quote from another group this morning>

There are actually other ways to make a high quality zero color
optical
system, one of which I will present at NEAF next week in a talk. It is
based on the Schupmann design. This system was first proposed in 1898
by Ludwig Schupmann as the answer to the problem of chromatic
aberration in refractive optical systems. Variations of the Schupman
by
Klaas Honders include Newtonians and Cassegrains which have unique
properties such as zero color, and corrected for all 5 Sidel
aberrations. This means they are able to cover huge fields without
field curvature, coma, astigmatism, spherical aberration, distortion,
or longitudinal and lateral chromatic aberrations. In effect, they are
totally diffraction limited over large fields. They can be made with
focal ratios from F2 to F20 in short tubes, and utilize only common
BK7
glass in their corrector and primary mirror.

New designs by Riccardi and Honders are so unique that I believe they
will revolutionize the world of imaging and perhaps also visual
astronomy. Are they refractors? Well yes, they do have a main
refractive element up front. Are they mirror scopes? Well yes, since
they also have a mirror similar to a Cassegrain some distance down the
tube, however, this mirror is actually a refractive element where the
light is bent by refraction as well as reflection. A Honders variation
of the Schupmann can be thought of as a Taylor Triplet lens, but the
center negative flint element is replaced by a negative meniscus with
back surface mirrored. As such it has all the properties of a Taylor
Triplet (wide flat coma-free field) but none of the chromatic
aberration.

If this all sounds too complicated, come to NEAF and see what is in
store for the future.

Rolando