Fixing Star Edges with Images Plus

Mike Unsold, the author of  Images Plus, has added a fantastic new tool to Images Plus called the feature mask. This tool allows the user to split an image into two separate images, one that is stars only and one that is everything else. This means that you now have the option of doing things to the stars that you never had before because the processing will not interfere with the rest of the image. This page deals with the artifacts that show up around the stars in some images. In particular the colour rings that can be formed from stretching images captured with some SCT's and refractors. The technique can be used to fix up any variety of star edge artifacts and relies on a layered technique that replaces the stellar edges with data from another layer filled with the background colour. When this is done with classic (non-split star) techniques it is very difficult to accurately model the background, but with split star processing the background of the star only image is generally a solid colour so it is much easier.

Let's start with the following image of M81.

Here we see some annoying blue halos on the left side of the stars embedded in the galaxy. These are very difficult to correct using normal techniques, especially  the stars embedded in the galaxy as the information that has to be filled in for the halos comes from the galaxy itself. Split star processing in Images Plus makes these relatively easy to fix with a little effort put into making a mask to target the technique to the stars with the problem. The first step is splitting the stars from the rest of the image using the feature mask tool (see the Images Plus site for details).

 Next a star mask is made by thresholding the star only image. One method that works well in Photoshop once you have the initial star mask, is to make a second version   with smaller stars made by eroding a copy of the original mask and subtracted from the first star mask. This leaves a ring surrounding the stars that can be used to apply corrections to the stellar edges and halos. A similar technique using the selection tools is shown in this star reduction technique.

In Images Plus the process of generating a star edge mask is easy. Once you have made the star mask using the luminance mask tool and the threshold tool then just use the edge mask function under the special function, mask tools menu. All that is required is to use the tool with Deriche selected and adjust the edge width slider to make a mask with the required edge width. as shown below.

This forms a mask that will be used to select just the edge halos of the stars in the following processing.

Use the statistics tool to sample the background of the star only image (lower right in the screen capture below). Use the new image tool to create an image filled with the background colour (image in the top right of the screen capture below). Using the combine images tool make a stack of the star only and the new image made with the new image tool (new filled image on top) and set the blend mode to normal. Now apply the ring mask that you made to the top filled layer. This covers over the stellar edges and halos with the fill colour from the top layer effectively removing the stellar halos and rings in the stars included in the mask.

Flatten the image stack to produce the new star only image with the blue halos removed. Again using the combine images tool, recombine the new star only image with the galaxy image produced in the first steps and flatten the stack as shown below.

Take a look at the zoom below to see the effect in greater detail.