Vanes for producing Diffraction Spikes
with a Telescope that has no
In spite of Roland's well-known feelings about central obstruction artifacts, I really like the look of diffraction spikes on bright stars in open clusters and similar targets. These spikes are an inescapable part of the image when using any instrument with vanes supporting a central obstruction (eg, a Ritchey-Chretien or Newtonian scope) but you have to put something in the way of starlight to achieve the same effect with a refractor. Some people have used string criss-crossed in front of the dew shield. The problem with this approach is that it's hard to get the strings crossing at exactly 90 degrees and to keep them that way if you accidentally brush the front of the scope.
I decided to try something that would always give exactly 90 degree spikes and be easily removed and replaced without modifying the scope in any way.
The vanes are made of 1/8 by 5/8 aluminum strips of the sort that you buy in 6 foot lengths at the hardware store. I cut them to be just slightly shorter than the inside diameter of my A-P 130 EDFS f/6 scope and notched each precisely in the middle, half-way across the 5/8 width of the strip, making the notch just slightly narrower than 1/8 inch. I then forced them together notch-to-notch in a vise. (Tip: make the notches tight but bevel the edges enough so that forcing them together doesn't cause the metal to break.) After a bit of filing, I had a smooth vane cross that press-fit into the felt lining the A-P dew hood (a VERY GENTLE press-fit). Because the vanes weigh very little, a tight fit is definitely not required or desired and we don't want to mar the felt. I finished the vanes by sanding with fine emory paper and spraying with a flat black paint. When the paint wears off, I'll probably have them anodized.
To achieve consistency of placement, I placed pencil marks on the outside of the dew hood at 90 degree intervals with two marks falling exactly midway in the Starfire labels on the sides. Then I place the cross so that one vane lines up exactly across the Starfire label marks (not visible in the over-exposed image below). You can see a sample result at M45. (Please don't tell Roland I did this.)