PixInsight Background Extraction - How to Remove Gradients and Tints

If you don't have access to truly dark skies with no light pollution anywhere, and you don't own a genuinely flat-field telescope (do they even exist?), there are high chances that your stacked astro-pictures have some gradients and tints (uneven field illumination). Do you have to live with that? Well, it depends on your PixInsight skills! This tutorial will show you the PixInsight background extraction: how to eliminate light pollution gradients, color tints, and the effects of lack of proper flat frames in your stacked astrophotography pictures.

How do those gradients and tints look like?

Look at this picture:

Trifid and Lagoon nebulae, unprocessed. PixInsight background extraction — Trifid and Lagoon nebulae, unprocessed (only stretched for display purposes). Nothing extraordinary, just an astrophoto.

This picture is an astrophotography stack before further processing. It's not important how many subs there are and other exposure settings like ISO, aperture, or shutter speed. Two important factors here are:

There is a significant vignetting gradient
There is a significant light pollution gradient

Vignetting comes from lens/telescope imperfections, and light pollution comes from our civilization. Can we do something about it? Of course, we can.

Eliminate uneven field illumination using PixInsight's Automatic Background Extractor (ABE)

PixInsight has an amazing (yes, no other word can express it better) tool called Automatic Background Extractor (there is also a manual version, called the Dynamic Background Extraction, but this is a topic for another post. Usually, the automatic tool produces fantastic results without any manual steps involved.). Open your stack in PixInsight, and go to Process -> Background Modelization -> Automatic Background Extractor.

PixInsight Automatic Background Extractor.

As for a start, the default values are good, except the Target Image Correction -> Correction parameter - be sure to select Subtraction instead of None. Let's apply this to my Trifid and Lagoon stack. PixInsight, as a result, gives me two pictures.

PixInsight extracted gradient — First: extracted gradient. You don't need this; close the image editing window in PixInsight.

PixInsight background extraction — BOOM! Second picture: the background-extracted stack. After just a few clicks, automatically.

This method is not perfect - there is still some vignetting and uneven field illumination. But if you look at the base image and then on the background-extracted one - it's just hard to force me not to use this tool for astrophotography post-processing. And keep in mind that background extraction is one of the first steps you should do when you post-process astronomy images. So the image above is just a starting point (without those gradients - a good one).

If you are not satisfied with the first extraction, run the second one. Also, play with the function degree parameter under the interpolation and output submenu. The greater the number, the more ABE extracts. Experiment.

This is the final picture of Trifid and Lagoon nebulae. I like it. I shot Fuji X-T20, Rokinon/Samyang 135mm f/2, and Fornax LighTrack II from my balcony. There are five minutes of the total integration time (f/2 FTW), sadly no darks, no bias, and no flats.

More examples of PixInsight Automatic Background Extraction

Let's run ABE on a few more images. It's strangely satisfactory for me to use this tool in my astrophotography post-processing workflow. I know it's science, not magic, but it feels like it.