Sunday, February 13, 2005

Fun & Games With The 17.5" Reflector

It turns out that I don't need to replace the mirror cell, at least not immediately. Like a lot of Dobsonians built slavishly to a cookbook, this used three three-point pads attached to 3/8" bolts that stick through the bottom of the mirror box, two side blocks to position the mirror east and west, and a sling to hold the mirror in position north and south. John Dobson's original design was driven by the need to keep costs down, and to build from junkyard components--there are better ways to do this, even on the cheap.

Anyway, I removed the sling, added two more blocks to position the mirror north and south, and then replaced the 3/8" bolts which had detached from the pads with 1/4" bolts, wingnuts, and springs. The 1/4" bolts are epoxied to the back of the pads. Now the adjustment screws are free to move a little, taking tension off the connection to the pads (perhaps why they had detached). I'm a little concerned that these springs aren't going to strong enough to hold up the mirror--it is very heavy, even though it is a thin mirror. I squeezed a couple of fingers pretty good returning it to the storage container.

I also removed the diagonal mirror, and cleaned it--although it was surprisingly clean, considering the number of insect bodies and spiderwebs elsewhere in the tube. The diagonal mirror for a telescope this size is huge.

The finder was a reasonably nice University Optics 8x50mm, head in place by a homemade wooden dovetail. unfortunately, nothing held it in place except gravity and hope, so I drilled a hole in the side of the dovetail receiver, and added a thumbscrew to hold it more securely, and more repeatably.

The person who was storing this behemoth got away on vacation before I noticed that the ground board was missing. It may be in his shed still, or it may have been lost. No matter. I bought a 24" x 24" x 1/2" piece of flooring grade plywood to use as a ground board. A little sanding, and a hole drilled, and it seems okay. I probably need to put a sheet of Teflon on it to reduce friction adequately, but for initial optical quality testing, it is good enough.

I was going to respray the interior of the tube flat black, but that has to be done outside on the lawn--and it is now raining.

No comments:

Post a Comment