Re: [ATM] Mirror Cells - Springs vs. Push/Pull

[Ross Sackett <rsackett00@yahoo.com>]

--- jeffmorgan@mindspring.com wrote:
> I was wondering if there was
> any comments on using collimation pull bolts opposed
> by springs (which seems to be the predominate
> method) vs. using push/pull bolt pairs 

Don's idea sounds great but might be more hi-tech than
you need.  You can simplify things a bit i think. 
Both push-pull and spring-tensioned bolts require a
backing plate attached to the mirror that adds
back-end distance to your tube and interferes with
mirror chill-down.  Springs also do a great job of
storing vibrational energy, which can be annoying. 
You might consider letting gravity do the work of
sticking the mirror to your collimation bolts.  For an
8" all you really need is three collimation bolts
threaded through the wooden cell, with three steel
mirror clips bent into a squared-off J shape.  Put
round felt pads on the floor of the Js at about the
midplane of the mirror as edge supports.  They should
look something like this:

* *
*
*
*
*
*          
* * * * * * * * * 

Two of these clips can be permanently screwed to the
cell, but make the last one adjustable from behind the
cell so you can release the mirror when you need to. 
Use a playing card to shim the edge support so you
aren't pinching the mirror.  (Well, use 3 clips if it
is going to be on a dob mount and always oriented to
gravity the same direction; 4 might be better if it is
on a GEM, with 2 adjustible clips.) By the way, if you
use this cell for an 8" I suggest you ignore the PLOP
results that put the three supports quite close to the
center of the mirror--if you put them at less that
about 75% of the radius then the mirror will tend to
get hung-up on the edge supports as you collimate it. 
For a full thickness 8" the additional error of moving
the supports closer to the edge is trivial IMHO.

Ross


		
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