Wanted to make the secondary mirror holder out of aluminum but could not find any 1 1/4" round bar stock in town for less than the minimum cut fee of $175.00. After spending 4 hours one Friday driving to all of the metal yards in Tulsa and not finding any scrap I decided to make it out of a hardwood dowel.
Learned a big lesson on this one. While it is ok to make the holder itself out of hardwood, DO NOT make the support/adjustment wafer out of wood! I used bicycle spokes, an idea from my buddy Shane, for the spider and while tightening them the wooden wafer they clip into broke. Shane had a brass disk 1/4" thick in his toolbox and we remade that part of the mount. Anyway, I'll get on with the description of making the mount using wood, it will be the same using aluminum or other metal for the support wafer.
I cut a 1 1/4" hardwood dowel 1 1/2" long for the mirror mount and a 1/4" wafer from the same dowel for the spider mount. See the drawing for details. The drawing should be very self explainatory as to the construction of this part.
The spider mount is a 1/4" thick disk from the same material as the mirro holder. Three holes are drilled 120 degrees apart and tapped 6-32 for the adjustment screws. Three more holes were drilled 60 degrees off of these also 120 degrees apart for the spokes to clip into. I purchased black anodized spokes from a local bicycle shop complete with nuts. The standard nuts used in a bike rim work very well to attach the spokes into the tube. Simply calculate the distance from the spoke mount holes to the outside of the tube with the wafer centered and cut spoke lengths sufficient to bend a 90 degree end and clip it into the wafer. I calculated spoke length to come out flush with the outside edge of the tube when inserted in the holes. This provides about 1/4" of adjustment in either direction to center the secondary under the focuser. Close tolerance on the drill size will keep the spokes aligned straight. Be sure and mount the spokes pointing out the end of the tube to prevent the secondary holder from falling onto your primary mirror if the spokes are loosened.
Remember, these pictures show a wooden spider mount but in fact I used brass. Aluminum would be better because it is lighter.
Some very important considerations for the mounting of the secondary mirror and the spider. I am by no means an expert, if not for Shane I would not have gotten this. When attaching the secondary mirror to the holder with silicone it can be put on two ways. One way blocks more light than the other. Look at the photo and pay attention to the angles on the mirror and their relationship to the outside top end of the scope. The machine screw points towards the top end of the scope.
When it's time to drill the holes for the spider and mount the secondary holder you must have already calculated the location of the focuser hole in relation to the primary mirror surface to obtain the correct focal length. I used the Newt software program for this. I made a mistake and measured to the center of the wooden secondary mirror holder to calculate where the spider holes would go.Arrggh.... my secondary mirror was too deep in the tube to align with the focuser because of the thickness of the secondary mirror and its' mount position on the holder. You must measure from the center of the face of the mirror to the spiders to determine how far up from the center of the focuser hole to drill for your spiders. I had to redrill mine and now have extra holes in my tube.