[ATM] Re: PLOP 101

[fabio@voreas.com]

Ross Sackett writes: 

> Bob, there's something I've wondered about.  I assume
> that PLOP is looking at the worst-case scenario with
> the mirror flat on it's back facing Dobson's Hole at
> the zenith.  Is it the case that the deformation would
> be less when it is pointed at lower elevations?  I'd
> imagine it would vary with the sine of the
> altitude--does that sound about right?

The moment you begin tilting a mirror, other factors come to play that will 
change the nature of the deformation. So you're right in saying that the 
deformation Plop calculates decreases with altitude. The problem is that the 
mirror now will tend to slide off the cell, which it won't because of 
mechanical constraints. These mechanical constraints--silicone blobs used to 
attach the mirror to the cell, metal band around the mirror, or any other 
kind of edge support--will cause the mirror to deform in different shapes. 
Plop, as is today, can't calculate these other deformations. Several 
people/institutions have developed the required calculations to characterize 
the problem you pose (I don't have my references handy, but a Google search 
can yield some interesting gems). 

Bottom line: the more extreme one's design becomes, the more careful one 
should be in designing and constructing one's mirror cell and for that 
matter the rest of the scope around it. 

Fabio
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