Re: [ATM] Mirror Cells

[ecairns <ecairns@mcmaster.ca>]

Jerry Reddell wrote:

>.. Some people say it equalizes the load to each
>point, I seriously doubt that it does that exactly. 
> 
>
If  you wish to SUPPORT your large thin mirror without distortion the 
supports MUST BE DESIGNED to deliver a FORCE rather than to define a 
position. It is like trying to approximate an air bladder ( or a double 
pan balance,almost) to take up the weight (a force) of the mirror 
locally at many points thereby avoiding bending.  Ideally the mirror 
should then not move, relative to its support system, if alignment 
provisions are provided to tilt the mirror support rather than  the 
mirror itself.  (Of course we must not overlook the obvious, that in any 
true balance system, the load is quite free to drift anywhere within the 
free range of the balance arm if disturbed. Something more must decide 
where the balance rests.The chemist's double beam balances do not 
generate perfect balance in all positions of the arm, they exert forces 
as if they were  very flexible springs You could if you wish consider 
them as a combination of a perfect balance force and a weak spring.) 
This lack of motion between mirror and support might suggest that the 
pivot arms could be done away with by careful design and craftmanship. 
However, such a rigid mount ( all support points are now verrrry stiff 
springs) would distort the mirror if there were the slightest 
differential temperature distortion, unless of course, the mirror were 
much stiffer than the now possibly many(any number greater than 3) 
POSITION defining points. At this point the inevitable solution for a 
thin flexible mirror becomes clear-- take up the majority of the 
mirror's weight ( ie. balance at many points the force of gravity on the 
mirror which will change with telescope pointing) on the force 
generating  support system and leave  only enough  weight  on three 
bottom points and two side points  to define the mirror pointing without 
distorting the mirror. THIS IS HOW EXACT THE LOAD AT EACH POINT MUST BE 
CALCULATED---   WITHIN ERROR,THE RESIDUAL LOAD EXERTED BY THE POSITION 
DEFINING ELEMENTS MUST NOT DISTORT THE MIRROR BEYOND YOUR DESIGN 
CRITERION. 
    The silicone solution  is an interesting mix of force and position 
defining elements which without carefull design probably only works for 
stiff mirrors where it really acts as a position defining system. If 
your alignment does not constantly run amuk during observing ask what is 
defining the mirror alignment without distorting the mirror as it cools? 
Keep in mind that the mirror and it's support will inevitably have 
different rates of thermal expansion.

>Without the pivots, one would have to machine the support points to a very
>high degree of accuracy to get all of them in alignment with the mirror...
>.
>

Three bottom position points however crudely fashioned are very safe for 
a thick rigid mirror or a thin mirror with most of its weight "floated" 
on a force generating balance system.

>
>Jerry Reddell
>Lubbock Tx
>
>-----Original Message-----
>From: Ulhas Deshpande;
>Subject: Re: [ATM] Re ;Mirror Cells
>
>  
>
>>All mirror cell designs for 6 point upwards seem to have support points
>>mounted on bars or triangles. Is this essential? why not just put dabs of
>>silicone at the positions given by PLOP and paste the mirror on? Am I
>>missing something?
>>    
>>
>
>
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