Mid Cheshire Astro Group Demonstration

Simple light pollution removal with paint.net

Paint.net is a free software package, that was initially created as an exercise to improve the very basic MS Paint program that came with generations of Microsoft Windows versions.

It can be downloaded from http://www.getpaint.net. It provides a couple of functions that are very useful for removing light pollution from astro photographs; the two functions being the use of Layers and the ability to apply a Blur effect to an image.

Download paint.net and install it. Open the program

I recommend bringing up two of the tool views, for Tools and Layers

These are top right of the screen

Open the original image (available here – right click and Save Link As… )

Start processing with the Layers Menu


– and select Duplicate Layer
You can now see there are two layers – you have the top layer selected

With the top layer selected, locate the Effects Menu – with Gaussian Blur

Apply the slider

Until the stars disappear

If there are only stars in the image (and no diffuse nebulae that are still showing), you can skip the next few steps. Rejoin at the instruction to go back to the Layer tool

Otherwise, select the Oval tool, because we want to set the paler area to the background.

Then use the Dropper tool to select a colour close to the average colour of the area you have selected

and then use the Flood fill tool

to colour in the selected area with an even level of colour

Now press Ctrl+A to select the full layer area – you can see that the layer isn’t perfect as there’s a bit of an edge to the flood fill area

So go back to the Blur tool (in Effects) and repeat the Gaussian blur with the same value as before.

Now, go back to the Layer tool Menu and select Layer Properties

The next thing we want to do is to change the Blending Mode from “Normal” to “Difference”

– this subtracts the smoothed out layer from the star filled layer below, and gives the background sky the expected black colour.

The last two stages are to Merge the layers down

And save the image in whatever format you prefer

As you may be aware, saving as JPEG loses some of the detail as the format includes compression, so saving in a format such as PNG or TIFF may be preferred.

The same technique can be done with any image processing software that has decent Layers handling, such as Photoshop and the GIMP. I have demonstrated in Paint.net because it’s got a relatively straightforward interface and an easier learning curve than some of the others.