[ATM] Re: Second Thoughts on Mirror Support

[jonah at eecs.berkeley.edu (Jeff Anderson-Lee)]

Dale Eason wrote:

>You are mixing to different processes here.  The
>carpet used to grind/polish the mirror works because
>we repeatedly pick up the mirror and put it down in a
>different orientation.  Thus using a semi random
>process to even out the high areas as we work the
>mirror.  This process will not work for a stationary 
>mirror in the telescope.
>  
>
The original John Dobson style scopes use carpet, not wiffle trees to 
support the mirror.  Thus it does appear to work to some (as yet 
unquantified) degree.  Think of  it as a 10000 point variable force cell.

>This discussion has forgotten first principals.  That
>is when you place two relatively flat surfaces
>together they will only touch at the three highest
>points between them. Because three points define a
>plane.  You can try make more points but it will still
>be only three most of the time.  Thus your nine point
>cell made on a flat plate will only be using three of
>the points.  
>  
>
That assumes that the two surfaces are stiff.  If they are not, one or 
both will sag slightly until more and more points touch.  By using thin 
metal over carpet, we are trying to make it the metal that sags more 
than the mirror and let the carpet balance the forces.

>Try this experiment.  Take three ball bearings the
>same size.  Place them on a slightly rough surface
>like a weathered board or a brick (this will simulate
>the metal plate at the scale of roughness we need to
>control to.) ...
>
Do the same on a carpet.  Does it still touch the carpet at just three 
points?  I don't think so. 

>Now this was all done at a different scale than what
>we need for our mirror cell.  It needs to not rock on
>the order of less than 1/10 the wave length of light.
>  
>
It needs not to sag on the order of more than 1/10 wave legth of light 
-- not quite the same thing.

>A whiffle tree arrangement can do this, has been doing
>this for years and is easy to make.
>  
>
Small wiffle trees are easy to make (4 to 9 points definitely and up to 
27 points with some care) and I will likely use them on the mirrors I 
deal with, but I'm thinking now of the monster mirrors like Jean-Guy's 
36 inch fused plate mirror.  It would likely require an 81 point cell: 
not an easy thing to assemble.

>Two flat plates (a mirror back and the cell) and more
>than three RTV globs can not do it.  You can't get the
>plates optically flat enough to know what three of N
>points are going carry the load.  So if you need more
>than three support points a whiffle tree is your
>better bet.
>  
>
I'm not convinced, given Don's seeming counter example, and I'm open to 
learning something new.

>Why do people spend so much time polishing a mirror to
>exacting standards and then expect to hold it with
>devices that are not up to the same standards?
>  
>
We are just trying to meet the standards in a new way.  Technological 
advance requires periodic innovation as the status quo never advances 
without it.  Besides, isn't that part of what ATMing is all about?

Jeff