Stacking and processing image of M101 to remove light pollution in GIMP

It is amazing what instructions you can find on YouTube and Google! I downloaded DeepSkyStacker and stacked 7 images of M101 I took last night to create single image with equivalent exposure of 40 minutes and then carried out the instructions at to remove light pollution – results were amazing as you can see below!

(All images below are TIFF files)

This post follows from my previous post & is using images from that session:

First deep sky images with QHY10 camera


I had taken 7 images of M101 “subs” last night – in my previous post I only presented a single frame. Today I stacked all 7 in DeepSkyStacker ( to create single image equivalent total 40 minutes exposure (below):


The following image is my first iteration of applying curves in GIMP:

I then applied curves again to above image each time narrowing down the top and bottom so that it bridged the data seen on the histogram:

Another iteration of curves:

Now when I looked at the histogram the edges of the curves touched the data both sides:

I used despeckle with standard settings in GIMP on above image to get following image:

I now followed the instructions at to remove light pollution.

The first step was to create a copy of the layer and on this copy I used despeckle with largest radius and maximum black and white points to remove stars, then used the clone tool to remove the two galaxies as best I could to create a layer showing image of the background light pollution.

I then used subtract on the layers to remove this light pollution layer from the original image to give following incredible image (seems incredible to me that this can be done so easily):

TIFF version:

PNG version:

BMP version:

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