Friday, December 17, 2010
My old spider just wasn't making it.
The spider is the part of a Newtonian reflector telescope that holds the secondary mirror suspended exactly in the center of the telescope tube so that the light rays from the main mirror are redirected towards the focuser attached to the side of the tube. The secondary mirror is mounted at a 45 degree angle from the main, or primary, mirror.
The first spider design was comprised of a CPVC ring with 2" washers acting as the adjustment mechanism. The secondary mirror was attached to a CPVC pipe section cut at a 45 degree angle with resin epoxy (it will never come off).
The spider assembly was mounted to the telescope tube by three 1/8" threaded rods.
This set up was not very sturdy. The threaded rods had very minimal contact with the spider and the tube so the sturdiness was not very good.
An additional problem with the original design is that the 2" diameter dimension of the washers over profiled the 1.83" dia of the secondary mirror. This made the secondary mirror "light print" to large. Not a good thing.
The mirror is much more sturdy using a truss type design to hold the spider assembly at the very center of the tube.
The new spider is made from almost all aluminum so it is relatively light. The main adjusting mechanism is made from 1/4" aluminum bar stock, 3/8" aluminum rod and 1/2" aluminum.
The photos show how the new spider come together.
The only thing left is painting it a nice flat black.
The new spider is very sturdy and easily adjustable.
I collimated the mirrors and although it is not mechanically perfect, I get good images.