Re: ATM question: sling or pads for edge support?

["Bell, Andrew" <AMB2@pge.com>]

	Herb's question motivates me to describe the not-quite-by-the-book
that I use for my 14" porthole glass mirror (plate glass, one-inch thin).  I
made the mirror with John Dobson some years back, apparently even before the
advent of sling supports.  In those days, the standard sidewalk mirror cell
was simply a square piece of wood with rounded corners that you bolted in
place at the bottom of your sonotube.  You drove three bolts through the
back of that board in an equilateral triangle, and then glued and/or
thumbtacked a matching "Y" cut from heavy cardboard to the front of the
board.  The mirror rested on the ends of the cardboard Y and you turned the
bolts to collimate.  To keep the mirror from falling down the front of the
tube, you just used one more short section of sonotube as a "collar" right
in front of the mirror -- this costs you a little bit of aperture around the
outside, but it also doubles as a convenient mask just in case you might
have a long-focus edge.

	I improved on this (just a little bit) when I rebuilt the scope last
year.  The sonotube has been replaced by a modified truss design, so I've
basically got a Kriege-Berry mirror box.  I stuck with three bolts through
the bottom of the mirror box for the basic mirror support, but replaced the
cardboard Y with three hardwood triangles that are strung together with
piano wire to keep them from twisting when I turn the bolts.  There are
thick felt pads at each tip of the three triangles, and those are what the
mirror rests on - making that a nine-point support, sort of at least!  To
hold the mirror in place and to keep it from falling out towards the front
of the box I made a short collar out of two concentric rings of
black-painted sonotube.  The first one is very nearly the same inside
diameter as the mirror and is held in place by pegs and friction, the second
one masks the last quarter-inch or so of my edge and is just a little in
front of the surface of the mirror.

	This set-up will never win any mechanical engineering prizes.  But
it does seem to hold collimation pretty well down to within about 30 degrees
of the horizon (as checked by star testing at 450x and watching what happens
in different regions of the sky).  The mechanical slop definitely becomes
noticeable within 20 degrees or so of the horizon, but the seeing's already
terrible down there, anyway.

-- Andrew Bell 

Herb Kasler wrote: "I'm hoping to get a variety of opinions from the group
on what type of edge support is best for my mirror. [snip]  From what I've
read, seatbelt-strap slings seem to be the most favored setup, but when I
think about how to build one into my primary cage, I get a headache. Also,
it seems that the mirror will have a tendency to swing about when jostled
and bang into the safety clips."