A New Telescope!

Look at this, folks.  On this page you will find a telescope design seldom seen by the observing public.  It is a hybrid telescope that combines the best of both refracting and reflecting telescopes. 

Usually we see many observers interested in either the classic refractor (known formally as a dioptric design) or some variation of the reflector (known formally as a catoptric design).  The hybrid design on this page is that of a catadioptric, a blend of the refractor and the reflector!

Such hybrid designs have been commercially available for decades, either as a Schmidt-Cassegrain or a Maksutov, and, more recently, a Schmidt-Newtonian.  All are good and are quite portable.  The names of these catadioptrics derive from those who contributed the design idea.

The catadioptric pictured here is the Mersenne-Nasmyth, and it is not a household name.  It is a telescope that I constructed from available parts and with the help of expert machining.  To keep it looking nice over time, I had many of the metal parts either powdercoated or anodized.

To my knowledge, few have attempted this design, though the ideas for it have been around for hundreds of years!  The name derives from the two historical personages that lend their names to the scope's operation.

Marin Mersenne was a 17th-century French mathematician and ardent champion of Galileo and his discoveries.  He was also interested in optics and in improving the kind of refractor that Galileo had used to make those discoveries.

He figured that confocal (or, optically matched) mirrors would obviate the need for lenses in a telescope...and he was right.  But his good friend, René Descartes, talked him out of any further pursuit of this telescope design! 

Instead, Mersenne is likely better known for experiments in tuning musical instruments and in music theory, and, for his Mersenne primes, a special category of prime number important in solving complex computational problems.

James Masmyth was a 19th-century Scottish engineer, known for his development of the steam hammer, which quickened the manufacture and shaping of metal forgings.  He dabbled in astronomy, and especially liked viewing the Moon.  He even wrote a detailed book about it.

He also developed a type of telescope that afforded him a comfortable view of the night sky while seated next to his instrument.  By inserting a flat mirror in the light path of a typical Cassegrainian design, he diverted the light path through the declination (or, altitude) axis of his telescope.  This allowed a steady and convenient place for the eye to observe.  This innovation, known as the Nasmyth focus, is used currently on large telescopes in large observatories.

The design I've constructed combines the best aspects of these two gentlemen's designs.  With a set of large optics I have crafted, I make use of Mersenne's confocal mirror concept to gather light and bundle it into a tight, magnified beam, and then to feed that bundled beam into a well-made refractor placed solidly at the Nasmyth focus (as shown in the photo, immediately below). 

The result is no less than a stunning view through a fine refractor that has the large aperture of a fine reflector!  Sounds implausible, but it does indeed work.  As a side benefit, the telescope affords a comfortable, seated view of the night sky, which makes its unusual optics available to parents with a small child seated on their lap and to persons in wheelchairs, since the eyepiece (below) is placed for easy access.

At present, my telescope is nominally portable when on pneumatic wheels, but does take a little while to set up if it were to visit a star party.  Collimation is necessary but not impossible to accomplish, even though this catadioptric system utilizes a total of four internal reflections, which results in a normal (i.e., inverted) telescopic view.

For those who must know, the 17.5"-wide main mirror was originally made by Coulter Optical, but had unfortunate polishing flaws prior to its aluminization, which were clearly visible with a Foucault tester.  (The figure looked like an old vinyl phonograph record, with hundreds of rings left from incomplete machine polishing!)  I had the optical services of Clausing's in Skokie, Illinois, remove the old aluminum coating before I set to repolishing the big glass.

With the expert help of Jim Seevers at the former Optical Shop of Adler Planetarium in Chicago, I repolished and re-figured the big glass to a fine optical surface better than one-tenth wave.  The four-inch wide secondary mirror took time to complete, but it's a confocal match to the main mirror, which means its optical figure is an exact complement to the main mirror's figure.  

Clausing resurfaced both with a bright coating and all of the components were fitted into the assembly prepared by Industrial Machining in South Chicago Heights, Illinois.  By the way, the heavy-duty tripod from Houston Fearless that everything sits on was once used in a television studio; years ago I bought it in a camera store in Urbana for $100.

This pair of hand-made mirrors allows distant starlight to be gathered, bundled, and magnified in a way that lets it be funneled into the fine optics of a well-made refractor.  That beam is directed by two flat mirrors, one above the main mirror and a quartz flat inside the refractor. 

In the photo, that's a sky-blue Brandon refractor shown, one of the first 50 manufactured by Roland Christensen.  One must have a fine refractor to make this all worthwhile.

This scope is my engineering prototype; the proof-of-concept model was built of plywood nearly ten years ago and displayed at various star parties around the Midwest.  That concept model won design awards as well.  Already I've learned much about how to utilize the design better and how to improve its overall operation.

I've written articles about this design, which you can find in back issues of Amateur Astronomy Magazine (see the links below) and I've presented many talks about its increasing usefulness as a stable platform for astrophotography and for simply viewing in comfort.

There's more to tell about this design.  It's likely that certain large Dobsonian-style scopes can be fitted with the right components to convert them to this catadioptric design.  Also, there is potential for making fine refractors perform even better.  Let me know what you think.

So, for additional info: 

You can scroll down here and here to find back issues of AA Magazine that feature articles on the Mersenne-Nasmyth telescope.  I wasn't the first to build this kind of scope; I'm the second.  But I am the first to make improvements that can make this scope even more useful to both casual stargazer and serious amateur astronomer.  More on that at another time.

The first person who I know to have built this kind of scope is one named Clyde Bone.  He is pictured (below) at a star party with his own 20-inch homebuilt Mersenne-Nasmyth telescope.  Note that he can enjoy the night sky while seated in comfort alongside his instrument.

His story is found in an article published by Sky and Telescope Magazine in 1999 which you can look for here but you might have to pay for a reprint.  A good library might have back issues bound and on the shelves.  Or, you can still find an old thread of posts here and here that describe this unusual telescope in all of its glorious, geeky detail.

More photos and progress reports to follow.