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6-Inch f/6 Fork-Mount Newtonian

by C. J. Park, VAS, of South Burlington, Vermont

I was motivated to try building a telescope after joining the Vermont Astronomical Society and attending the 1997 Stellafane convention. For a first attempt, I thought a reflector would be best, but one which could be used in both altazimuth and equatorial modes. Not a "typical" Dobsonian. I had seen an article in Sky and Telescope a few years ago describing such an instrument, but mine has some design differences.

My design allows for adjustment in declination from 55 degrees latitude to 22 degrees latitude when using it in the equatorial mode. In this mode, I can approximately polar align it in daylight by using a compass which can be adjusted for the local magnetic declination to obtain true north. The telescope rests on a flat support with three adjustable feet for leveling in this mode. The whole telescope can be rotated plus or minus five degrees for tweaking in on the pole after dark while the support stays stationary. In altazimuth mode, the base is tipped to rest on one adjustable foot and two casters. A slow motion control in right ascension is accomplished by a set of three gears that deliver a 12.6 to 1.0 ratio.

The declination axis bearings are needle bearings mounted on PVC piping adapters with internal threads. Each has a disk of Formica mounted on it with velvet craft ribbon glued to the inside surface for friction breaking. These are screwed onto stainless steel threaded adapters mounted to the lower half of the split saddle. When the tube assembly and saddle are mounted on the fork arms, two saddle blocks are mounted on top of the bearings and secured by threaded rods. The two declination knobs are then screwed in until the desired friction is obtained to prevent movement from ant desired elevation. There is no need for counter balances. The four circular surfaces of the split saddle also have the velvet friction ribbon attached. Four threaded rods secure two halves of the saddle on the tube. Again, desired friction is obtained to allow the tube to be rotated for easy viewing through through the focused yet still star in place where positioned.

The base, fork and saddle are made of poplar and the tube is sonotube covered with self-adhering birch veneer. The telescope base and form arm assembly are both doweled and glued, except for a support below the rotating fork assembly designed for possible future motorized operation. This is assembled with screws. One additional feature is a leather on leather brake between the fork assembly and the base to facilitate polar alignment or for determining field of view of an eyepiece.

The finder scope is made from a piece of automotive tailpipe extension, a PVC piping adapter and a rack-and-pinion focusing lens form an old slide projector with friction ribbon for a tight fit for the eyepieces. There is no need for a thumbscrew. The finder will accept and 1 1/4" eyepiece. A 15mm eyepiece produces the equivalent of a 6.7 x 30 finder.

This telescope was awarded 3rd place in the craftsmanship competition and 4th place in the mechanical design competition at the 2000 Stellafane Convention. See additional photos here.

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Page last revised on Thursday, 2017-Sep-07 11:29:26 -0400