When processing a photograph, with any software, the premier
sharpening method is Unsharp Masking. Very effective but it has limitations. It
draws the light colours into a star from the background to more concentrate the
star, it works well indeed, but if done to any excess it produces black rings
round the now brighter and bigger star, Panda Eyes it’s sometimes called, so it
can be quite a coarse method. High Pass Filter is better but can give you a
similar result. Any “global” sharpening method will produce similar
results.
The problem is that you’d like to sharpen the nebulosity or
dust lanes in a galaxy but not the stars, and sharpening the whole picture
improves the nebulosity and galaxy arms, but ruins the stars.
But there is a solution.
This is in Photoshop, but any layer using software will do
it.
With your photo on the screen, go to Layers and make two
copies of it, they will be on top of each other, with the background layer at
the bottom.
Make the middle one active and sharpen it as heavily as you
wish in favour only of the areas you want to sharpen. In a galaxy photo sharpen
the dust lanes to their best, ignore what this does to the stars (it will ruin
and enlarge them badly). At the same time enhance the colours of the dust lanes
or star forming regions in the galaxy, by enhancing the colours throughout,
again being concerned with the dust lanes and nebulosity, ignoring the perhaps
over colouring of other areas.
You now have a stack of separate layers, the bottom
background, the middle excessively sharpened and coloured, and the top
layer.
Make the top layer active, and with the erasure tool set to
perhaps 25% opacity and flow, with a soft edged brush tool, carefully paint over
the areas you want to highlight, dust lanes nebulosity etc.. Be very careful not
to brush over the stars. You are actually rubbing out parts of the top layer to
expose the middle layer in chosen areas.
If you have the History panel open you can step back to change
what you’ve done if you want to change it. Use layer opacity too to smooth the image.
Top layer - no sharpening or colouring |
Bottom layer - heavily sharpened and coloured |
Top layer erased in chosen areas to expose the Bottom layer |
Finally, merge the layers, to produce a single
layer.
It takes some practice and some enlarging and reducing the
size of the image to feel comfortable with it.