Friday, June 1, 2012

The Advantages of Having a Machine Shop At Home

The finderscope on my refractor suffered a crosshair failure some time recently.  (A finderscope is a low power telescope attached to a large telescope to help you aim the main scope at an object that may be too faint or too small to see with the naked eye, but that you will never likely never find with the main scope, even at your lowest power.)

This has happened before; instead of using a glass reticle, it literally uses two extremely fine pieces of thread suspended very near where the focal point of the finderscope objective and finderscope eyepiece meet.  (In the old days, these were actually pieces of spider web.)  The last time, I took the finderscope eyepiece apart, and replaced the threads.  Sewing thread is a bit thicker than the original, but against a dark sky, these thicker crosshairs are actually something of an advantage, as they knock out a star.

This time, I was just a bit too clever for my own good on the repair.  Like most Chinese-made finderscopes, the materials are cheap and designed for cheap assembly--not for repair.  The eyepiece screws into the back of the finderscope tube.  This is a Kellner design eyepiece (which is actually better than this class of finderscopes usually gets), and the two pieces of glass are held in place by the tube that contains the crosshair.  The crosshair tube screws into the eyepiece tube.

So, when I unscrewed the crosshair tube, the two pieces of glass (one of them a cemented doublet) and the spacer when flying.  This took a while to find.

After I finished putting in new thread through the holes, I epoxied the thread in place.  From my experiments with fiberglass a couple of years ago, I knew that I could harden the epoxy more quickly and more solidly by putting this assembly into the oven at a low temperature for a couple of minutes.  When I pulled it out, I discovered something that I should have known: the plastic of the crosshair tube was doing its best Wicked Witch of the West, "I'm melting, I'm melting!"  It would still screw into the eyepiece tube, but the part that melted inward meant that it would no longer be particularly useful for an eyepiece.

I tried to find a replacement.  No luck.  So, I machined a piece of 7/8" OD .065" wall aluminum tubing to the right dimensions, drilled the holes, and put the thread through.  Along the way, I discovered that some annoying aspects of this finderscope were actually repairable.

From the beginning, I have had a problem getting a sharp focus from it.  Because the eyepiece adjusts by screwing, the only position that seemed right was on the edge of falling off.  I discovered that the front lens was loose in its mounting--and by fixing this, I solved the focus problem.  And the crosshairs are again operational.

1 comment:

  1. It's not just having the machine shop at home... It's handy to be handy!