Four-Speed 17.5-inch Newtonian

Sky & Telescope Magazine, February 1986, p199

Gleanings for ATMS, Conducted by Roger W. Sinnott

To work on a large telescope for over 10 years, anticipating its completion, would probably try the patience of most amateur astronomers. But I was bitten by the "Big Band bug" way back in grade school, when my mother gave me a 40mm refractor as a birthday present. Ever since, the desire for better and larger telescopes has been almost insatiable. It has now culminated in the 1000-pound reflector seen here.

Blue,  as I've come to call it, was built for the rediculously low cost of $100, less the Coulter optics bought secondhand at a considerable savings. The reinforced-concrete slab, wood, paint, and motorized roof of my temporary observatory added another $300. Yet the total investment, including optics, came to no more than $1,000.

I wanted a telescope that could track, scan, slew, and talk. I settles for everything except talk (though a computer interface with voice synthesizer is not so far fetched these days!). the basic motors for star tracking in right ascension and guiding in declination have magnetic clutches that disengage them automatically whenever a "servo function" (scan, drive, or slew) is activated. What I call servos are 120-volt AC three-phase self-synchronous transmitter-receiver sets, obtained surplus. Perhaps they were originally used in missile guidance systems.

The control panels came from a commercial laundromat. they were heading for the junkyard when I intervened and rescued them. After gutting the panels, I devoted a lot of thought and sleepless nights to redesigning their interiors. the goal was a system having everything that could possibly be necessary or convenient, as well as ease of operation. I also wanted total remote control of the telescope, because I hope ultimately to move Blue to another structure. The controls will stay where they are, which will become a warm room for viewing, on a video monitor, the telescope's output as picked up with an image intensifier or charge-coupled device (CCD).

With Halley on its way, expediency interrupted these grandiose dreams to some extent. the present quarters are some what cramped, and they do have their hazards. Once while observing, my head became rather tightly wedged between the west wall and the westward-tracking telescope. I could almost picture my wife, coming to see if I were spending the night in the observatory, and finding my head impaled by a 20mm Plossl! It's a good thing the drive has friction clutches.

DRIVE OPTIONS: The main control panel has a switch marked "variable frequency generator," which controles the main synchronous drive motor. during normal star tracking, the telescope's aim can be adjusted slightly with lightweight joysticks or a heavier control paddle. Also, sidereal, solar, and lunar tracing rates may be selected on the main panel.

There are slave servos mounted on the right ascension and declination worm shafts.  Through a network of relays, any of the three master servos mounted remotely on the main control panel can be activated to drive a slave servo. The result is an extremely versatile speed control offering a variety of drive rates.

For example, a rate I call "manual servo scan" is available on the control paddle.  Minuscule corrections at extreme magnification are made by depressing either the right-ascension or declination push button and turning the knob on the paddle's top.

Another option, labeled "servo drive," is invoked with either or two joysticks, one at the viewing station and another at the main control panel. Pressing a button on a joystick causes the tracking function to drop out and a faster drive rate to be cut in. This is useful for general image centering at medium to low magnification.

Finally, a "fast slew" control on the main panel shifts the telescope quickly to a new set of sky coordinates. At present such a movement is more conveniently done by hand, thanks for the friction clutches. But showing off the fast slew impresses visitors at star parties and is a lot of fun. This function will become important when Blue is eventually placed in its own dome.

Other convenient features include dimmer controls for the setting-circle lights, a panel blackout button for astrophotography, a digital clock with 0.8-inch-high numbers, and a reversing switch for correct operation of the declination controls when I shift the telescope from one side of the pier to the other. There is a switch that allows alignment of Blue to the meridian, where right ascension equals local sidereal time. Initializing the electronic coordinate counters when the telescope is in the meridian avoids the need for direct observation of a particular star.

TUBE ASSEMBLY: With a truss-type open tube, a telescope's primary mirror rapidly reaches thermal equilibrium with the cool night air, minimizing convection currents along the light path. A truss is also inherently light and strong. But I wanted a rotating tube as well, so that the Newtonian eyepiece could always be turned to a convenient location.

This problem was solved by using two rolled angle-iron bands for the tube's center section. These bands sit on four support bearings, mounted on each corner of the C-channel tube cradle. With the addition of two thrust bearings and two spring-loaded bearings to keep the tube from falling out of the cradle, I created a ringless truss system capable of rotating up to 120 degrees - quite adequate in most any situation. The flywheel gear from an automatic transmission was welded to one of the angle-iron bands. A gear motor and microswitches for end stops give me a rotating tube at the push of a button.

I have not been very impressed with the focusing methods provided on commerical telescopes. Rack-and-pinion focusers are often not smooth, and the popular Schmidt-Cassegrain telescopes that move their primary mirrors sometimes have intolerable image shift. Many focusing mechanisms seem to get their smoothness from heavy grease, but performance worsens dramatically when the grease ages.

Therefore I planned and built my own focuser. it is a modified Crayford design, which derives its smoothness from ball bearings and a minimum number of contact points rather than heavy grease (S&T: September, 1974, page 182). This effort really paid off. I have used many focusers at star parties, but mine is far superior in every respect to commercially made ones available today.

For versatility, I equipped this focuser to accept eyepieces with barrels 1 1/4, 2, and 2 1/2 inches in diameter. A drawtube gives an additional 3 inches of adjustment over the 2-inch travel of the focuser itself; the resulting 5 inches have proved very convenient for astrophotography, especially when alternating between first-focus work and eyepiece projection.

I also have a rare and treasured Nikkor eyepiece of the Erfle type in a 2 1/2-inch barrel. Imagine viewing the Andromeda galaxy M31 and its companions M32 and M110, with field to spare, through a 17 1/2-inch telescope. The focuser includes a built-in off-axis guider and a super-smooth motor control operated from the hand paddle. A three-eyepiece turret is an added convenience.

THE DRIVE GEARS: My right-ascension worm gear was machined from a 10-inch aluminum blank that I got for a few dollars at a local scrap-metal yard. To hob the teeth of this gear, I mounted a 3/4-inch tap with eight threads per inch in the chuck of my drill press. A jig allowed the aluminum blank to revolve smoothly against the tap at low speed. Practice, care, and many hours of patience went into completing this gear. I've made several others by the same technique, dispelling the notion that an expensive milling machine is needed for such a job.

I was very lucky one day at the scrap yard to find a 9-inch bronze worm gear with 50 teeth and a matching stainless steel worm assembly. This set became my declination drive. The friction clutches for this and the right-ascension worm gear were made on my 6-inch metal lathe.

THE MOUNTING: When weight is no deterrent, I consider the standard German-style equatorial to be the most convenient type of mounting for a telescope. I made mine some years ago in the Nuclear Power Facilities machine shop at Fort Belvoir, Virginia. The stainless steel polar and declination shafts are 2 1/4 inches in diameter. These shafts had been part of obsolete and discarded control-rod drive assemblies, and the housings are 4-inch high-pressure steam pipes with 1/4-inch walls. Couplings were welded on each end after being machined out to accept 5-inch ball bearings.

When the mounting was finished, it was mothballed and moved around the country for about four years. It finally came to rest in Colorado Springs, and Blue was completed last summer. We are both thriving now, here in the Black Forest at an elevation of 7,500 feet.

First light through the 17 1/2-inch verged on a religious experience for me. Now I understand why so many persons "of the cloth" are amateur astronomers. I could perceive countless individual stars of M13, the famous globular cluster in Hercules, more clearly than in any photograph I've ever seen. there was a strong sense of depth, the stars across the cluster seemingly backlit by the central glow.

My other deep-space encounters that night, and every clear night since, have been equally memorable. I know how to design and build a good telescope; now I will learn the heavens. All my previous telescopes had been awkward to prepare for observing, seriously dampening incentive, but now my most strenuous act is pushing the button to open the dome.

Paul Boone Van Slyke

Some pertinent pictures that were not in the S&T article