Wednesday, December 20, 2017

The Irrational Rebuild of my Little Reflector

I have been busy putting together final grades for the semester, so my hobby projects have fallen by the wayside.  But today I was able to resume the effort on the diagonal mirror holder.  The spider is the 3 or 4 legged assembly inside the tube that holds the device that holds the diagonal mirror.  Here's an example:
Astrosystems makes very nice ones but a bit too big for this application.

The holder goes into the spider.  This holder is partly mine, partly cannibalized from a crummy one that I originally used in the 1990s because it was cheap and the right tube size.
The mirror holder just holds the diagonal mirror in place by adhesive.  Not pretty, but I am not prepared to put the effort into making something more sophisticated.  There is a very long 1/4"-20 screw that runs from the 45 degree part up through the spider.  The original had no easy way adjust the mirror position up and down--or at least in a manner that would let me stuff my big hand down a 3.92" ID tube and do so.  So I put a compression spring around that screw.  By tightening the wingnut, I can position the mirror precisely under the eyepiece focuser.

The original holder had three flat head screws that you turned to position the mirror for collimation.  But there was no spring tension on them, so it was largely luck that it ever worked.  I cut a .5" thick piece of aluminum, drilled and tapped three 8-32 holes precisely 120 degrees apart (more precise than needed), then drilled a .25" hole through the middle for that 1/4"-20 screw..  The three socket head screws are against compression springs.  Because the inside 1/4" hole is very slightly larger than the screw, there is room for adjusting the mirror holder.  Getting into them with an Allen head wrench may be a struggle, but hand turning will be enough. 

The hole visible in the side of the aluminum cylinder: That has a 1/4"-20 setscrew that will be used to prevent rotation around the screw once the mirror is pointing the right direction.  I have a damaged 25mm diagonal mirror that I will use for a test run to verify optical path, then I will order a 28mm diagonal mirror (not expensive on eBay), flat black paint this holder and the mirror cell.  I may paint the holder, the spider, and the inside of the tube in one operation.  Adhesive tape over the eyepiece focuser opening inside the tube, then spray madly.


  1. Do you have any dead hard drives laying around? People building their own laser engraving machines have found that sections of the platters make excellent front surface mirrors.

  2. No dead hard drives. The cost of a 28mm elliptical mirror is so low, the time to disassemble a hard drive makes no sense.