[ATM] Thermal Pre-Distorting

["Richard Schwartz" <richas@earthlink.net>]

See Roark's Formulas, Table 24, case 8 to calculate the shape of an annular
disk under a front-to-back temperature difference from the edge to a certain
radius.    For a circular plate, see table 24 case 15.   I believe you can
consider two such circular plates with different radii to find the
deformation when the center zone is heated.  (You might want to cheat by
applying negative heat to the outer zone.)   For mirror polishing, the glass
is simply supported (no moment applied to the edge), so use case 15a.

Roark's text Article 15.6 deals with thermal distortion.  Example 5 says
that a flat plate with uniform temperature difference front to back will
assume a spherical shape with radius that can be calculated by a formula
that is undecipherable.

Roark provides reference to "Elements of Thermal Stress Analysis" by D.
Burgreen, C.P. Press, 1971.   I have not had seen this book.

----- Original Message ----- 
From: "matt" <electro_optic@bellsouth.net>
To: <atm@atmlist.net>
Sent: Friday, June 17, 2005 4:22 PM
Subject: Re: [ATM] Too Small Optics-( and flex produced optics)


> there are some quite old literature references and newer web references to
> predistorting optics in order to achieve some aspheric figure after
> polishing the predistorted surface.
>
> I posted earlier in this thread links to the Stewart Obs. Mirror Lab and
> their stressed lap method to generate very large aspherics .
>
> I recall reading about Couder figuring several large mirrors through
thermal
> predistortion by applying controlled heat through the mirror back with an
> electric resistor and polished it spherically. More elegant than gluing ,
> pulling with harnesses, warping with mechanical means, just let thermal
> expansion do the work.
>
> I also recall reading somewhere in Texereau some references to
parabolizing
> a mirror by polishing it spherical and using thermal distortion .
>
> For all the above, power levels seemed to be relatively low, in the 10
Watts
> or so range , for large mirrors ( the Couder references were talking about
> 1m class mirrors ) . All these mirrors were made a half century ago or so
.
>
> There was I recall a more recent article (maybe past 10-15 years) in S&T
> about someone adjusting (in the field) his telescope figure by adjusting a
> heater , basically the guy was turning a knob/potentiometer to adjust how
> much his mirror back was getting heated and controlling its figure .
> Primitive but allegedly it worked .
>
> Returning from ancient history to more modern times, there's at least one
> company called Okotech who makes a thermally adjustable flexible mirror
> commercially. They have a thin facesheet mirror , supported by simple
axial
> resistors and programmable current sources for the resistors. The copper
> leads and resistor body are the actuators , the other end is solidly
placed
> on a thick and rigid metal base plate. Cost for ATM's would be very low,
> resistors are a dime a dozen literally .
>
> Meade is vacuum flexing their corrector plates for the new RCX400 scope
> line, which is advertised as pseudo-Ritchey-Chretien , and the plate is
> different from the regular Schmidt variety in their venerable all
spherical
> SCT line .
>
> I'm sure it could be tried using 2x4's, chewing gum, hose clamps, plywood
,
> maybe even patented given the ineptness of our USPTO .
>
> best regards,
> matt tudor
>
>
>
> -----Original Message-----
> From: john sherman <atm@mail.johnspics.com>
> To: atm@atmlist.net <atm@atmlist.net>
> Date: Friday, June 17, 2005 4:59 PM
> Subject: RE: [ATM] Too Small Optics-( and flex produced optics)
>
>
> >
> >hi Jerry,
> >
> >>you could tighten a large industrial strength
> >>hose clamp around the circumference making it smaller.
> >> ...
> >>OK, OK, I see now. It is feasible. I think this idea
> >>could warrant a patent.
> >
> >
> >Go ahead, get the patent. But try it first, before spending the money.
> >
> >I once had a beautiful sphere that I tried to parabolize with hose clamps
> around the edge (there is one on this list who remembers). The force
exerted
> by the clamps has to be perfectly even at every point on the edge. In my
> setup, it was not. So there was some atrocious astigmatism.
> >
> >Good luck,
> >
> >John
> >
> >
> >
> >
> >
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> >
>
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