Re: ATM Perferating Cassegrain (slightly off topic)

[Del Stanton <sd20@earthlink.net>]

I would say not off topic at all.  Exactly the sort of
question that 
should be asked here !!

The procedure recommended one place in the original ATM
books (published by 
Scientific American) was to partially cut through - from the
rear - using a slotted 
tubular tool in a drill press with a slurry of abrasive and
water, leaving perhaps 
a 1/16 inch thick wall (figure the sag) between the circular
groove and the finished 
face of the mirror.. Then grind, polish and figure. I think
they suggested then 
filling the circular groove - perhaps 
with ordinary canning paraffin (white wax). 

Then grind, polish and figure the mirror.  That annular ring
will expand and contract 
during polishing - expand because the heat isn't conducted
away - so it will be high 
while polishing and sunken when the area cools.  I remember
seeing photographs
of the mirror being Foucault tested.  When the mirror is
finally done you cut through 
the thin web, from the rear, with the original tube.

This is all remembered from reading it from twenty years ago
- so take with a large
dollop of salt.  I felt that posting this would at least get
the discussion off to a 
good start.  On rereading this it seems it would be
appropriate to hog out the curve 
first - to avoid any possible error made in calculating the
sagitta.

Del Stanton  sd20@earthlink.net

StarHoper@aol.com wrote:
> 
> What is the correct procedure when making a cassigrain type mirror, to core
> first before grinding or after polishing?
> 
> David