Re: [ATM] 7 inch spherical mirror

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

On Tue, 15 Apr 2008, David Harbour wrote:

> Thanks; just wanted to have at least some hazy idea of what is going on. The
> entrance pupil to any system employing a paraboloidal primary (and also a
> Ritchey Chretien, as well as a Classical Cassegrain, and the ever popular
> Newtonian, is of course the primary). The entrance pupil to a Schmidt, is,
> on the other hand, at the aperture where the corrector lens is, or should be
> (lenseless Schmidt).

Sounds right, albeit this is the pupil diameter, not location. I
might state that the entrance pupil is located where a corrector
is not needed. I think entrance pupil may be located at at/near
the focal length for a Newt, but at ROC for a Schmidt. I deduce
this not from any academic formality, but what I have seen in
some Oslo files. For my Schmidt designs (lensless or with
corrector), it is situated at/near ROC. In examining some
longer F/ratio spherical mirrors, like F/10, I have also
looked at smaller entrance pupils, because I am used to
working Schmidt designs. In my case, it has been issue of
giving my existing lensless Schmidt designs in Oslo a longer
F/ratio, such as F/10. This was some years ago, but looking at
the posts from this list, I think this explains why I do not
see much coma on a spherical lensless design at higher F/ratio,
and also not so much spherical aberration. It is very easy
to change the aperture stop as desired.


> Does anyone know that Schmidt himself was reputed to have told someone that
> an attempt to figure a corrector plate for any faster system then f/2 by the
> vacuum method can, and usually will, break the plate?

I have heard this over and over again. I have a couple of
articles by Schmidt, or by his collegues. I need to check
these references again to be sure. I remember something
to the effect that beyond some critical cutoff, a corrector
can be added. The deduction is that below that cutoff, no
corrector is needed. I suspect lensless "Schmidt" (i.e.,
spherical primary with aperture stop at ROC) designs may have
once been quite common. I need to check the references I have
to see if Schmidt specified the vacuum technique, but I do
not think I do. I would be quite happy to see any reference
actually from Schmidt.

Does anyone know for sure that Schmidt ever used a vacuum
method?

Dominic

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