Re: [ATM] Schmit corrector grinding

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

On Tue, 13 Feb 2007, matt wrote:

> I accidentally came across a very interesting alternative to a custom field
> flattener for a curved focal  plane scope a couple of years ago .
> It is a coherent fiberoptic faceplate.
> I have no idea of their original cost but they're available from Surplus
> shed for $5 each .
>
> http://www.surplusshed.com/pages/item/l3172.html
>
> These might make a great distortion and aberration free field flattener .


Yes, I have seen this before and was not real sure how to implement
this as a field flattener. The blurb on that site does not specifiy
if this is plastic, optical glass or some other, nor any spectral
data. Clearly it was intended to be used with things like CCD, which
is plastic makes plastic possible, since mainstream commercial CCD
market is not interested in UV or near IR. Have you or anyone else
here successfully made a flattener with this? I have at least one
Schmidt scope that could utilize a corrector of this diameter,
maybe.....

There is a proviso here.... atl east some of my Schmidt designs
gain (probably) nothing from a flattener. I stumbled on this when
I actually built a curved film support that used vacuum to force
film onto a curved glass surface. It was quite easy to make
actually. I just used the tool from grinding a small convex of
appropriate sagitta (took a few minutes, actually). Eyeball analysis
of the surface revealed an important reality. The curve was much less
than the thickness of the film! Examining some defocused spot digrams
covering the thickness of photographic film for some of my designs
with curved focus made me suspect I am wasting time on on flat
flattener to replace the film with a CCD because very little (and
probably nothing) was gained. I think my rationale was based on
comparison of grain size of film and pixel size on a CCD chip. I had
the idea of someday having a really massive 1" to 2" square CCD with
massive pixels, but market seems to have gone the other way, with 1/4"
and 1/3" chips with many millions of tiny pixels. Having learned to
be realistic about how market economics operates, I now design
according to market forces. My most recent Schmidt camera project
(a 310 mm F/1.9) can use 35 mm film or common CCD chips.

>From the test images on the primary without the corrector, I am
not sure how much will be gained using a flattener. Along the
same lines, in my work, we have techniques like deconvolution and
I have contemplated merging images from a few focal positions in
order to capture a perfect focus for the entire field using similar
algorithms. Basically, this would eliminate the need for a
flattener. In a lensless Schmidt config, this removes all lenses.
Has anyone done this or thought about this?

Dominic-Luc Webb

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