To: 
From: Steve Follett 
Cc: "Bill Russell, Bull Russell" 
Reply-To: Steve Follett 


Hi all,

A local astronomy club, the Valley of the Moon Observatory Association,  operates a
public observatory and we are making a 40" scope replace an existing
one.  We would like it to be a Nasmyth design or something similar so that
we can keep the
eyepiece position near the ground and stationary, if possible.  The scope we're
replacing is a conventional Alt-Az and requires the use of a ten foot high
ladder/platform.
Given that this scope is going used by the public in the dark, we feel that a ladder
  is too dangerous.

The primary mirror will be 40" F:3.6 (this is the mirror that Jeff
Baldwin posted here a
few months ago).  Another thing we're trying to do is keep the secondary size down to
below 30% of the aperture, so one idea I had was to make a long FL Cassegrain and
then use a focal reducer to bring the overall focal length back down to
where commercial
eyepieces can be used for relatively low powers (5-6mm exit pupil).

I'm contemplating making an 16" F:30 classical Cassegrain/Nasmyth as a test bed
for that idea and I've got some questions about focal reducers.

1).  How do you calculate the effect of a focal reducer on system focal
length?  If you start with 12M focal length and you use a 1000mm lens as a
reducer what is the resulting focal length.  Does the relative position of
the reducer and the eyepiece effect the result (as with a barlow lens) ?

2).  How much reduction is practical?  Does f:30 to f:8 make sense?

3).  Is a simple lens like an achromatic doublet adequate or does a focal
reducer really need to be designed from the ground up as such?  This scope
is strictly for visual use, so things like field flattening are not as
crucial as they
would be for photography.  Are commercial designs (Celestron, Mead)
specific to those scopes or are they usable as general-purpose reducers?

4)  How does one learn to use OSLO.  Are there any resources available
beyond the program documentation?  Are there any good introductory texts on
optical design available for those who are too feeble-minded (me) to grok
OSLO by itself?


Steve Follett 

--- BBBS/NT v4.00 MP