I’ve been spending some time reading about techniques for post processing digital images.  There are certainly a lot of information about digital filters and their effect on the picture.  But there is not that much information on the net about which filter to apply to an astrophoto that will make it look more “pleasing”.

I guess for astrophotography, the ultimate goal is to capture the sky as sharp as possible and with as much detail as possible.  So when I processed the pictures I took from the last star party, I simply tried as hard as I could to bring out as much detail as possible.  However, the more I look at the picture, the more “busy” it looks and everything just looks messy.  When I looked back as some of the constellations pictures at astropix I realized that Jerry had used a diffusion filter on his pictures.  Actually, he said in his tutorial that a fog filter will soften the picture and bring out the bright stars of the constellation better.

I couldn’t really just take another picture with a fog filter so I decided to see if I could simulate the effect in photoshop.  After a lot of trial and error (there are a lot of filters in photoshop, and none of them is labeled diffusion), I found that the effect I was looking for is call the soft focus.  Soft focus is used mostly in portraits to give the subject a softer look.  A softer look reduces the effects of harsh lines and intensifies colors.  To simulate the effect in photoshop (no, there isn’t a soft focus filter like they should have), you have to use the Gaussian blur.  Here is a great tutorial on how to properly use the Gaussian blur in photoshop to simulate a soft filter.

I applied a very strong soft filter to the image I took last star party and here is the result:

Scorpius and Sagittarius with heavy soft focus applied

Scorpius and Sagittarius with heavy soft focus applied

As you can see, a lot of the background stars have been wiped away.  But this is different than simply turning down the brightness because the bright stars kept their brilliance and is much more pronounce than before.  You can now very easily pick out the major constellations in the picture.  The star’s color shows up much better now that it is not fighting with the background as much.  Overall, I think it looks pretty good.