RE: ATM 18 inch telescope

["Mel Bartels" <mbartels@efn.org>]

>>>
3.  Even if you reduce the blockage from 50% diameter to, say,
30% (thereby only reducing the tube length by 30% or so), the
diffraction image isn't very wonderful.  About 68% of the light
goes into the central disk and 22% into the first bright ring.
That's also unacceptable for planetary viewing, but probably OK
for low power viewing and wide-field photography.
<<<


I've seen superb planetary images in R-C scopes with 1/3 obstruction.  A
20 inch f/5 scope with 5 inch flat to fold the light back 2 feet still
has only 1/4 obstruction - better than many small scopes that are meant
for planetary observing.


>>>
5.  Baffling, Michael Lindner suggests, could be a problem if
the first mirror is tilted, and the eyepiece made to look at
sky.  This is similar to the baffling needed for a Cassegrain,
and would require a long tube extending inward from the eyepiece.
And this tube would cut into the light path badly (causing all
sorts of mayhem to the diffraction image) for a tilted folding
mirror.  If the first mirror isn't tilted, then the problem is
similar to Cassegrain systems with a "Nasmyth" tertiary focus,
and solvable using a suitably long baffling tube.
<<<

The focuser of the typical large dobsonian also faces the sky.  With
focuser and diagonal baffles, the sky background even with lights
shining on the scope is very dark.  I don't see why a folded reflector
would have more trouble (unlike a cassegrain, which has a lot of trouble
baffling for wide and narrow angle views).

Mel BArtels