Many people assume that since the Newtonian is such a simple and popular design that all its variants have been fully exploited. I hope that this article will serve as a challenge to the thesis.

John Dobson always advised the use of longer focus mirrors in his plans. But f: 6 - f: 8 ratios seem impractical to most ATMs building 16-25 inch scopes.

With this design I wish to demonstrate how portable and practical an 18-inch f: 8 scope can be.

Folding usually implies the use of a third mirror not needed in the Folded Newtonian. The key of course is a 6-inch circular flat set at 15 degrees instead of th more usual 4-inch elliptical set at 45 degrees. This and a much longer upper cage assembly bring the focal plane down to a very manageable height while cutting the length of the truss in half.

The greatly reduced figuring effort of a f:8 mirror allows the use of a plate glass mirror, coated by peacock labs spray silvering process. Because the cost of coating is about one dollar a coat, it can be used on cosmetically flawed or poorly figured mirrors to speed up the construction process. The coating can be replaced more simply than cleaning the mirror.

The figure itself is stabilized by a trick developed years ago for stabilizing glass lasers eliminitaing any need for Pyrex at least for mirrors of this size. If heat is prevented from leaking out at the circumference of a disc, the disc will remain flat in spite of large heat fluxes. More about plate glass.

When structures are scaled up non-linear scaling laws force us to shift design strategies. As optical telescopes approach one meter most designers feel a need to switch to Cassegrain designs to reduce structure.

There is another option; the Folded Newtonian deserves consideration. The Folded Newtonian is actually a radical departure from accepted design resulting in huge cost and performance advantages.

It has features of both the Newtonian and the Cassegrain. For a Cassegrain to achieve a speed as fast as f: 6 usually require a 50% secondary obstruction. A Folded Newtonian can do this with only 30% obstruction. A straight Newtonian can use a smaller diagonal but it is not very helpful above 16 inches. And the size of the secondary is much more an issue with a six or eight-inch scope than it is with a 16 - 40 inch scope. Like the Cassegrain the Folded Newtonian has a long baffle tube reducing skylight on the focal plane. But because it is off axis it can be much more efficient.

On a 40 inch f: 8 the 8 ft. long back focus allows the focal plane to be brought into a rotating room, isolating the telescope from the heat of the equipment. With a f: 8 it is easy to compress, collimate or correct the field for other instruments and great performance is available even with cheap oculars. Binocular viewing is easy to set up.

Common 3 Mirror Newtonian

My 2 Mirror Newtonian

Folded Schmidt Camera

A - Folding trust system (all captive fasteners)

B - Baffle tube

C - Diagonal ring

D - Intermediate ring

E - 2" Crayford focuser with adapters

F - Mirror box

G - Detachable trunions

H - Rocker box and ground board