Re: [ATM] Temperature sensing and adaptive optics

[Dominic-Luc Webb <dlwebb@canit.se>]

On Thu, 10 Nov 2005, Peter C. Chen wrote:

> Dominic-Luc:
>
> > Another thing I have been thinking about, more
> > like bad science fiction than science or engineering, is
> > the use of heat differentials to bend optical and other
> > surfaces. Surely people have done this, but I am not
> > personally aware of and actually application of such a
> > method of adaptive optics.
>
>             I guess great minds think alike,  That is not a bad idea at all.
>
>             Andre Couder of the Paris Observatory demonstrated (since the
> 1930s) that embedded heater elements could be used to control the figure of
> a telescope mirror.  (A. Couder, Thermal Distortions of Telescope Mirrors
> and their correction,  Vistas in Astronomy, v.1, 372-376, 1955).  The 1.2
> meter mirror at the Observatoire de Haute Provence performed very well for
> many decades using this method.  Until, if I recall correctly, someone who
> did not read instruction manuals plugged the heater into line voltage (220
> ac?) instead of 12 v or so dc.
q>



Wow, I had no idea anyone had ever done that (I mean, plugging in the
heating element for a telescope mirror without first consulting the
instruction manual). What an idiot....

:)

Just kidding.... Really, I had no idea anyone had ever built a
thermally controlled optical figure. I have toyed with this mainly
because I had components sitting right in front of me and gave it
a try, mainly just to try something silly when I was too lazy to do the
work I was supposed to be doing. I sensed maybe it could be used, but
never managed to convince myself it was sane.

So now that we have drifted from temperature in tubes to thermal figuring.
I note that my sci-fi application of using this approach was tried on
thin mirror, siting the link....

www.digilife.be/club/Johan.Vanbeselaere/atm/optics/figuring/thermally.htm

"The 1950 article out of Comptes Rendus confirms the experiments of
healing the cooling effects of a 81cm disk, 7.6 cm thick. The edge
effect is a kind of spherical aberration : overcorrection 3 times
the Rayleigh limit, a hyperboidal form, besides the change in focal
length. The heating procudure needs 3 W and for every Watt there's a
change of 0.81mm longitudinal aberration of the outer rays.
25W produced an enormous but very regular undercorrection. Half time of
the process healing the cooling-down effect was only 8 minutes. The
mirror is constantly losing 22W during a night, so a surplus 3W was not
sensibly detoriating the images by convection. Much more visible was the
amelioration of the images by the disappeared turned down edge ! Solar
coelostats are much more detoriated than a normal cooling-down mirror,
so they could benefit a lot of a appropriate heating system at the back."

Scaling this down to mortal proportions, this would be equivalent to a 200
mm (8") disk, 7.6 mm (1/3") thick. This is in line with many "thin"
mirrors we have frequently discussed in the past. The electricity required
is rather much in line with more powerful batteries. I note that kanthal
heating elements are cheap and operate quite precisely at lower
temperatures. I could envision winding some heating elements around the back
of a mirror in concentric ring patterns. I need to think of how to
control it, not sure if I would use a AC diac/triac type design or just
batteries and trim pot and/or frequency modulation  from 555 or some such.
I would beed some advice here.

I am very curious about more suggestions from the list. This is starting
to get interesting....

Dominic-Luc Webb

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