Re: big scope inquiry

Durring most of 1997 I was wondering, designing, and planning to build
a 20-25" scope. Durring this time, I designed the truss and base and
waited for a mirror to show up on ebay that suited my purposes. It
ends up that a 20"-er showed up first, and I struck a deal with the
(then) current owner. The mirror arrived in Dec of 1998, and I got
started building the various stuff to make a telescope to fit the
mirror in hand. Along-about July the scope was finaly comming together
and was useable in my front yard. And by Aug it was ready for the
annual trip to Ft. Davis Texas.

At first there were the typical bunch of things that lessened the
telescopes efficiency, that got knocked off one by one until the only
thing holding back the telescope was the secondary support. The first
generation was a piano-wire 3 vane suppore that simply vibrated and
almost never stopped vibrating. I tried various tensioning
experiments, thicker wire, and all sortf of things. I finally gave up
and built a 4 vane spider with 0.010 metal more typically used in
flashing of roof seams. This one works really well. The whole
telescope operates like a binocular with no vibration whatsoever, and
holds collimation at all angles.

Another mistake was using silicone glue to glue the mirror to the
mirror cell. This proved to be the source of some subtle errors in the
mirros surface, So I rebuild the cell to have a strap like most other
big DOBs. Still alter, I found that the 27-point cell was build on the
radius for an 18-point cell and had to redesign pats of the lower
support structure. The wrong radii caused some subtle spherical error
in the mirrors figure. This helped make the mirror perform.

After using this scope for 6 years and having it completely debugged,
I am glad I stopped at 20" and did not reach upwards to 25". At 56, I
can still pack and unpack this scope single handely into my SUV. I
would never have gotten a 25"-er down to a weight that could be single-
handledly packed and unpacked. The mirror, cell, base weight in at 72
pounds (50 pound miror) and the rest of the telescope runs the total
up to 111 pounds. About 75% of what a normal DOB weights.

I still ahve my C11 and my 6" AstroPhysics, but hardly use either any
more.

Mitch
.

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