Re: [ATM] 8" f/3.5 !

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

Francis J. O'Reilly wrote:

> Any thoughts on the usefulness of this mirror, or am I wasting my time 
> with
> something that will never give me much utility?

There's number of things that can be done with an f/3.5 sphere,
assuming one is willing to make or buy a full aperture corrector.
Even a simple Jones-Bird sub aperture doublet could do a decent job,
especially if achromatized.

Lenses aside, the mirror could be a part of an f/4.9  three-mirror
folded Cassegrain-Gregorian (that's what it is). "Three-mirror"
probably sounds a bit scary, but the optics are actually very
easy to make. The secondary is also a sphere, ~1.1" in diameter
 f/5.26, placed just inside the primary's focus, the small folding diagonal
can be anything from ~.25" to ~.75" minor axis, depending on
the size of the field desired, and the tertiary is a >4" f/3 ellipsoid
with -0.4 conic. Diffraction limited (by astigmatism) field is nearly
1/4 degree in diameter, although the actual visual field should be
considerably better, if conventional eyepieces are used (offset
between the mirrors and the ep). The field would be limited to
1.25" barrel eyepieces. Peculiar but usable design.

Vlad 

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