Re: [ATM] Batterson address change

[Woodchuck <djv@bedford.net>]

On Sat, 1 Jan 2005, Jeremy Batterson wrote:

>
> Address change for Jeremy Batterson is jdbaterson@msn.com

 [reformatted to 72 col wrap]

> I) Fresnel lenses are inadequate for any but the most crude
> applications, but, as a matter of fact, in these days of nanotechnology
> and laser machine tools, would it be possible to produce a fresnel
> lens as a spiral with millions of grooves, somewhat like an old-style
> record, but with optical quality. This would be as a flat lens.
> There would still be color aberration.

Don't know about the spiral cut, but such lenses with concentric
rings are made.  "Diffraction lens" or "Diffraction surface".
They are modelled, for example, in the OSLO program.  I believe
(memory is poor) there is discussion in Warren Smith's "Modern Optical
Engineering".

> II) For those who want to produce F2 or F1 mirrors, the only
> problem is parabolization, which is very difficult at such ratios.

No kidding. For any "meaningful" diameter, the work seems to require
doing at the fine grinding stage.

(For those doubting this, an idea of the amount of glass (depth)
to be removed can be gotten by comparing the sagitta from the formulae
s = r*r/2R  (approximate for a sphere, exact for a parabola) with
s = R - sqrt(R*R-r*r)  (exact for a sphere)).

> In these days of computerized corrections, what of a CCD which takes
> the image from a perfect spherical F1 or F2 mirror and corrects for
> aberration?  Thus, the hard part could perhaps be circumvented.
> Would this be possible?

Dunno.  It's not clear what you mean.  It sounds as if you want to
produce an image with aberrations, then correct it for the aberrations
after it is recorded by a CCD, by using a priori information about
the image.  Nope.  Info lost is info lost.  I think this would violate
Shannon's information theorem, the equivalent of unshuffling a deck of
cards without expending energy, or knowing the pre-shuffled order.
(Sorry for the thermo example, but that's where I learned this sort
of thing.)  Yup, you could certainly take the image of a star
with S.A. and massage the data to make the image have a perfect
Airy disc, but you have used the a priori knowledge that it is the
image of a star to do this.  (This, of course, is how all the
"Hollywood" image enhancement in detective stories is done.  "Zoom
in on his wristwatch, Todd, see where it was made!" [looking at a
surveillance video from a 7-11 holdup].)  Maybe there are cases
when a priori information is valid, say in a spectrometer where
you *do* know the shape of a slit being imaged. (This is a thin ice
region of my brain, sorry).  It's tempting when thinking of star
images because we "know" what they should be.  How do we know
image X is a star, though?

On the other hand, by monitoring the image, active devices can
control the figure on a mirror.  (This was proposed to fix the
Hubble, but the range of the actuators was not great enough).  This
technique is, I believe, used to correct for bad seeing (and other
things) in certain large active-optics ground-based telescopes.

Figuring large fast mirrors is today much more routine (but hardly
a "piece of cake") than in the 1950's -- I mean in the professional
astronomical workshops.  Certain very clever polishing machines have
been invented.  Volume II of my Christmas present (thanks to my cat),
R. N. Wilson's "Reflecting Telescope Optics" has some discussion of
these issues, with many references to the literature.

Dave

>
>
>
>
>
> Jeremy D. Batterson
> Ridgefield Park, NJ
> (201)441-4888/(877)305-8856
> Literary & Astronomy Page:
> http://members.lycos.co.uk/jeremybatterson<http://members.lycos.co.uk/jeremybatterson>
> _______________________________________________
> ATM mailing list http://www.atmlist.net/
>

-- 
        In each of us, there burns a soul of a woodchuck.
        In every generation a few are chosen to prove it.

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