Re: [ATM] Mirror Cell Question

[Jeff Anderson-Lee <jonah@eecs.berkeley.edu>]

David Cohen wrote:

>I'm new in the ATM world, so please forgive me if
>these are stupid questions. Firstly, some background
>information:
>  
>
There are few stupid questions in the world, and almost none are asked 
in the spirit of the quest for knowledge.

>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.
>  
>
Mirror clips have been in vogue for years.  Some commercial 
manufacturers still use them, incorrectly I might add.  We now know 
better how to use them.

>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.
>  
>
Consensus isn't always right.  You would probably be OK with a simple 
3-point cell assuming the mirror is Pyrex.  If it is a very good quality 
mirror you might want to increase that to four or six points, either of 
which should be sufficient.  Nine point cells are harder to build than 
six point cells and usually not worth the extra effort.

What would help to be able to give you a better determination is the 
following information, only half of which you provided:

diameter: 12.5 inches
focal length or f-ratio: f/4.5
edge thickness: ??? (1.625 in? 2.125 in?)
material:  ??? (plate glass, pyrex, etc.)

>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?
>
The mirror should only move significantly if there is significant 
mechanical slop in the joints/bearings or you have a stretchy sling 
(assuming you use one).  Otherwise, the stiffness of the glass keeps it 
balanced on the flotation points.

>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?
>
The clips themselves are not bad for the image as they take up only a 
little area.  What hurts is cranking them down tight to hold the mirror 
in place.  Thus, the mirror should be held in place primarily by gravity 
with edge supports (e.g. sling) or with RTV to keep it from sliding 
sideways, with the clips kept a hair-width above the mirror to keep it 
from falling out should the tube go upside down.

>I would like to do a floating mount, but I don't want
>a "floppy" mirror.
>  
>
A well made floating mount is not floppy.

Jeff Anderson-Lee

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