Re: ATM Diagonal Mounting Procedure.

[akelly@ghgcorp.com (Albert F. Kelly)]

> Could anyone tell me how to mount a diagonal mirror on a holder. If I use a
>homemade mount do I need to make it so that the entire diagonal rests on the
>mount, or can the mirror edges "overhang" the mounting surface? In Berry's
>book there is an illistration of a fixed position diagonal holder/spider
>that shows the mirror overhanging a considerable distance. Any advice
>appreciated!  JIM

Jim,
        If you are using a precision-ground elliptical secondary, then
(depending on its size) I would let it overhang the mounting surface so that
the edge of the mirror defines the central obstruction.  The reason I said
"depending on it size" is that the usual way of mounting the mirror to the
surface is to glue it using a silicon-based RTV adhesive (RTV....Room
Temperature Vulcanizer).  This sticks very nicely to glass, metal, and wood,
making a durable and somewhat flexible bond, and can safely hold mirrors up
to 3 inches m.a. or so without having to worry about edge retainers. 
        I would make the amount of overhang relatively small, so that there
ia a large surface for adhesion.  Spread the RTV generously and evenly
across the mount surface and "rake" it with a deep-toothed saw blade so that
it has a constant depth.  Make sure and clean the back of the mirror with
alcohol or acetone to ensure that no oils or other contaminants remain
before adhering.

Al Kelly