How to Remove Light Pollution in Astrophotography

Recently, I’ve been seeking how to remove light pollution from my astrophotography images. In this post, I am going to compare several different techniques for doing this and try to decide which is the best. To improve your astrophotos, read on!

I will compare the following:

Gradient Xterminator – a paid tool.
Graxpert – free
Siril background extraction – free
Manual methods to remove light pollution in Photoshop (adobe.com)

Two kinds of images

The first type of image I need to fix has a small, deep sky object and a larger area of sky. This area has excessive light pollution. This image is easy to work with. I can easily remove the background gradient or skyglow while keeping the nebula or galaxy intact.

The second type of astro image has nebulosity covering large areas. This makes it harder to remove without losing details of the object we want to enhance. I am currently seeking an effective editing solution for this type. Up to now, I have been using background extraction in Siril with varying success.

It can be tricky to select the sky’s background and find a way to make the software ignore the nebula, but it is possible.

When and how to remove light pollution and gradient

There are two possible points in your astrophotography post-processing workflow when you want to remove any light pollution or gradients you see. According to many, removing gradients from your linear image is best. In other words, before you stretch your image. The reason for doing this is to prevent stretching from making the gradient or haze from light pollution worse and, therefore, more challenging to manage.

The second point is later during your editing workflow, when you have already stretched the image, maybe several times, but then find that there is still some light pollution that needs to be reduced.

I wanted to compare the different methods and tools I have. This will help me find the best ways to reduce light pollution in my images. I also want to know when I should change my approach. Should I use different methods depending on the image or always the same one?

When removing light pollution from a linear image, we need to stretch the image to see the gradient. However, do not apply the stretch to the image after it is saved. Fortunately many tools, such as Siril or Graxpert, for example, allow us to apply a temporary stretch like this.

I need to be careful when using the same stretch on all my images. This will help me check how well the different programs work. My images should be as similar as possible for editing.

Two types of light pollution removal tools

Some tools are easy to use with just one click. Other methods require us to follow several steps, like those done manually in Photoshop. If a one-click tool works best, then why do it manually? Let’s see if this holds true.

See how I did my experiments and compared these tools and methods in my video:

Here are a few conclusions from my experiments with these different image editing techniques:

I have been told that Siril and Graxpert share the same process and algorithm. So, it is no surprise that the results I got with these two were almost identical.
The manual method for removing light pollution in Photoshop works quite well. The main focus of this technique is to adjust the black point. We use the eyedropper tool to do this. This helps the image stand out more against the background sky. Check these before and after images to see the difference:

before and after manual light pollution removal in Photoshop — Before

GraxPert

I tried to use background extraction on a difficult image with poor light pollution. Again, check these images for the before and after:

This was a great improvement! Background extraction is a great tool.

And in Siril it’s virtually the same:

how to remove light pollution with background extraction — Before Siril

Gradient Xterminator

I have been using Gradient exterminator for a few years now and it is still a great tool. I find it levels the gradient and removes colour casts from the image. With the use of the lasso tool in Photoshop it is easy to select parts of the image to work on while keeping the other parts such as your target object untouched.

Let’s compare the images before and after use of Gradient Xterminator:

It’s clear that this tool has removed the colour gradient associated with the light pollution in the image. The difference is huge. Don’t forget that the final step is always to adjust the levels and this is a matter of taste. Some prefer a very black sky but I think it is better to leave some brightness in the sky. It just looks more realistic to me.

There are other tools I would like to try out including Pixinsight and Seti Astro’s suite of tools. They also have a variety of tools for removing gradients often due to light pollution.

These tools and techniques help us after we have taken our images under poor conditions. The best strategy for improving images is to prevent this problem in the first place. Find a dark sky if you can. I have flocked my telescope to avoid reflections of light inside it, and I always try to shield my equipment from street lamps and other light sources.

I also use filters like the L Pro from Optolong and the ZWO Duo-band Filter. Narrowband is a great way to image and minimize light pollution. Another thing to consider is to image well away from a bright moon, especially during the new moon cycle.band is a great way to image and minimize light pollution. Another thing to consider is to image well away from a bright moon and especially during the new moon cycle.