this accordingly. It reads the folder, measures the session, runs all eight steps from those measurements, and then lists every decision with the number behind it so you can overrule any of them. A hundred-frame session takes two to three minutes, and the free tier covers sessions up to 25 frames.

Cost: free up to 25 frames a session, then one payment. What it asks: a folder. Where it stops: the adjustments are global. No masks, no selective editing. The same wall PixInsight exists to get past.

The combination most people should actually use

Here is the arrangement that gets least attention and suits the most people, including plenty who already own PixInsight.

Use something automatic for steps 1 to 4, and finish wherever you like.

Calibration, grading, registration and integration are mechanical. There is a correct answer, it is determined by the data, and doing it by hand buys you nothing except the knowledge of having done it by hand. Akastroid’s Save the stacked image, unprocessed writes the integration before any finishing is applied, which is exactly what PixInsight or Photoshop want as a starting point.

You then spend your time on 5 to 8, where the judgement actually lives, in whatever tool you are fluent in - and you spend it on data you already know is worth the effort, because the automatic pass produced a finished version in three minutes and told you what was wrong with your night.

When you do need to learn the deep tool

Be honest with yourself about which of these applies:

• You want masks and selective corrections. Nothing automatic gives you these.
• You are producing images for competition or publication.
• You are doing narrowband combination with custom channel mapping and want control over every stage.
• You enjoy processing as an activity in its own right. This is a completely legitimate reason and quite a few people are here for it.

If none of those describe you, the months are not obligatory. They were never obligatory - they were just the only route anyone had for a long time.

Where to start today

Take one session you have never managed to process. Run it through whichever route above matches your constraints. Get a finished image out of it, however imperfect.

Then read what each step was doing, because the fastest way to understand processing is to look at a result and work backwards - not to learn the theory first and hope an image appears at the end.