Re: [ATM] Large Schmidt Camera?

["vladimir sacek" <vla@toast.net>]

vladimir sacek wrote:
> It won't work like Schmidt camera, but maybe something useable could
> be done with a split reversed meniscus. [...]
> It can correct spherical aberration and astigmatism of fast spheres quite
> well, with acceptable chromatism induced.

Well, I did take a closer look. Split reversed meniscus can reduce
horrendous spherical aberration of a 16" f/2.5 sphere to acceptable 
levels (between about 1/4 wave and 1/2 wave p-v at 0.2f and 0.5f
from the focal plane, respectively). But it adds coma, and it can't be 
reversed, only minimized. Corrector separation of 0.2 f from the focal 
plane results in more than double the coma of the mirror alone, while 
at the 0.5f separation it is 20%-25% greater than that of the mirror alone.

With both lenses of the same glass (BK7), secondary spectrum can be
eliminated (at the expense of unequal radii), while relatively low 
spherochromatism (~15 microns violet blur) remains. Split meniscus
of about 50% the aperture, in combination with Paracorr, should be
able to produce field with about 25 microns sagittal coma at 10mm
of-axis. That would be an effective ~f/2.9, with the field comparable
to an f/5 parabola. 

It would likley work better if split meniscus would be combined with
a tailor-made coma corrector. It should allow for smaller lenses, and
might result in somewhat wider useable field. That would be four
lenses altogether, a split meniscus and a coma correcting doublet, 
most likely a negative and positive meniscus combo (although the
positive element could be biconvex, as in the Ross doublet). Having 
the possibility of varying the amount of spherical aberration with the
split meniscus would make designing the coma correcting doublet
easier.

Vlad

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