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Richest field system

RFT systemThis RFT system couples Fujinon 10x70 binoculars and a 6" richest-field telescope. A University Optics 28mm Pretoria eyepiece provides the optimum-sized exit pupil and low, 21x magnification. The mount is all-aluminum and features a Nasmyth focus for observing convenience.

 

 

 

 

Introduction

Inspired by S. L. Walkden's definitive chapter on the richest-field telescope in Amateur Telescope Making (Book Two), published by Scientific American in 19491, I made a 6-inch f 4 mirror in high school. I mounted the mirror twice over the intervening 47 years since, but none of the mounts were satisfactory.

I've owned a pair of Fujinon2 FMT-SX binoculars for awhile. Each time I used it, my tired arms and shoulders invariably suggest I needed to mount it. I decided to build a single mount that would support both a 6" telescope and the binoculars.

CAD rendering, using IMSI's DesignCAD software.I designed the mount described here in January, 1997. I finished it in time to win a merit award at the 1997 Riverside Telescope Makers Conference3 (RTMC). The judges called it an "innovative low power observing system."

 

Why an RFT?

As Walkden puts it, "Many users of telescopes have probably experienced their most exalted feelings of admiration of the heavens while sweeping aimlessly, with low powers, in the region of the [Milky Way]." As he points out, what makes an RFT "richest" is a combination of optics that, for a given aperture, delivers the most stars to your eye.

Walkden shows that the ideal RFT in clear, dark skies is a 2½". This is pretty close to a pair of 7x50 binoculars. He also points out that, for larger aperture RFTs, the small loss in real field of view is offset by the brighter appearance of individual stars.

Today's large-aperture telescopes provide great views of deep-sky objects and areas rich in very dim stars. Some star fields, clusters or galaxies, however, are too large to fit in their real fields of view.

For most users, the small aperture RFT provides the best views of large objects such as these:

  1. Praesepe cluster (M44)

  2. Pleiades cluster (M45)

  3. Perseus double cluster (NGC 869, 884)

  4. Great Orion nebula (M43)

  5. Milky Way in Sagittarius, Scorpius

Design

I designed the telescope with these objectives in mind:

  1. Keep the telescope's eyepiece height fixed at 45 inches. This position, similar to a Nasmyth focus,  lets the user sit comfortably regardless of the telescope's altitude angle.

  2. Incorporate the binoculars with the mount and locate the eyepieces of the telescope and binoculars as close as possible.

  3. Make the telescope transportable in the back seat of a car.

  4. Use a simple alt-azimuth mount.

  5. Use waterproof, maintenance-free aluminum and plastic.

  6. Keep the telescope as light as possible and avoid counterweights.

 Construction

The optical support structure consists of two ½" diameter aluminum rails, which in turn pass through a ¾" aluminum hub around the altitude axis. The arm, end rings and hub all slide on the rails and are fixed in position with set screws. The focuser is mounted on the outside of the hub.The altitude and azimuth bearings are machined from a Delrin and Teflon composite, sometimes called Delrin AF. The telescope lifts off the pier at the altitude axis.The mirror is glued with silicone RTV at three points to a 6" ring. The ring has three bolts that align with holes in the end ring. Separate thumbscrews adjust the mirror's position, and provide a push-pull collimation system. The zero-magnification finder is a Daisy9 #7809 Electronic Point Sight. I built an adapter that attaches it to the center post of the binoculars. The Daisy is unmodified, except for the addition of a homemade LED filter.

  1. The moving parts -- telescope, eyepiece and binoculars -- weigh only 16 pounds. The system's total weight is 33 pounds. Most parts are made from 6061-T6 extruded aluminum plate, rod and tubing; a few of the parts were then welded. All metal parts were anodized.

  2. I made the mirror from a kit purchased from Edmund Salvage Company5 in 1947. According to Tinsley Laboratories6, who applied the aluminum and silicon dioxide overcoat in 1950, it had a turned down edge. Operating at 21x, the figure is acceptable. The original coating is still intact after many years of banging around in closets, garages and moving vans.

  3. The two-inch 28mm Pretoria eyepiece was sold in the late 1980s by University Optics7 (it is no longer available). H. W. Klee and M. W. McDowell of South Africa designed it to overcome spherical aberration and coma, especially when matched with f 4 Newtonians. The Pretoria provides a 50° apparent field of view; when used with the 6" f 4 it has a 7mm exit pupil, 25 mm eye relief, 21 power and a 2.5° real field of view.

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Internet links

  1. Willmann-Bell, Inc. now publishes the ATM book series
  2. Fujinon Specialty Products Div.
    Fujinon binoculars
  3. Riverside Telescope Makers Conference
  1. Edmund Scientific 
  2. Tinsley Laboratories precision, aspherical optics
  3. University Optics Pretoria eyepiece
  4. Tele Vue eyepieces
  5. Daisy gun sights