Perhaps influenced by maritime tradition, we soon found ourselves saying "her" instead of "it" when talking about the telescope. We named her Christine. And while many telescopes look like space probes or laboratory equipment, Christine is different.
The hardwood design came together while I was grinding the primary mirror. By the time I switched from No. 80 to No. 120 grit I had already fed the first planks through the table saw. I ran the saw and router in the early evenings, or by day on the weekends, saving the late nights for quieter work on the primary mirror.
The tube was the first structural part made. I wanted 1/2 inch of clearance around the mirror to avoid vignetting, and 1 felt that the light baffles should be at least 3/4 inch deep. That meant a tube with a 15-inch inside diameter. Strips of oak flooring were ripped on the table saw to just over 1/4 inch width and beveled by just under 3 [degrees] on each side. Sixty-five of these strips, glued edge to edge, would form a polygonal tube of 15 5/8-inch outside diameter.
To prepare the form I cut seven 15-inch disks and made a concentric groove 13 1/2 inches in diameter most of the way through each one with a router. These disks were then center-bored to fit tightly on a length of thin-walled steel pipe (3-inch electrical conduit). With the disks spaced out along the pipe and the latter supported on sawhorses, I glued the oak strips in place to form the tube. I also installed walnut and mahogany accent strips and a maple "sighting rail" during this phase.
The strps were glued full length to each other and to the disks. Fast-setting adhesive held them in place while the slower-drying epoxy resin between the strips cured. Using this technique I could work quickly, without having to clamp each strip mechanically and without leaving screw holes in them.
Once the glue had cured, the exterior was rounded with a small block plane and hand-sanded in steps to No. 400 abrasive. Bands of Honduran mahogany were cut, steamed, and bent around the ends of the tube. Then the center section of each disk was punched out and the conduit removed. The outer 3/4 inch of the disks remained in the tube as light baffles.
By now the finish was really beautiful. Many hardwoods acquire a polished but natural appearance when sanded with No. 400 or finer grit. I did not want the tube to look as if it were encased in plastic, but I knew it would have to be sealed. The whole affair was impregnated using epoxy resin of the WEST System ("wood epoxy saturation technique") developed for building wooden boats. The resin is a low-viscosity, low-surface-tension liquid with a remarkable ability to penetrate wood.
Resin applied inside the tube was drawn through the wood by capillary action. It appeared as glistening liquid beads after a journey of almost 3 inches through solid oak! Wood treated this way becomes stronger and harder and no longer absorbs water. I wrapped the tube's inside with fiberglass cloth and more epoxy.
Next a thin, slow-setting resin (but no cloth) was applied to the outside of the tube, forming a very hard and completely waterproof surface. A clear, marine-grade ultraviolet blocker was applied over the epoxy.
There are no metal fasteners anywhere in this telescope or in the mounting except for those that secure metal hardware. The wooden joints are doweled, splined or mortised, and then glued. If the ends were closed, Christine could probably go to sea!
An altazimuth mount is compact and convenient because the eyepiece stays parallel to the ground. But an equatorial design would be easier to motorize for high-power views of the planets. I realized that I could have it both ways, with a convertible mount!
Essentially, an equatorial is like an altazimuth tilted over to suit the observer's latitude. But it takes an unusually strong altazimuth to survive, let alone function, off the vertical. So I decided on a fork mount that would use opposing, preloaded, tapered roller bearings. An automobile front-wheel hub and brake assembly were easily converted for the task. Having been forced unexpectedly to drive a car on two wheels some years ago, I was confident that the telescope was a small fraction of the load this hub could carry.
My hub assembly is mounted in a platform of oak and walnut 1 3/4 inches thick. The heavy planks were doweled together and wrapped in fiberglass cloth and epoxy. I then mortised the disk-brake rotor into this platform. It is bolted through with stainless-steek socket-head cap screws and bedded in epoxy resin thickened with milled glass fibers.
The rotor carries the hub casting. A 3/8-inch-thick steel disk was mounted on the other end of the spindle and mortised into the bottom of the fork, where it is bolted and glued in place the same way.
In altazimuth mode the platform stands on four casters made of stainless steel and urethane. Brakes on these casters lock the pivots as well as the wheels when I am observing. To operate in equatorial mode I tip the platform over so the instrument rests on extensions of the side panels. These pieces, cut from oak planks 12 inches wide and 1 1/16 inches thick, extend well beyond the telescope's center of gravity; their ends are joined by a walnut plank for extra stability.
The fork is made of solid oak planks 1 1/16 inches thick. Two such planks, glued and doweled together, form the bottom and each side. The two planking layers overlap at the corners, where they are cross-doweled and gusseted. The fork's inside surface has a fiberglass-cloth overlay. In my shop I placed the fork on one side and set a dial micrometer between the ends. Stacking 80 pounds on the other side closed the fork ends by only 0.02 inch - it's solid!
The declination bearings have caps made of oak, walnut, and brass. Bucking the trend of huge disks so common on big Dobsonians, Christine's side bearings are only 6 3/8 inches in diameter. My idea was to minimize inherent drag and control the precise amount of friction with a brake. On each side two small Teflon buttons 3/4 inch across and 3/16 thick carry the load; they sit in 1/8-inch-deep "button holes" at the top of the fork.
This mount has worked so well that only now, after four years, have I started to work on the drive. Guest observers are often surprised at the ease and smoothness of the telescope's motions. Either axis can be adjusted from almost no friction to as much drag as anyone would want. When the instrument is used in the equatorial mode, following Saturn at just over 500x is a thumb-and-forefinger task.
Paul Zurakowsky of the Chabot Observatory Telescope Makers Workshop has more than 25 years of experience with the Foucault test and its many variations. With his valuable advice, some guidance from Jean Texereau's How To Make a Telescope (Willmann-Bell, 1984), and a lot of perseverance, I managed to reduce the primary mirror's surface error to a calculated 1/32 wave.
For the final test I took four sets of measurements on each of five zones, then rotated the mirror 90 [degrees] and repeated the operation. The values in any one zone differed no more than 0.0005 inch, indicating the errors of measurement were small. My tester has a micrometer thimble reading to 0.0001 inch and uses a green light-emitting-diode (LED) source. Paul's readings were in good agreement with my own. Nevertheless, he points out that the Foucault test cannot be relied upon to measure surface errors smaller than about 1/20 wave, corresponding to 1/10 wave on the wavefront.
With coatings having 98 percent reflectivity on both mirrors, Christine delivers the same image brightness as a standard 14-inch Newtonian. The diagonal mirror has a 2.14-inch minor axis, obstructing 17 percent of the primary's diameter. At the focal plane a field 1 inch in diameter (2/3 [degrees]) is uniformly illuminated.
The primary-mirror cell is an integral part of the tube. Since an equatorial mounting subjects the mirror to various orientations, I could not employ a simple Dobsonian sling. Instead I drilled three 1/4-inch holes 1/2 inch deep into the tube from the inside and placed a small metal disk in each one. The rest of each hole is filled with RTV (room-temperature-vulcanizing) silicone. Behind each disk is a machine-thread insert, allowing socket-head screws to be inserted from the outside to apply gentle pressure against the circumference of the mirror.
For collimation, brass thumbscrews in the backplate push on three "pistons" of birch plywood that bear directly against the back of the primary mirror. I suspect that this method may not offer enough support points for a very thin primary, but Christine's full-thickness Pyrex disk is, in the words of one observer, "some very happy glass."
FINDING THE "FAINT FUZZIES"
The maple sighting rail runs along the top of the tube. It does the job most folks do with a Telrad, except that no light is lost to reflection, it never fogs up, and there are no batteries or glaring red bull's-eyes. But, like the ubiquitous plastic box it replaces, this sighting rail works best for locating naked-eye objects. To pick up the faint fuzzies Christine needed help.
The finder was made from brass tubing and an f/3.75 objective 82 millimeters (3 1/4 inches) across. I threaded the inside of the tube and made a threaded PVC insert to serve as a lens cell. The first light baffle in the finder is at the point where the tube diameter changes to 2 inches, and the next is at the end of the 2-inch section. The 2-inch tube is internally threaded, and the whole interior is painted flat black.
A big Amici prism from a World War II Navy MK-47 gunsight has 1.32-inch faces, allowing me to use an eyepiece with a 13/16-inch field stop, The prism's bronze cell was fitted with a custom-made helical focuser and silver-brazed to the back of the finder. The combination provides upright, correct-reading views spanning 4 [degrees] at 12 x with no vignetting. The finder looks right at home on Christine and makes Messier hunting much easier.
The finder is mounted well down on the tube, close to the balance point, to reduce the need for counterweights. The low mounting point also lets me guide the scope while someone else is observing. This helps to get the most people to the eyepiece in a given amount of time at a star party.
Christine was a popular Merit Award winner at the 1992 Riverside Telescope Makers Conference. At the same meeting in 1994, where the finderscope and its magnetic mount received honorable mention, people waited a half hour in line for a view through Christine. One fellow (an optician) said, "It's been so long since I looked through a long-focus Newtonian that I forgot how nice it could be." Frequent comments like this validate at least some of the thinking behind the scope's unusual design.
It took more than a few hours to build Christine, but I don't feel a minute was wasted. She has redefined pride of ownership for me, and I've turned down offers for her running to five figures. (If someone were willing to wait three or four months for delivery, however, a sibling could be made to "look as if it belongs" in another home.) Just having a telescope like this in the house is tremendously satisfying. And, yes, Mary does lets me keep Christine in the living room.
PHILIP A. ALOTIS 29 Peralta Ave. San Francisco, CA 94110 e-mail: firstname.lastname@example.org
A member of the Fremont Peak Observatory Association, Group 70, and the Chabot Observatory Science Center, Alotis has years of professional experience in wood- and metal-working. He is a contractor in San Francisco.
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|Title Annotation:||home-made reflecting telescope|
|Author:||Alotis, Philip A.|
|Publication:||Sky & Telescope|
|Date:||Sep 1, 1995|
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