Just A Little Project
By: Rev. David J. Ross
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Back in June of 1995 members of the Wilderness Center Astronomy Club tackled one of their most ambitious telescope making projects to date. One of our members boasts a fine 12" long focus reflector, another a 14" truss tube behemoth, not to mention the current project of restoring and automating the Keller 16" cassegrain. But all these projects pale in comparison to what transpired that year: building 25 small reflectors at a one day teachers' workshop for a local school system.
First, a little history. Founded in 1983, the WCAC is related to The Wilderness Center in Wilmot, Ohio, on the edge of Central Ohio's Amish country. We share the Center's dedication to nature education and have taken on all sorts of projects to promote our interest in astronomy. The Center and its environs offer us a comfortable meeting place and suitably dark rural skies.
Back during the Halley Days of the mid-80's one Astronomy Day project that enjoyed some public popularity was a workshop on building little copier-lens refractors using PVC plumbing parts. Dubbed our "MX", the scopes were easy to build, simple to mount and, above all, inexpensive. Over the course of about a year we built around 80 MX scopes with participants ranging from teens looking for science project ideas, to birders wanting a low-cost spotting scope, to genuine Halley enthusiasts, a few of whom actually went on to join the club as active members. The MX even allowed the club to earn a small profit to pay for other Astronomy Day and related activities. Difficulty obtaining suitable optics, especially a good dirt cheap eyepiece, eventually caused us to discontinue these workshops. Sometime in the early 90's we happened on a cache of military surplus binocular eyepieces that had been packed away since the early 50's. We snatched up twenty of these easily adaptable Kellner's for less than $5 each! Could our next project be far behind?
MX telescope plus our small reflector projects.
Club members planning for our 1994 Astronomy Day event with coordinator Robin Gill began to discuss reviving the MX workshop to go along with our goal of providing hands on astronomy activities. Our then president, Eric Mast, urged us to explore possibilities for building a scope with a bit more muscle, something a little more worthwhile as an introductory instrument. Cost would still be a prime concern as few of us believed we could interest our particular public in a scope costing more than around $100. With that as one of our specifications, several of us set to work. We finally presented the prototype of a 4.5 inch f/8 reflector with a PVC tube mounted on a fairly conventional Dobsonian mount. The imported spherical mirrors came as a set complete with a 1 inch diagonal from IDAI out of Baltimore, MD. These non-overcoated mirrors proved to be far too delicate for our tastes (we ruined a couple to prove it to ourselves) so we eventually paid to have them recoated and protected. Even so, with our surplus Kellner we could offer what we believed would be a perfectly serviceable little scope at a cost of just under $100. That summer the little scope clearly showed the dark impact spots on Jupiter when used at about 75 power.
We offered the Astronomy Day telescope workshop, held at the Center in October, for $135 and were surprised when all ten slots were snapped up within the first week! Forty other people went on our "maybe later" list. The workshop took six hours from start to first light when participants took the scopes outside to check the image of distant trees. All seemed thrilled with what they saw and with the prospect of what they would see come the next clear night. As part of the workshop participants received a one year membership in the club and at least two have since become enthusiastic regulars at our meetings and star watches. (Gerry Powell and Barb Vaughn)
Observing the workshop that day was a representative from Canton City Schools who had expressed an interest in talking to us about a teachers' workshop being planned for that summer. "Well, how many telescopes are we talking about," I asked. "We were thinking one for each of our buildings; that would be 24," she answered.
25 read to go tubes.
Twenty four soon went to twenty five when her colleague in the curriculum office saw our prototype. But, twenty five telescopes! Despite the schools' generous offer to make the workshop worth the club's while, some of us had already concluded that given the several orders of magnitude difference between the MX and the current project that those on the "maybe later" list would just have to be disappointed for the foreseeable future. Ten had been taxing enough!
Determination, Teamwork, Success.
It took several weeks of recuperation before discussions resumed. Gradually the club's leadership began to see was that this was really quite an opportunity. Helping teachers to make these little scopes for use in their science classes might well make a lasting contribution and excite more students about astronomy than we could ever hope to reach individually. The curriculum representatives offered to put many of the school system's resources at our disposal and accepted our design changes which we hoped would simplify construction and make for an even better scope. A shop teacher, who supervises developmentally challenged vocational students, took on the job of fabricating the square plywood tubes of the new design as well as a variety of other wooden parts according to our specifications. Thus, the actual amount of club time was reduced, apart from my time as project coordinator and the team actually supervising the forty some teachers on the day of the workshop. We'd like to share some features of our design that may be of interest to readers.
Eric Mast working with teacher Pat Regan.
The tube: Although 6" PVC pipe is a good choice for small reflectors, being very durable and easy to saw and drill, it does offer some drawbacks. To get a nice clean finish on the ends and the exterior requires a great deal of power sanding, although the results can be quite nice. But, fitting basically flat fixtures to a round tube presents some complications. Attaching the 4" PVC end caps to the tube for altitude bearings required sanding each cap by hand to form a little saddle that would allow the cap to rest securely against the tube when bolted. Going to a square tube, made of 3/4" plywood sides and 1/4" plywood on the top and bottom, was a simpler alternative. The desanco trap adapter focuser and the 4"x1 1/2" reducing bushings we chose for the bearings in the new version attach flush with no problems. Aesthetically, the tubes may not be as pretty but our suggestion to the teachers was to let their classes decorate both the tube and mount with suitable astronomical creations, perhaps supervised by the school art teacher.
The diagonal assembly, focuser and sight rings.
The diagonal assembly: A dowel cut at a 45 deg. angle and mounted to a 1/8" brass toilet tank rod is all that we found to be necessary. The mirror is attached using a pad of silicone. To provide a set screw to secure the holder on the rod we used a threaded brass insert at the rear. The eyelet in the tank rod allows it to be bolted to the top side of the tube. Aligning the necessary holes takes some care and fixing up a suitable jig would be a must for turning out a number of these. However, for the teacher workshop we ended up jobbing this out to a local machinist who produced the holders out of aluminum for a small charge. These turned out to be much more uniform than the wooden versions we used earlier and hence were easier to measure and position accurately during assembly.
The focuser: Using an ABS (black plastic) 1 1/2" desanco together with a matching slip extension of suitable length provides a very light weight, very inexpensive focuser that does the job just fine. Mold burrs and other obstructions on the outside of the slip extension were removed relatively easily on a lathe. We shim the inside of the desanco using strips of plastic label tape to remove any play. To adjust the tension on the slip extension just tighten or free the nut atop the desanco. The eyepiece slips inside a standard 1 1/4" washer that is secured with the similar nut atop the slip extension. A small piece of label tape may be needed to shim the eyepiece barrel so that it's snug in the washer. The inside of the tube is deadened using black flock paper. The range of motion with this system is limited to about 1.5" so it's important to know the focal length of the primary and to lay out the light path so that the image plane falls where desired.
The sight rings: The 1x finder for aiming the scope is made from two eye-bolts, a 1/4" a the front and a #10 at the rear, spaced about 8" apart. Alignment is pretty straight forward- adjust the relative heights of the eyes and then yank the #10 left or right as required. The best that can be said for this feature is that short of adding significantly to the cost, to say nothing of off-axis weight, it's better than nothing.
The mirror cells
The mirror cell: This was right out of Richard Berry's book, Build Your Own Telescope, and uses the push/pull method for collimation. We put acorn nuts on the ends of the push screws to prevent their gouging into the mirror disk unevenly. As with the diagonal, the primary is attached to its plywood back using three air spaced pads of silicone well in advance of assembly, of course. For small mirrors the technique works very well.
The mount: The azimuth bearing is a vinyl LP which rides on three Teflon pads. Teflon also supports the altitude bearings in the fork's V-notches and was salvaged from scraps of gasket material, commonly used in the chemical industry, at no cost.
The optics: In the earlier workshop we used our fine surplus eyepieces and the imported mirror sets. A spherical mirror can be "diffraction limited" given a certain minimum focal length, given by the formula f3=88.6D4 (in inches), as described in Texereau's How to Make A Telescope. Although a 4.5" spherical mirror at f/7.8 just barely meets the Rayleigh criterion, careful visual tests reveal the optical advantage of exceeding the criterion. So, for economy with less compromise, we decided to go with a spherical mirror of 4.25" diameter at f/10.6 for the teachers' workshop. Supplied in Pyrex and overcoated from the start, such a mirror approaches 1/8th l. Edmund Scientific offered us a generous discount in the quantity we required. We also selected a 25mm imported Kellner from IDAI giving 45 power for around $25.
Collimation: Admittedly, the prospect of having to collimate 25 telescopes at one go can be a bit harrowing. But, the f/10 optics lived up to their reputation and were relatively easy to align. Having a team of experienced collimators on hand to patiently walk the teachers through the process was a godsend. Each telescope was supplied with a sight tube made from a plastic film can. Fitted into the 1 1/4" washer in place of the eyepiece, the view through a little hole drilled in the center of the can's bottom shows where to bend the diagonal rod and holder and then how to adjust the primary.
Tom Kolar helping a teacher.
During construction the teachers were arranged in five workstations overseen by one of our club members. A few members floated from station to station to help solve problems and deliver materials. I led the process step by step and monitored progress. At the end of the day long event the teachers were obviously proud of these new resources for their schools that they helped construct. All the scopes worked and the participants were thrilled to view Jupiter and its moons that evening.
The scopes and teachers in action.
Each scope was released with a brief maintenance and usage manual and will spend the summer with the teacher who will take charge of its use this fall. The final cost? With the new design and upgraded optics the scopes came in at under $150, allowing the club a reasonable margin of profit to put toward other outreach and education projects.
Since 1995 the W.C.A.C. has been retained by Canton City Schools to continue to offer maintenance and training services to support the scopes. We've used the opportunity to experiment with providing beefier altitude bearings, made from #40 6" end caps and "permanent plugs", the addition of a Rigel Quickfinder for easier sighting, and the provision of a selection of various eyepieces. The science curriculum coordinator, Ms. Claudia Bower, seems satisfied to continue this mutually beneficial relationship with the club. It's been a uniquely astronomical opportunity to reach out to teachers and their students.
We'd be happy to take questions and comments from anyone interested in our design or in launching similar efforts in their communities. I'd like to thank the club's three past presidents, Dave Gill, Kent Rothermel and Eric Mast, for their interest and support for these projects over the years, and, of course, the many other WCAC members whose labors were indispensable to both their success and fun.
Rev. David J. Ross
Joann Ballbach, Education Director
TWC School Programs
The Wilderness Center
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Wilmot, OH 44689
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