Sunday, September 25, 2022

Why I Have Been Fairly Silent

I am rebuilding an 8" f/7 Newtonian reflector test was originally built by my father and I in 1969.  

First of all:  a Newtonian reflector uses an elliptical mirror (which at a 45 degree angle is a circle) to reflect the image from the primary mirror to the eyepiece focuser.  The elliptical mirror is held on a diagonal holder by the spider, so named because it has either three or four legs holding the diagonal holder.  The legs attach to the tube.

The diagonal holder should have adjustment screws to collimate the image from the primary mirror to the rest of the optical components. 
The closer it is to collimation, the finer the detail and the higher the contrast of the image.  When Coulter first started making mirrors at an absurdly low price (one of the first to do large scale automated production: the factory in North Hollywood looked like no other telescope maker of the time), they advertised them as +-1/25th wave.  The surface was within 1/25th of a wavelength of a perfect parabolic.  There are times the telescope has given me reason to believe it.  During one spectacular Rohnert Park heat wave I used about 700x on the Moon with a very sharp image, far above what is considered possible with this size of reflector. 

When my father and I built this we used an Edmund Scientific spider/diagonal holder that had no adjustment screws.   The best that I could do adjustment collocation was brute force bending the assembly until the laser collimator was very approximately correct.  Let me emphasize how approximate that was.

I finally broke down and bought an AstroSystems spider with the adjustment screws.  I have mentioned my efforts to make 5" long 10-32 thumbscrews to replace the SHCS with which it comes.  There is nothing intrinsically wrong with their design but the diagonal is pretty far from the end of the tube; holding an Allen wrench to make adjustments above a irreplaceable mirror makes me shudder.

I am finally working on the installation.   First of all the elliptical mirror was glued to a piece of paper glued to the diagonal holder.  The AstroSystems unit lets you slide the mirror into the holder where a metal lip keeps it from falling out.

Removing the elliptical mirror from the old diagonal holder involved gently slicing through the glue with an Exacto knife.  The AstroSystems assembly is 1.5" longer than the Edmunds so the old holes will not do.

I measured the proper distance from the end of the tube, then used a cloth sewing tape measure to put ink dots at the appropriate locations for four equally spaced legs.  Then I used a center drill in the power screwdriver to make precise pilot holes, then a twist drill to go through the fiberglass tube trying to minimize dust production.

The next step was to patch the old holes.  I tried mixing flay black paint with 5 minute epoxy to full those holes.  Too much paint; the epoxy flowed too much, leaving black drips on the outside of the tube.  Then I realized that I needed something on the inside of the tube to hold epoxy while it hardened.  But what? Black construction paper is close to flat black.  I epoxied small pieces to the interior of the old holes and filled each hole from the top, rotating the tube 90 degrees after the epoxy started to harden.  Then I put painters tape on top of the new holes and around exposed parts and sprayed a small amount of flat black paint on the interior and glossy white on the exterior to cover over the globs of epoxy in the old holes.  

Now it is drying.  Tomorrow I put the new spider and make sure that I measured correctly. 

In case you are having trouble visualizing this assembly (and feel the need)
I finished installing the new assembly this morning.   If I have the energy this evening, I will collimate and may be even make another attempt to get to Go To working.

Bending over repeatedly to get installed is showing up in my lower back so tomorrow night.

No comments:

Post a Comment