Spoke based spiders (was Re: ATM Wiring Spiders)

[k_patel@vsnl.com]

Hello, all,

Spiders is where my telescope is at, at the moment.

The 6" mirror I was polishing has come back after being aluminized, and
to my biased, subjective, eye, it looks beautiful. To my unbiased,
objective, eye, the outermost half-inch is definitely unfinished, and
needed an hour or two of extra work on the pitch lap. But I started
grinding in May, last year, and the family was starting to wonder if,
not when, it would be finished.

So now I am making the spider, which is being planned as follows: 
The diagonal mirror, whose minor axis is 1" in dia,  will be stuck to a
PVC pipe, also 1" in dia, whose end has been cut at 45 degrees and
smoothened flat. This will be inserted into another, larger pipe, which
will be suspended with the help of bicycle spokes. The spokes, each 2mm
thick, will be inserted through this larger pipe, and their ends will be
attached to the frame by wooden clamps. The small pipe slides smoothely
in the larger one and a tiny screw will fix it in position once aligned.

This arrangement will allow the diagonal mirror to be rotated as well as
moved towards and away from the main mirror. By suitably positioning the
ends of the spokes before clamping it will be possible to position the
diagonal perpendicularly to the main mirror as well. Once collimated,
small adjustments to the main mirror (with the help of its own alignment
screws) should suffice to maintain alignment.

Advice on the finer points of spoke based spiders and other comments
from the list will be gratefully received.

Sincerely,
Kartik Patel
Bombay, India
k_patel@vsnl.com