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4-Inch f/22 Refractor

by John Avallone, Northern Virginia Astronomy Club (NOVAC)
of Alexandria, Virginia

This scope was constructed over the period from November 1998 to July 2000. I realize that most people who display their home built 'scope at the Stellafane convention build reflectors, but hoped that any lurking refractor-nuts present might enjoy seeing this example! Most of the glass came from one estate sale of a vast hoard of surplus optical treasure. For only $60, I acquired the unmounted objective lens elements, a big penta-prism, and a nice 3" f/4 cemented doublet (for the rich-field finder). The objective glass must be old. The f-ratio of 22 is high for a 4" diameter (87" focal length!) lens. If anyone has knowledge of when and where this design was common, I would appreciate hearing from them!

The lens cell was worked up from PVC plumbing parts and a 13 oz coffee can. A 5 foot length of (salvaged) sheet iron stovepipe was on hand for the main tube. Two-inch PVC DWV pipe forms the drawtube, with the mechanism from an old photographic enlarger for the focuser. Brass lavatory drain tubing, scrap aluminum plate and thin hobby plywood were used to construct the prism housing. The 3" finder objective matched up well with an expired "KIDDIE" fire-extinguisher bottle. The main eyepiece is a 37 mm symmetric in 1 1/4" brass drain tube (60 X, 40' FOV). The finder eyepiece is a 5-element super Plossl in a plastic 35 mm film can (10 X, > 5 degree FOV).

The German equatorial mount is built from 1 1/2" cast iron pipe fittings. The motion is smooth on the iron plain bearing surfaces, hand filed and lapped to close tolerances. Two 4.4# exercise weights counterbalance the 16# OTA. For stability, the base of the mount is a broad triangle constructed of 3/4" plywood and 2" x 4" timber. The tapered (classic 1 in 10 ratio) rectangular cross-section vertical pier of 1/4" plywood (actually the rear-half of the OTA of another refractor project) is narrow enough to allow the scope to see the zenith. The rear face of the pier contains three storage compartments for lens-caps, extra eyepieces, my eyeglasses, etc.

The scope is pleasant to use, if somewhat retro. It gives sharp color-free views of the Moon, planets and double stars. The focal-length is perfectly matched to 35 mm prime-focus photography of the Sun and Moon. It's not quite stylish enough to be a "garden telescope". Maybe just a good "backyard scope"?

This telescope was displayed in the mechanical competition at the 2000 Stellafane convention, and there are additional photos on that page.

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Page last revised on Thursday, 2017-Sep-07 11:16:49 -0400