Nasmyth Telescope Designs

You are right about it being AltAz. The mounting for Nasmyth's own reflector sat on a circular rail and the telescope was mounted using two geared trunnions, with which the focuser was coaxial. The workings of the mount were pretty much the same as barbette gun mountings of that period.

But the mounting was simply an application of existing technologies. Nasmyth's important invention was the optical layout of the telescope itself, which allowed him to operate the mounting and look through the eyepiece without ever moving from the seat he had integrated into the mounting. This was a significant improvement over equally large reflectors of the time, which required the user to stand on a ladder or set of steps (in itself a little risky for middle-aged astronomers on freezing cold nights) that had to be moved with the telescope to track an object.

(Nasmyth's alt-az design only needs two extra mirrors - secondary (circular) and tertiary (elliptical) flats.)
The down side of these designs is that you loose about 3% or 4% of the light with every reflection or lens... so you want bags of light to start out with or 99% reflectivity coatings on everything!
Phil.

Here is an image of layout of the Nasmyth design:

If the design, is using flats for the secondary and tertiary mirrors, is there no effect on the focal length?
Is there a set of formula to determine the distances between the mirrors and the focuser?
How much short can the tube assembly be, typically with a Nasmyth compared to a Newtonian Reflector?

If you have a copy of Jean Texereau's "How to Make a Telescope" then turn to page 142-3 for a discussion on Nasmyth and Coude telescopes.

I don't have that book, could I get a photocopy of those pages if you have that book?

Thanks
Darragh

Hi Darragh,

No, the flats do not affect the focal length of the system - they simply fold the light path, so it is very easy to draw your intended optical layout to scale or even 1:1 and see exactly how big the flats (and therefore the central obstruction) needs to be. The Nasmyth design is problematical at short/fast focal lengths as you end up with a massive central obstruction. If you go for an f/10 - f/15 primary you can make the central obstruction quite small... and you'll end up with an excellent high-contrast planetary scope.

I'll scan in the pages from Texereau's book tonight - remind me if you don't get them by PM by tomorrow :roll:

Phil.

So, in theory, the Nasmyth design could be beneficial for high aperture scopes in perhaps a Dob design, but not eqatorial?

The reason Nasmyth gave for his design was that he wanted to build a telescope with a large aperture but a focuser that was more conveniently located than in traditional reflectors. So I guess the answer is yes.