Nasmyth Telescope Designs

Hi,

I am looking for any material relating to designing and build a Nasmyth telescope.
Searching on google results in lots of hits on Nasmyth platforms for very large telescopes and very little on amateur nasmyth designs and construction.

Any links to websites on designing and building a Nasmyth telescope would be greatly welcomed.

Thanks
Darragh

Hi Darragh,

I think you are looking for plans for an Equatorial Platform rather than a Nasmyth telescope?

A Nasmyth telescope is simply any alt-az mounted scope that has its optical axis exiting the tube along the altitude axis so that no matter what altitude you have the scope pointed at, the eyepiece always remains at the same height - very useful for wheelchair bound observers.

An Equatorial Platform is a motorised 'table' that your dob-scope sits on and allows you to track the stars for an hour or so before you need to reset it. I'm thinking of building one myself for my 8.75" travel-scope.

Check out:
members.tripod.com/denverastro/dsdfile/dspfile.htm
www.atmsite.org/contrib/Shaw/platform/

Hope this is what you are looking for!

Phil.

Hi Darragh,

I think you are looking for plans for an Equatorial Platform rather than a Nasmyth telescope?

I was looking for information on designing and building a Nasmyth telescope as I like the idea of building a large reflector but having the
focuser at the front of the tube with such a long focal length could be impractical and dangerous with a ladder in the middle of the night.

A Nasmyth telescope is simply any alt-az mounted scope that has its optical axis exiting the tube along the altitude axis so that no matter what altitude you have the scope pointed at, the eyepiece always remains at the same height - very useful for wheelchair bound observers.

Is the Nasmyth restricted to alt-az mounts? I would have thought it wouldn't have mattered which mount type was used, whether it was dob, eq or alt-az.
From some of the ray diagrams on the web, I thought the mirror combination was slightly more complex as it looks like the secondary mirror was convex and with a tertiary flat mirror close to the primary to all the focuser to quite close to the back-end of the scope.
This seems to impose those some limits on the focal ratio of the scope due to the combination of the concave primary mirror and convex secondary mirror. I am particularly interested in trying to understand the implications of this setup in terms of the image quality.

An Equatorial Platform is a motorised 'table' that your dob-scope sits on and allows you to track the stars for an hour or so before you need to reset it. I'm thinking of building one myself for my 8.75" travel-scope.

Check out:
members.tripod.com/denverastro/dsdfile/dspfile.htm
www.atmsite.org/contrib/Shaw/platform/

Hope this is what you are looking for!

Phil.

The equatorial platform is an interesting idea and one I hadn't really considered.

Thanks
Darragh

I get the point. Nasmyth's reflector was designed so that the focuser was at the Alt axis and he was able to sit in one position all the time, using wheels to move the telescope as required, I've seen an etching of it. The use of secondary and tertiary mirrors to redirect the light to the focuser position on the side of the tube meant that the light loss was comparable to a Schmidt Cassegrain. The scope would have a long tube in comparison to an SCT of the same ratio, but would be shorter than a comparable Newt, making it possible to use quite a large primary.

I actually had built an 18 inch Cass a long time ago that had no hole in the primary. The cone of light from the secondary convex mirror was directed to a flat secondary mirror and reflected at ninety degrees on the opposite side of the declination axis at f/12. This was similar to a Nesmyth system.
The other option was to use it as a Newtonian where the secondary convex mirror is replaced with the flat. This worked at f/4. The mounting was a german equatorial.

Eamonn A

Is the Nasmyth restricted to alt-az mounts? I would have thought it wouldn't have mattered which mount type was used, whether it was dob, eq or alt-az.
From some of the ray diagrams on the web, I thought the mirror combination was slightly more complex as it looks like the secondary mirror was convex and with a tertiary flat mirror close to the primary to all the focuser to quite close to the back-end of the scope.

This seems to impose those some limits on the focal ratio of the scope due to the combination of the concave primary mirror and convex secondary mirror. I am particularly interested in trying to understand the implications of this setup in terms of the image quality.

Nasmyth's design was (I think) based on an Alt-Az. Andre Coude came up with a similar optical arrangement for equatorials, bringing the focal plane out through the polar axis for his large spectrographic camera. This arrangement requires three auxillery mirrors to work, and you probably need a long light-cone if you want to limit the size of the secondary obstruction in order to avoid vignetting. (Nasmyth's alt-az design only needs two extra mirrors - secondary (circular) and tertiary (elliptical) flats.) Alternatively, you could use a couple of acromatic doublet transfer lenses to produce a collimated (parallel) pencil of light, which the second lens would then re-image to form a focal-plane. That way you could actually make the central obstruction very small.

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!

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.

Phil.