Image 1: Star field for M33, nebulosity removed
Divide and Conquer – processing the stars
Once an RGB image has been assembled from the channels the fun begins.
I now run the (free) “starnet” software to remove the stars. Generally we want to do the opposite thing to the stars (reduce them, drop the dim areas) as we want to do with the nebulosity (draw down the brightest parts and enhance the dim areas), so the best way I’ve found to manage this is to separate them into two files and then combine them later. “starnet” takes some computer resources, so you may need to wait a while. When complete I load the full image and the starless one into GIMP and subtract one from another to create a star-only image.
For the star-only image I run “Astra Image” and use the “levels” sliders first. I draw the black (leftmost) one to the right to kill the low light level noise. I then take the middle slider and move it to the right slightly to draw down the intensity a bit. Work with this until the stars are smaller and sharper. The stars should have colour, so typically I increase the colour saturation and perhaps alter the hue/tint a tiny amount to enhance this. I run deconvolution but look carefully at the results to make sure the image isn’t corrupted. If so, try a few options or drop it.
When satisfied with the stars I hit “apply permanently” (don’t forget this) and then save. By default it saves in the folder previously used and NOT where the image came from… If the stars are still too large I may load this image in Photoshop and run the toolkit script “Reduce Star Size” a few times, always checking that the image isn’t getting corrupt. Things to beware of include stars becoming donuts or a background full of noise.
That should be it for the stars but sometimes the sensor is not at the right location and the stars on the periphery of the frame have become lines perpendicular to the radius. Or if the problem was very poor polar alignment the stars may all be lines in one particular direction. A dirty trick is called for…
<Begin dirty trick>I load the star-only file in GIMP and make a copy of it as a second layer. I make the second/top layer “darken only”. If the problem is circles around the centre, then I rotate the top layer about the centre (of the circles) by a tiny fraction of a degree. The stars at the centre will not be affected and the ones at the periphery will shrink slightly. I don’t do too much, just a tiny reduction is enough to make a bad image look good. If the problem is that the stars all look like parallel lines then instead of rotating the top layer I slide it every so slightly in the direction of the line. The stars will be reduced and you might be able to make then nearly spherical. You need to combine the two layers (“flatten image” command in GIMP) and then save this new star image. Look closely at it and compare to the original to make sure you haven’t accidentally made a mess of it. <end>
After this work the star-only image should be nice and clean, ready for merging.
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