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Trick to Reduce Coma in Images

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This is a bit of a sly trick to get rid of coma if you don't have a coma corrector, based on the fact that coma is only bad at the corners of an image.

Inspect stars near the corners of the image and see how many pixels they are elongated smeared outwards by compared to if they were round. Let's say 5 pixels.

Create a copy of your image, using 'maintain aspect ratio'. Assuming your image is 3:4 ratio, reduce the width in pixels by the stretch amount multiplied by 8/5 =  5*8/5 = 8 pixels (that's a sneaky bit of Pythagorus). You can always experiment to get the best amount.

Paste this copy as a layer on top using 'if darker' or 'darken' (PS). Align the top layer so two stars right at the centre of the image are lined up.

The egginess of stars around the edge will now be hugely reduced. You can fine tune the reduction amount and alignment of the layers, and reduce the transparency of the top layer if the effect is too stark.

You will see that this also takes all the sparkle out of the centre of the image, so create a mask or just lighten the centre portion of the top layer - I used a quick dab of 'airbrush' white with a gig soft edge and set to be about the height of the image in diameter. This will limit the effect to the corners of the image where the egginess is more objectionable than the loss of sparkle.

I won't post the demo image, as it was someone else's - I have a coma corrector.

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