Re: [ATM] Basic mirror support question...

[Dale Eason <atmpob@yahoo.com>]

My son is a senior in high school and taking physics. 
It is one of his favorite classes because he get to
play with stuff.  So I have a warm spot in my heart
for physics teachers.

Vlad already responded with the correct answer. 
Basically it surprises most people that two plan
surfaces no matter how smooth will only touch at three
places.  From math we know that three points define a
plane.  The two plans will define a third plane where
they touch.  They will touch at the mutual high points
on their surfaces.

Because of that we cannot determine easily where those
three points are.  For mirrors that are thick enough
it does not matter.  But as mirrors get thinner and
start to sag and need support then it does matter.

Remember for a perfect mirror we are trying to keep
the surface from sagging by only 1/8 the wavelength of
light.  If we could make a flat surface and the back
of the mirror was perfectly flat then the flat surface
would need the same accuracy.

One way to support the mirror accurately is to apply
equal force to all the points that PLOP calculates. 
Some people wonder then if it would be alright to use
a stiff flat surface and a spongy pad to assure that
the mirror is supported at all the necessary points. 
Yes it would if the supporting flat plate and mirror
back are flat to 1/8 wave and the spongy material has
a uniform spring constant to 1/8 wave all across its
surface. None of that is likely.  So we use wiffle
trees or astatic supports to provide proper support
were necessary.

All of that applies only if you want as close as you
can get to a 1/8 wave sytem.  You can get away with
less if you settle for less accuracy.  What most of us
really have is somewhere between our lower limit and
1/8 wave.  From reports of experienced observers it
would seem most are closer to the lower limit than 1/8
wave.  So many get away with poor cell principals.

Dale Eason

--- "P.K. Ferrick" <pferrick@tows.moric.org> wrote:

> Hi all,
> 
> I'm a high school physics teacher from Northern NY
> and, recently, the 
> recipient of a donated 17.5 inch F4.5 primary
> mirror.  My science club 
> students and I are planning on building a Dobsonian
> scope with the 
> mirror.  Since I've been hovering around the fringes
> of the ATM 
> community for quite a while, I think I have a pretty
> good idea of what 
> the scope will look like, but there are a few
> questions that I haven't 
> been able to find answers to in the archives, so
> here's one:
> 
> Regarding mirror cells:  it seems to me that the
> more support points one 
> uses, the better able a program like PLOP is to
> generate a low-flexure 
> design.  Why then wouldn't a flat, continuous
> surface (ideally an 
> infinite number of points) give the best support? 
> 
> thanks,
> Patrick
> 
> _______________________________________________
> ATM mailing list http://www.atmlist.net/
> 


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