Idea Transcript
Issue 12 12.1 A 22-Inch Portable Telescope By John Lightholder At one time or another the quest for more aperture has driven most of us to increase the size of our instruments. At the time I was planning such a move, the limiting factor was the door to the Bob Dottle Memorial Observatory. We purchased our home from Bob and Barbara Dottle. The “observatory” site is where Bob formerly raised his pigs; but with a little sheet rock, and according to the neighbors, some “weird red lights,” we put together the makings of an observatory. Since the access door was only 3 ft wide, a 22-inch telescope was the limit. My past telescopes have all been portable, but I thought, “the 22-inch will need wheels.” So, off I went into dreamland to come up with a working model. I had Dave Kriege’s Obsession design in mind as perfect in function and form—although Tom Clark’s early Tectron telescopes seemed to have advantages, with their closed tubes made of foam. After working a while with some cardboard and hot glue, there it was—a model telescope with wooden dowel poles and a rough, unfinished look. Making physical models is a great way to design, and cardboard and wood lend themselves very well to telescope making—thank you very much, John Dobson! The primary mirror is f/6, and my design uses a minimal 2.6-inch minor axis diagonal (11.8% obstruction) for the absolute best in performance. Normally a 3.1-inch would be used—this 14% obstruction delivers a 0.6-inch fully illuminated field and is useful mainly for those who want to observe with wide-field 2-inch eyepieces. Enhanced coatings throughout give this 22-inch package the light grasp of a 24-inch scope with basic coatings. “Get the most from what you’ve got,” is the motto around here. The next things to consider for this “high tech wheelbarrow” were weight and the tracking platform. Enter Steven Overholt’s “door skins on foam” method with Georges d’Autume’s tracking platform, and you have unoriginal makings for an original design with excellent form and function. 12.1.1 Optical Tube and Rocker Box The diagonal cage has a 25-inch inside diameter and a 30-inch outside diameter. This allows plenty of clearance around the mirror. The cage rings are approximately &/8-inch thick, spaced 13 inches apart. I glued fluorescent poster board to the rings’ perimeter to trim the edges and cover the exposed foam. The color 1
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Fig. 12.1.1 The author with his 22-inch f/6 featherweight Dobsonian. The design features composite materials and is optimized for high visual contrast.
scheme of dark mahogany and fluorescent orange looks good day and night. The “solid tube” is rolled foam with the seam taped. The foam used for the tube runs !/2- to #/4-inch thick, and has foil glued on one side. When the foam sheet is rolled into a tube, the foil restrains the outer surface, forcing the inside to compress, while the outside stays smooth and does not crack. This gives a good appearance to the tube, and it can be painted if some other look is desired. The tape is a foil also and almost disappears. A ring is glued into it to form a hold, because an 8-ft tube is an awkward thing to deal with; and even though it weighs only 5 lbs, you still have to get your arms around it. Some Velcro strips painted silver hold the joint together. The tube keeps dust, dew, and the lookie-lews from getting on the mirror; and the foam enables it to reach ambient temperature on the spot. Foam is the perfect tube material, because it neither holds nor gains heat. The inside is painted with Dutch Boy flat black latex paint. The poles are anodized aluminum, 1!/2 inches in diameter with a 0.035-inch wall, and sit in split mahogany blocks made in the Obsession style. Since the poles are 8 ft long and would ring if struck, I filled them with foam to keep them from vibrating like violin strings. They are covered with black foam pipe insulation to keep observers’ hands from sticking to them on cold winter nights here in Lake Tahoe.
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The mirror box is a sandwich of door skin (!/8-inch hardwood plywood) and foam with foil applied to the outside. Radiant infrared is reflected, and sensible heat can get out. It is 28 inches square and 26 inches deep, with a 25-inch clear-diameter interior baffle. The foam-and-door-skin sandwich material weighs about the same as !/4-inch plywood, but has the rigid strength of solid plywood for a given thickness. The rocker box sides are 1!/2 inches thick, and the bottom sandwich is about 2!/4 inches. It weighs approximately 25 lbs total. The top arch of the rocker box is trimmed with fluorescent paper. During the day, the orange reflects off the white formica, which looks like it is lit up from underneath! The altitude bearings are 1#/4-inches thick and 30 inches in diameter, faced with pebbly Formica. The scope’s motion is very smooth, and requires only 3 lbs of force to start. Since the foam sandwich has no compression strength or density, I put wooden spacer blocks in anywhere there is a screw and to hold the Tee-nuts for the bolts. That way a firm, solid spot is available for the fasteners to grip. I installed pine cleats inside the sandwich at the top and bottom, with cutouts for the #/16 x 2-inch aluminum stock of the handle and wheel assemblies. To make it more rigid, the bottom has ribs extending from each corner and around the 30-inch circle that the azimuth bearing pads ride on. The rest of the space is filled with foam cut to fit. Like the altitude motion, azimuth takes about 3 lbs force to move. All Teflon pads used should run with a pressure of about 15 p.s.i. for that “butter feel.” Just figure the total weight resting on the pads, and divide by the number of pads. Divide this by 15 to find the right surface area in square inches for a single pad. The use of bolts, heavy upper-tube blocks, and other items makes the telescope very strong, but thanks to the composite foam structure, there is really no “extra” weight. The fully assembled weight of the cage comes to only 4!/2 lbs. Adding the focuser, spider, glass, and heavy pole fittings brings the total to 12 lbs, including the 32-mm Widescan eyepiece. The total weight of both bearing halves is 9 lbs. With the split blocks and all the hardware attached (less mirror and cell), the mirror box weighs only 40 lbs. The whole optical assembly less mirror, cell, and platform weighs 96 lbs. The glass is 53, the cell 24, and the platform with motor and wheels another 21, for a grand total of 194 lbs. It could have been even lighter; however, my goal was staying below 200 lbs! 12.1.2 The Driving Platform The driving platform is designed after the “Classic d’Autume”, shown in the September 1988 Sky & Telescope. The platform is very easy to make. It rides on bearings, and is constructed of 1-inch plywood using redwood segments covered with 18-gauge steel. The secret is to set up a jig angled to your latitude and sand the surfaces smooth. I used redwood primarily to save weight, but it also makes the sanding much easier. The drive sector is %/16-inch x 18 t.p.i., all-thread, steel-rod stock epoxied onto the curve and driven by a worm running at 2 r.p.m. The curve has an effective radius of approximately 25!/2 inches, which works out to the King rate at 1,438 turns. The drive is excellent and tracks for an hour and 4 minutes without resetting.
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Fig. 12.1.2 The aluminum bars of the wheel set are mounted in reinforced cutouts in the rocker box for transportation.
Fig. 12.1.3 The rocker box, showing the “wheelbarrow” handles and fittings. Teflon-pad bearings support the 96-pound tube.
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Fig. 12.1.4 The author’s method of reinforcing the telescope’s base includes wooden crossribs and supports along the tracking area of the 30-inch diameter turntable. Materials are plywood and plywood/foam sandwich, forming an ultra-light but rigid support for the 200-lb instrument.
The mirror cell is of the Obsession design, and the side cams have to be very close to the sling so the 7!/2º tilt of the platform will not shift the mirror. Even though these side-cams touch, there is really no lateral stress to the mirror as the tilt does not allow enough weight to distort the mirror’s figure. The mirror is from John Hall of Pegasus Optics, and sports an excellently smooth surface with beautiful crisp images and a textbook appearance of defocused and focused star images. I feel as though it is his best work. It is a treat and a joy to have such images impressed on one’s mind when the atmosphere will allow. There is always much curiosity about a telescope built in such a light manner. Still, I had vowed never to pick it up! The wheels and handles go on and come off without any tools. The appearance of handles was not in the original dream. However, after many workouts with an 18-inch reflector, I did not want to pick these things up any more. There are two battery boxes to the rear on the mirror cell. One 15-amp gel cell puts the scope balance 20 inches up from the bottom of the mirror box and will probably run the muffin fan for weeks. The other battery is used when the scope is taken to a really dark and remote site (about 45 minutes away, elevation 8,500 ft). With a 10-gauge wire threaded through the pole and an RCA jack at the top, the other battery allows a 12-volt hair dryer to chase the dew from the eye-
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piece or allow the use of heat ropes. It also counter balances the binocular viewer when the seeing is good and a planet is up. Two eyes really increase the detail and contrast beyond belief; you must try it. This telescope is everything that I have always wanted. Now it is just a roll away to the stars; and big, bright, crispy planets await. The latitude here is about 39° north. If the platform is level and pointed correctly, the objects stay put, even at 800x. This is absolutely wonderful, and necessary at our star parties. Anyone wishing to build a telescope such as mine, or get further details on its construction, should not hesitate to contact me days or evenings. Many of the difficult details are missing, since I believe it is best for all of us to come to our own methods of construction. That is what telescope makers and telescope making are really all about.
12.2 Choosing a Wide-Field Telescope By Norman G. Oldham When choosing a telescope, consideration must be given to aspects of light grasp, portability, preference for planetary or deep-sky work, and other related matters. If you have a particular interest in deep sky objects such as star clusters and nebulae, you will require a specific type of telescope. Clusters and nebulae are wide and faint compared to planets, which have narrow angular diameters and are relatively bright. Their faintness will require a wide field telescope and a camera using plates or film to pursue their study. 12.2.1 Off-Axis Aberrations of Newtonian Reflectors The Newtonian primary is figured to a parabola to cure spherical aberration at focus. This gives excellent image quality on-axis, at the center of the field of view. The image quality of Newtonian reflectors, however, suffers from what are known as off-axis aberrations. In practical terms, when one looks at an image near the edge of the telescope’s field of view, he sees light rays that are received from off-axis. The image appears as an elongated, pear-shaped smudge. Coma is the worst type of off-axis aberration for two reasons; the comatic image is not symmetrical about its center, and the light intensity is not uniformly distributed (as can be seen in image-spot diagrams.) As the focal ratio (F/D) increases, coma decreases; but so does the angular field. You cannot have both a wide-angle view and an aberration-free field with this system. Another aberration present in the Newtonian image is astigmatism. This is smaller than coma, and symmetrical in shape and light intensity. Astigmatism cannot usually be seen in a typical “fast” Newtonian, because coma swamps 1 it. 1
This is true only if you are using an anastigmatic eyepiece. The astigmatism in many eyepieces can swamp the coma of Newtonian and is often mistaken for coma.
Section 12.2: Choosing a Wide-Field Telescope 7 Fig. 12.2.1 Newtonian Telescope Tube length -