Star Shrink Processing

One of the common problems encountered when processing astrophotos is star bloat caused by the extreme stretches applied to our images. This bloat is caused by the the gaussian profile of a star. As the brightness of the dimmer areas is increased, more and more of the star saturates and the result is larger stars.

Fortunately with a little processing we can reduce the size of the stars with little impact on the rest of the image. Take a look at the stars in the image below.

Using the magic wand tool click on a bright large star. Play around with the selection tolerance to get the selection correct. There is no need to select all of any particular star, just try to center the selecton on as many stars as possible. Now enlarge the selection to cover the stars and a little extra as shown below.  

Now create another raster layer on top of this layer and use the fill tool to fill each selection with black on the new layer. This will create small black circles on the new layer. Now hide the new layer and reduce the size of the selection to cover just the central area of the larger stars. In Paint Shop Pro this is done using the selections, modify, contract menu item. Make the new layer visible and active and hit the delete key. This will leave black circles around the outer portion of each star as shown below.

Select the top layer and using the magic wand select the black circles. Feather the selection by three or more pixels. Hide the top layer and make the image layer active. You now have a feathered selection around the stars and can now use erosion or a minimum filter to reduce the star sizes. Finally delete the selection and the top layer and save the final image. Compare the image below to see the difference. Moving your mouse over the image will display the original with larger stars.