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
Open the original image (available here – right click and Save Link As… )
Start processing with the Layers Menu
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.