Thursday, May 6, 2010

My 8" F/4 travel scope: Secondary mirror

I get confused by three-screw collimation arrangements, and I couldn't figure out how to make one. So the secondary mirror collimation is done like in my Coulter scopes, by rotating along two axes (and one can adjust the position up-down because the support stalk fits in an elongated hole in the focuser board). Basically, the cell is two pieces of softwood that swivel around a screw. The gray square knob at was made by putting a nut in a square made out of painter's tape, and pouring JB Weld into this cast.

The stalk started as a piece of threaded rod. Unfortunately, the support wasn't solid enough--collimation shifted too much with elevation. So I JB Weld'ed (there is a theme here, isn't there) a small square steel rod in parallel with it to reinforce, which helped.

And here is a picture of the cell before it was painted and the mirror was glued to it (with silicone glue). I later cut and sanded it smaller.
I did have one problem with the scope--when I viewed Jupiter, I would get a long smear in one direction. It turned out that the secondary mirror had a distorted edge on one end. I tested by looking through the telescope at an LED flashlight with aluminum foil and a pinhole, if memory serves me. I blacked out the bad part, and the smear disappeared. It might be a good idea reorient the primary so that the blacked out part of the primary aligns with the blacked out part of the secondary, but currently the blacked out part of the primary aligns with most of the secondary stalk, which is also good.

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