From: "Tony Owens" 
To: 
Reply-To: "Tony Owens" 


Tom Krajci wrote:

>Date: Wed, 11 Dec 2002 19:11:26 GMT
>From: tkrajci{at}san.osd.mil
>Subject: ATM Main mirror cell design - need help with bearings
>
>I've posted a discussion of the problem I am facing with main mirror
cell
>design.  I need to eliminate 'slop'.  See:
>http://overton.tamu.edu/aset/krajci/scope-design.htm )very bottom of
page) >and specifically:
http://overton.tamu.edu/aset/krajci/cell-bearing.htm
>
>I am trying to find ways to use rolling element bearings to solve two
>different problems:
>
>1.  Tilting of the pivot/balance member in the main mirror cell.
>
>2.  Movement of the 'lead screw' collimation bolt.
>
>All comments and feedback are welcome!  (Once I get some good ideas
>together
>on how to approach these design problems, I'll add them to the
website.)
>
>Tom Krajci
>Tashkent, Uzbekistan

Interesting concept doing such a big moving-primary focusser.  My instincts
would be similar to Don and Richard - flexures make a lot of sense here. 
This is why:
1. for non-visual use as you mention the scope will be used for, limited
range of motion need not be a problem.  Less than 16 mm or so should do,
with coarse adjust at the top ring.  Limited displacement is the downside
of most flexures.
2. in any kind of astrocamera, photometric or otherwise, you don't really
want any clearance or significant compliance in whatever connects the
primary mirror to the telescope structure.  Flexures can be a relatively
low mass way of making this connection while leaving the optic free to move
a bit axially, with accurate and repeatable straight-line motion.
3. a bit of thought usually results in a lower parts count solution when
designing a mechanism with flexures, compared with the alternatives.
Generally this leads to lower mass, which is good when doing a mirror cell,
because the thermal time constant of the cell structure is shorter,
minimising cooldown issues.

While there are lots of ways of doing it, a simple and elegant method I
favour is to use parallel disk flexures.  Imagine a drum, with a short
shaft through it axially, fixed to the top and bottom skins. On one end of
the shaft, the mirror is attached, by means of either a whiffletree cell,
or (preferably) a hub mount.  The other end of the shaft can be driven
axially to and fro, using whatever - a 1" stroke micrometer head for
example, available fairly cheaply. I've got some nice concept CAD drawings
and JPEGS of this arrangement for an SCT mirror focusing upgrade kit I
toyed with a while back.  The 16" version weighed 5 kg and had a
stroke of 12mm or so.  Contact me off-list if you're interested...

Tony Owens
London UK

--- BBBS/NT v4.00 MP