Re: [ATM] Mirror cell design

["Nils Olof Carlin" <nilsolof.carlin@telia.com>]

Wouter,

> > The best solution *I* can think of is here:
> >
> > http://w1.411.telia.com/~u41105032/sling/threepart.htm
> >
> > - it ought to work up to a maximum tube/cell rotation of +-45 deg from
> > neutral - I leave it to you to figure out if that would leave any part
of
> > the sky inaccessible to you (if so, you could perhaps extend the 90 deg
> > elements to 120 deg and overlapping, giving you +-60 deg - perhaps worth
> > thinking of a bit more). But I haven't actually tried it on an eq mount,
so
> > I can't make any promises.
>
> This looks familiar. You didn't post this link here a few days ago did
> you? If so, sorry, I didn't have a closer look then :)

I must confess I did - that page was just written then.

> The idea of this support is clear. When the mirror is close to being
> vertical, not much support to the sides is needed. As soon as the mirror
> gets tilted enough it gets in need of support to the side (now facing
> down) and this support must be constant over the complete lower side of
> the mirror.
> So, is there an alternative to using slings? I suppose creating a ring
> around the mirror would not leave enough room for the mirror to expand
> under heat, right? How about putting a rubber ring inside a metal ring?
> Wouldn't that give enough support and not too much friction?

I expect there is no really good solution for thin mirrors allowing as good
operation at any tube rotation, including mine. At 45 deg rotation, the
mirror will be supported around only the "bottom" 45 deg, not 90 deg as
(supposedly) ideal, but I believe the operation will still be decent if you
only make sure that the forces can't pull the mirror edge back-to-front.

Using whiffletrees with ball bearings for the edge, supporting at +-22.5 and
67.5 deg from the vertical diameter may also be good enough - and may even
allow a little more rotation before things get "decidedly prejudicial". Or
even at +-30 and 90 deg, by making the leverage somewhat unequal (though I
don't know how!!).

I suspect that the rotation of the tube (inevitable in an EQ mount unless
you actively de-rotate - one possibility but probably rather awkward) will
involve some compromise - if the mirror is really, critically thin, you
should perhaps avoid a true EQ mount and settle for either driven alt-az or
a platform.

Just some thoughts at the moment, all of it may not hold water for critical
thinking.....

Nils Olof


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