ATM Clyde Bone's Mersenne Nasmyth (long post)

[ceragioli@uh.edu (Roger C. Ceragioli)]

The mystery telescope that Steven Lee was referring to is Clyde Bone's
20-in. Mersenne Nasmyth.  It's described and pictured in "Amateur Astronomy"
magazine #4 for Winter 1994, pp.45-47.  Clyde had it at the Texas Star Party
(he's from San Angelo, TX) last spring.

It does consist of two confocal paraboloids each f/5, the primary being
20-in. dia., the secondary being 4-in. dia.  There is then a tertiary flat
in the Nasmyth position that directs light to a quaternary flat outside the
telecope.  The emergent beam is collimated light.  The quaternary mirror
directs this light into a fixed-position 4-in. f/10 Genesis refractor.  The
net effect is equivalent to a 20-in f/10 telescope, BUT the eyepiece
position is fixed at lawn chair height.  The telescope itself is a
truss-tube, mounted in Dobsonian style.

I never looked through the thing at TSP: I couldn't stand the smell of
Clyde's cigars!  Clyde is an older fellow, and he told me he wanted to build
this telescope so he would have a 20-in. he could still look through even if
someday he's confined to a wheelchair!

His telescope is certainly very compact.  The design of a Mersenne with
Cassegrain-like optics being viewed with a refractor has only recently
become practical, because you need a flat-field refractor of short focal
ratio to end up with a usable scope.  The final focal ratio of the system is
equal to the refractor's inherent focal ratio; moreover, since the Mersenne
is a species of Cassegrain, it produces an outwardly curving field (like a
Cass) that's quite curved.  If you try to combine a Mersenne with a
classical refractor (which also has an outwardly curving field) you end up
with a disaster.  The off-axis image has enormous astigmatism and field
curvature.

The chief peculiarity of a workable Mersenne is that it shows a low-power
field superimposed on a high-power field, unless it's fully baffled.  The
low-power field is produced by the sky that the refractor sees around the
secondary mirror; the high-power field is produced by the main part of the
instrument.  Only if you block all the sky background with a baffle will
this phenomenon disappear.  Rutten and van Venroiij discuss this problem in
their book.

Interestingly, it is possible to produce a Mersenne-Gregorian.  If you use a
concave primary and secondary, the curvature of the resultant beam (being
inward) nearly cancels with the outward curvature (and astigmatism?) of a
classical refractor.  If you use a good COMA-FREE refractor doublet for your
viewing telescope, the on-axis and off-axis images are excellent, the field
is much much flatter, and color is no worse than in the refractor by itself.

I raytraced a 10-in. Mersenne-Gregorian with f/5 primary and secondary (both
parabolized) and combined with an air-spaced 2-in. f/15 classical refractor
doublet.  This combination resulted in the equivalent of a 10-in. f/15
Cassegrain-like telescope with a 20% obstruction.  The spot diagrams were
excellent over a 2-in. field and had no more color than a decent apochromat.

This kind of telescope could be excellent, and I seem to remember an old
copy of S&T from the 60s in which someone built a telescope like this.  The
biggest problem with this telescope is baffling out the stray light so that
you don't get two superimposed fields.  That increases the obstruction
ratio.  Another potential problem is centering and aligning the viewing
refractor.  I don't know how sensitive the system is to misalignments.  And
finally, the distance between the primary and secondary is much longer than
an equivalent Cassegrain, since this combination is a Gregorian.

Roger Ceragioli