Here is a simple inexpensive way to greatly improve the diffraction artifacts around bright stars in your images. Here is the before and after effect both looking into the focuser drawtube, and what the resultant images look like:
The ideal primary mirror would have a perfect circular edge profile. But the edges of your primary mirror are not perfect. They diffract light and cause diffraction spikes/rays to extend from the core of bright stars in all directions. In addition, the three mirror clips that overlap the edges of the primary will cause gaps in these spikes where they mask the edges of the primary. The combination is a a bright area of diffraction spikes with three dark lobes in it. The trick to fixing this is to mask the entire edge of the primary mirror, including the mirror clips. This reduces the diffraction effect, and creates a nice even halo around the star core. Here is how to do it.
Here is what you will need:
- Material to cut a masking ring out of and a template to cut to
- Hobby knife to cut the material
- Flat black paint or flocking material
- Double-sided foam tape
The first thing you will need to do is cut out a masking ring. The outside diameter of the ring should be small enough to fit inside your tube easily, but larger than the outside of your mirror. The inside diameter of the ring should as large as possible, while being just small enough to cover the mirror clips when centered over the primary mirror. You can take measurements and then use a free tool like Sketchup or LibreCAD to create a 1:1 print with your inner and out diameters to use as a cutting template.
The mask can be cut from just about any material. I originally hand cut one from black art matting board which worked great for over a year. Eventually it sagged and lost its shape some, so now I am replacing it with a 1/16" thick aluminum water-jet cut ring from an online metal cutting service (metalscut4u.com). The service was very easy to use, their online form includes a ring shape and all you need to do is select it and then type in your ID and OD to get an instant cost on a multitude of materials.
If you cut your ring out of something reflective or lightly colored, you will need to paint or flock it to prevent unwanted reflections from entering the light path. In my case I will flock my ring front and back.
Once the mask is dark and flat, adhere it to your mirror clips with double sided foam tape, being sure to center it over the primary mirror. In my case, I've removed the flocking from the areas where the tape needs to make contact so I get good adhesion.
And that is all there really is to it. The only downside is that you do lose some mirror area, reducing your effective focal ratio by a practically unnoticeable amount.
