[ATM] Mirror Cell Question

[David Cohen <davelcohen2000@yahoo.com>]

ATMer's:

I'm new in the ATM world, so please forgive me if
these are stupid questions. Firstly, some background
information:

I am about to purchase an ATM GEM scope that was built
by a machinist.  It's pretty solid and well-built, but
the mirror cell design is weak.  Picture a 12.5" f/4.5
full-thickness mirror sitting inside a hefty square
steel rectangular tubing frame, which is about the
same thickness as the mirror. The "cell" consists of
three pieces of flatbar bolted to the bottom of the
frame, spaced 120 degrees apart, that the mirror rests
on.  At the top of the frame three mirror clips at 120
degrees apart hold the mirror in place, which protrude
onto the mirror's surface approx. 1/8".  There's no
collimation adjustment - that's done on the secondary.
 The main advantage I see to this mount is that the
mirror is held solidly, regardless of position of the
scope.

I've done some research on the internet that states
that this type of mount is very bad in that it causes
mirror deformation, and thereby image distortion.  The
consensus from what i've read would be to build a
9-point "floating" mount, using PLOP-generated
dimensions, with no protruding mirror clips which also
cause image distortion.  The scope's primary use is
CCD imaging, but I will do emulsion photography as
well.

Having said all that, here are my questions:

1. In a floating mount, wouldn't the mirror move when
the elevation of the OTA is changed, thereby degrading
collimation? If not, why?

2. Are mirror clips really that bad for the image? If
so, what is a good alternative to restrain a heavy
mirror? I see a lot of discusson on adhesives here,
but what redundant mechanical means as well? Can you
actually depend on any kind of adhesive with that
amount of weight?
 
I would like to do a floating mount, but I don't want
a "floppy" mirror.

Sorry for the length - I hope that I provided enough
information. The substantial wisdom of this group
would be greatly appreciated.



=====
Best Regards,
Dave Cohen
davelcohen2000@yahoo.com


		
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