Subtraction layer gradient removal refinements

[Ags]

I can only shoot pictures at 50 - 100mm focal length with my wibbly mount but with strong LP I always have ferocious gradients to remove.

My manual gradient removal process goes thus in GIMP. Firstly, if it is just a star field then you can apply a curve to increase the gap between the stars and the background in the histogram.

Then you create a dulplicate layer and use the Despeckle tool to remove all the stars (the curve applied earlier helps here as the tool can fail if the speckles to remove are similar in brightness to the background), then a gaussian blur to smooth away the star remnants. Use the clone tool to level any traces of nebulae or clusters.

At this point you can subtract the smoothed layer from the image and you have a gradient removed. But if, like me you have a very strong gradient, this proces can make the stars go faint in parts of your image.

To resolve this, set the smoothed layer to subtract and then make a new layer from the visible. Set the gradient layer back to Normal (not subtract). Set the new layer (effectively a star layer) to subtract.. Now you have an image of the gradient with the stars as black dots. Merge the star layer down into the gradient layer and set the gradient layer back to subtract. Now you are subtracting the gradient but not dimming the stars.

I've attached an image showing the difference between just subtracting a gradient layer and subtracting a gradient layer with a star mask subtracted from it.

[Black Knight]

Sounds useful . I'll have to try that. I use a mix of different techniques at the moment, some work better than others