Re: [ATM] Looking for information on a telescope design

[dave w <daze39@earthlink.net>]

Steve Chambers wrote:

> Hello,
> 
> I am thinking about building a large (~30") scope.  I remember reading
> about a nice design that kept the eyepiece at a comfortable height.  The
> scope looked like a large Dob but observing was done through a refractor
> near the base.  The light cone was somehow folded and collimated before
> entering the refractor.  Does anyone know what I am talking about?  The
> name of the design and a link would be great.


Sounds like you're thinking of something like what is often called the
"Nasmyth" configuration: it's basically a "Cassegrain", with a convex
secondary mirror near the primary focus that partially diverges the
focused beam and reflects it back toward the primary, but instead of
passing through a perforation in the primary, it is reflected, by
a small diagonal mirror, to an eyepiece location at the side of the
telescope near the primary. Nasmyth's original telescope of this type
passed the optical path through one of the elevation bearing shafts -
his mount had a seat mounted on the azimuth turntable and a pair of
handwheels within easy reach, with which he could aim the telescope
at any point in the sky while looking through an eyepiece that
remained stationary with respect to the seat.

This layout could be applied to a large "Dobson" style mounting, with
an eyepiece located at a convenient point on the side of the mirror
box... it would make more sense than the usual "Newtonian" layout
used with such mountings - which, for a large telescope, places the
observer on a tall ladder! (Would this configuration be called a
"Dobsmyth"?)

-dave w

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