From: "Richard Schwartz" 
To: 
Reply-To: "Richard Schwartz" 


One thing you can do with plop is work out a support whose diameter is the
major axis of the ellipse.   Then project that circle back onto the
ellipse.
Because you are shrinking one dimension, it is guaranteed that you will
shrink the distances between points and therefore you will shrink the
opportunity for the glass to flex, and thus you shrink the surface error.
So the surface errors of the large circle are an upper bound to the
deformation of the ellipse.

Similarly, project the ellipse onto the SMALLER circle, and you get a lower
bound for the deformation of the ellipse.

Next, project the ellipse edge wise onto a straight line.   Now you have a
beam with supports and can evaluate the deformation easily.   This is a
lower bound to the deformations of the ellipse.   (I don't know which lower
bound is more useful, but you can do beam calculations without finite
element software.)

. . . Richard

----- Original Message -----
From: "Dwight K. Elvey" 
To: 
Sent: Thursday, March 13, 2003 10:04 AM Subject: RE: ATM Large spider and
secondary holder


>
> >From: "Tom Krajci" 
> >
> >As scopes get larger, secondary mirrors get larger.
> >
> >Up to now, mechanical edge clips, supporting the secondary mirror, have
> >worked well for the amateur.  But as secondary mirrors get larger, and
> >probably thinner to keep weight down...edge support of a large secondary
> >mirror will distort the mirror too much and image quality will suffer.
> >
> >What then?  Upside down 'flotation cell' design for large thin secondary
> >mirrors...with RTV bonding to the secondary?
> >
> >What does PLOP say for edge supported mirrors?...at what size/thickness
> >does image quality begin to degrade?  (I know, secondary mirrors are
> >elliptical, but PLOP would give us a first approximation.)
>
>
> Hi Tom
>  Maybe we need to modify PLOP to find the best support for a
> secondary that allows the mirror to sag a little so that it
> always forms one of the family of hyperbolic off axis shapes
> that keeps the image correct. This way, a thinner mirror can
> be used and the only disadvantage is that the focal point
> moves slightly as the telescope is moved about.
>  We might find that the normal three point mounting is not
> really the best for a secondary. It might be that a beam
> 4 point was actually better.
> Dwight
>
>
>
>
>

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