Re: ATM Re: astigmatism with a sling?

["Bill T." <twentiethwave@hotmail.com>]

Here's a slingless mirror box I use.  I originally built this as a
"compromise" to achieve a highly portable scope -- for travel I could screw
a piece of plywood into the four positioning blocks and jam the mirror up
against it with the collimation bolts.  But it works quite well in general.
No support related funny business is visible in the star test until the
scope is pointed almost at the horizon -- even then it's no worse than with
a sling.

http://www.kupercontrols.com/misc/mirbox.jpg

The four blocks allow only about 1/16" mirror movement.  They extend
slightly above the mirror.  The two white teflon supports end well before
the top edge of the mirror.  A nice feature is that if the mirror starts to
tip out, it will snag in the top supports almost immediately.

The support triangles are carried by bolts threaded through brass inserts in
the plywood.  The triangles are registered with pins extending into slightly
larger holes in the wood.  This is a very easy cell to build.  I've made
a14" and a 12.5" this way.  I would not hesitate to go much larger with the
same scheme.

Since the mirror is so well registered relative to the support blocks, you
can use the blocks to support an easily removable, no-mirror-contact edge
mask.  When the Optical Police run the physical inspections just before
sunset, just hide the mask underneath the star charts.  Then just slip it
back in before they return for star testing ;^)

The hole at the bottom of the box is to give encoder clearance -- reduces
eyepiece height by 2".  As far as I can tell it doesn't cause astigmatism
from asymmetric cooling.

Bill Tondreau