Michael Fallwell's projects:

Drought Reduction * Glider powered windmill * Folded Newtonian Design * The secret of Brown Adipose Tissue (BAT)


18" f: 8 Folded Newtonian
Weight 70lb
Eyepiece Ht. at Zenith 5'4"
33% Obstruction
3 Min: Setup - Ultra Portable
Wide Field - Low Cost


Folded Newtonian Design

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

18", f:6, 33% obstruction, LOA 4'4"

My 2 Mirror Newtonian

18", f:6, 33% obstruction, LOA 4'4"

The eye position is actually very comfortable. It also allows more back focus for instruments, better baffling and a longer f:8 construct.


Folded Schmidt Camera


OpticalSchmidt CassegrainNewtonianFolded Newtonian
Field Width:NarrowWideVery Wide
Image Scale:LargeSmallLarge
Aberrations:SmallLargeVery Small
Induced Aberrations:SmallLargeSmall
Baffling:30 DBs10 DBs40 - 50 DBs


MechanicalSchmidt CassegrainNewtonianFolded Newtonian
Tube Length:ShortLongMedium
Secondary Shape:FiguredEllipticalCircular
Strut Length:ShortLongShort
Primary Mounting:MovingFussyRobust
Eye Piece Position:LowHighComfortable
Center of Gravity:HighLowLower
Instrument Mounting:DifficultDifficultEasy
Slewing Rate:FastFastVery Fast
Tracking Accuracy:PoorPoorVery High *
Weight:HeavyHeavyVery Light
Cost:HighLowVery Low Cost

*) a friction drive equatorial table has low periodic error.


Advantages of large secondaries

Professional Cassegrains rarely have secondaries smaller than 33% and there are some real advantages to larger secondaries:

  • Large field of view or image scale
  • Medium focal ratio allows the use of economical eyepieces
  • Better baffling
  • 30% reduction in tube length
  • 50% reduction in eye piece height
  • 4x more back focus
  • Low Cost Optics / More apperture per dollar
  • Safety - no ladder required
  • Easier equipment balance
  • Makes 1/10 wave optics easier to achieve
  • So the only real advantage of a small diagonal in a large telescope is a tiny improvement in contrast/resolution that can easily be recaptured with image processing.


    A few comments in response to threads on the web (slashdot)

    It took me many years to realize that secondary size is of much less importance than the quality of the primaries figure in 2 hours I can put a nearly perfect figure on a f:8 mirror. Its just not possible to achieve the same quality in a week of work on an f:4 . But because on some objects a small secondary may be useful, a longer upper cage can be used. In this case the trunions are used in their extended position to eliminate the need for massive counter weight.

    My binoviewer is better in at least one respect than the $1000 model. It gives a 100 degree field of view. I used a split field, not a beam splitter. If you place your fist on your forehead, you can study the effect of the transitions from one field to the other and each eye has twice the light to work with. The binoviewer is the reason for this telescope.

    Collimation of the primary is fixed - no adjustments possible. But the secondary has 4 degrees of freedom; all collimation adjustments are made at the secondary. The secondary can be stored without disturbing the adjustments. Setup does not require collimation.

    The tracking table has low periodic error for CCD work, but it is not designed for film photography. However, an autoguider driving servos on the secondary would meet even this need.

    I find little use for the star diagonal - even at the zenith you are not looking straight up.



    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



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