Issue A 22-Inch Portable Telescope. By John Lightholder

Transcription

1 Issue 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 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

2 2 Issue 12 Fig 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 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.

3 Section 12.1: A 22-Inch Portable Telescope 3 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! 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.

4 4 Issue 12 Fig The aluminum bars of the wheel set are mounted in reinforced cutouts in the rocker box for transportation. Fig The rocker box, showing the wheelbarrow handles and fittings. Teflon-pad bearings support the 96-pound tube.

5 Section 12.1: A 22-Inch Portable Telescope 5 Fig 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-

6 6 Issue 12 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 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 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 it. 1 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.

7 Section 12.2: Choosing a Wide-Field Telescope 7 Fig Newtonian Telescope Tube length - <F/L Field curvature - concave Primary figure - paraboloidal Off-axis image quality - very poor Fig Schmidt Camera Tube length - <2 F/L Field curvature - convex Primary figure - sphere Off-axis image quality - excellent The Newtonian Telescope When comparing different types of telescope systems, we should put special emphasis on their practical uses. In the examples below, I use a 200-mm-diameter primary with a focal length of 800-mm and F/D ratio of 4, yielding a 3.2 field of view with a 44-mm diameter. The 200-mm parabolic primary mirror is situated at the bottom of the telescope, see Figure An elliptical flat is fixed on a spider support about 550 mm from the primary. The flat redirects the converging beam to a focus outside of the tube at right angles to the optical axis, making the telescope a photovisual instrument. The overall tube length is, therefore, less than the focal length of the primary. Both coma and astigmatism increase in size as the image moves off axis, and are worst at the field edge. Coma is usually more serious. In the system profiled, the comatic plume grows to a size of 250 microns at the edge of the field. On a properly exposed photographic plate, the length of the comatic image would be about!/3 of this, not very acceptable for photographic purposes. Also, the image surface is concave as seen from the primary mirror. The result on a flat film surface is that star images increase in size progressively towards the field edge. This is due to the photographic plate being displaced from the curved focal surface, producing an image in which the out-of-focus stars have the appearance of small disks. Thus, flat-field photography with this Newtonian system would require a suitably curved film The Schmidt Camera The Schmidt camera s primary mirror has a spherical figure, see Figure

8 8 Issue 12 Fig Schmidt-Newtonian Tube length - <F/L Field curvature - convex Primary figure - sphere Off-axis image quality - poor Fig Wright Camera Tube length - 1 F/L Best field curvature - flat Primary figure - oblate ellipsoid Off-axis image quality - acceptable The corrector plate, situated at the center of curvature, has an aspheric curve figured on one or both sides. The distance between these optical elements is twice the primary s focal length. The image plane (in this example 800 mm from the primary) is convex toward the primary. The radius of the curved focal surface has the same value as the primary focal length. The film and holder are fixed to a suitable spider arrangement and attached to the camera tube. It is interesting to note that a flat photographic film could be used in this F/D = 4 system without objectionable images at the edge of the field. Flat-field photography with a faster system would require that the photographic plate be bent to match the curved focal plane. Overall performance is superb, giving a wide, diffraction-limited field of view of over 3 using a conventional 24 x 36-mm negative. Due to its design symmetry the Schmidt camera is an optically pure system, free of coma and astigmatism. In fact, the only function of the aspheric plate is to cancel the spherical aberration caused by the spherical primary Schmidt-Newtonian The primary and corrector plate are the same as in the Schmidt camera, see Figure The main physical difference is the introduction of a flat which redirects the beam to the side of the tube, making it a photo-visual telescope. The corrector plate is situated a little inside the primary s focal distance to enable fixing the elliptical flat to the plate. Because the plate is situated nearer the primary focus, the system suffers from coma and astigmatism. The coma is a little over half the size of that in the example Newtonian (Figure ); 150 microns at the edge of the field. This gives poor photographic images. The field surface is convex towards the primary surface, having approximately!/3 of the curvature of field of the Schmidt-camera (Figure ) Wright Camera This variant of the Schmidt camera was developed by F.B. Wright in 1935 and is

9 Section 12.2: Choosing a Wide-Field Telescope 9 also known as the short-tube Schmidt, see Figure The primary is figured to an oblate ellipsoid. The aspheric profile of the corrector plate must be twice as strong as that of the Schmidt camera to eliminate spherical aberration caused by the primary. The corrector plate is situated close to the primary focus. The oblate figure of the primary is necessary to suppress coma caused by rays entering the telescope at this position. This system cannot suppress astigmatism, because the plate is not at the center of curvature of the primary. Due to astigmatism, the diffraction-limited field of view is not as wide as that of the Schmidt camera (Figure ). This aberration causes star images to enlarge, but fortunately without destroying their symmetry. The size of the astigmatic blur spot at the edge of the field is about 30 microns. Thus, the best focal surface is almost flat, and a conventional photographic plate can be used instead of a curved one. The film and its holder can be fixed to the inside of the corrector plate Wright Telescope This system is similar to the Wright camera. The main physical difference is the installation of a flat which redirects the converging beam to the side of the tube, making it a photovisual telescope, see Figure The corrector plate is situated a little closer to the primary than in the Wright Camera of Figure , which enables fixing the elliptical flat to the plate. Because of this de-spacing, the oblate figure on the primary needs to be a little stronger than the Wright camera s to correct for coma. The size of the astigmatic blur spot at the edge of the field is about 30 microns Lensless Schmidt Camera This system has a spherical primary and no corrector plate. The tube length is equal to twice the focal length, making it a long instrument, see Figure The entrance to the system is open. Not having a corrector plate makes for ease of fabrication. The system is free of coma and astigmatism. This is because of the distance from the primary at which the rays enter the camera, as shown in Figure Since there is no corrector plate, the system suffers from spherical aberration. The spherical aberration is symmetrical in shape and smaller (100 microns in diameter) than the coma seen at the field edge of the Newtonian system described above. This aberration occurs over the whole field, both on and off axis. The problem of the blur size can be overcome if the spherical aberration in the telescope can be reduced to produce 25-micron size images. This can be done by placing a 120-mm-diameter annulus at the entrance to the camera. The photographic film will need to be curved in the same way as in the Schmidt camera. For convenience one can use a flat film plate and still keep within the 25-micron size criterion for the edge of the field. This is done by further reducing the diameter of the annulus to about 100 mm. The disadvantage of stopping down the aperture is that it yields a slower photographic system.

10 10 Issue 12 Instrument Advantages Disadvantages Newtonian Telescope A tube half as long as the Schmidt Camera. Good images near the center of field. An accessible focal plane, making it a photovisual system. Poor off-axis images and a curved focal surface. Schmidt Camera Schmidt-Newtonian Wright Camera Wright Telescope Very wide, diffraction-limited field. Tube length same as the Newtonian, accessible focal plane; therefore, a photovisual instrument. Good on-axis images. Good images over all the field although not quite as good as the Schmidt camera due to astigmatism. Tube only a little longer than the Newtonian. The focal surface at best focus is flat. Good images over all the field, although not quite as good as the Wright camera due to astigmatism. Tube only a little longer than the Newtonian. The focal surface at best focus is flat and is accessible, making the instrument photovisual. Lensless Schmidt Camera Good images over all the field, provided that the entrance of the telescope is stopped down, reducing the spherical aberration to an acceptable size. Lensless Schmidt Telescope Good images over all the field, provided that the entrance of the telescope is stopped down, reducing the spherical aberration to an acceptable size. Inaccessible focal plane, a curved focal surface, and a tube twice as long as the Newtonian Poor off-axis images, although better than the Newtonian. A curved focal surface. Focal surface is visually inaccessible. Images a little larger than the Wright camera A tube twice as long as the Newtonian, with a curved image surface. Photographically slow. Focal surface inaccessible. A tube twice as long as the Newtonian, with a curved image surface. Photographically slow. Focal surface is accessible Lensless Schmidt Telescope This has the same optical arrangement and tube length as the lensless Schmidt camera, and the off-axis images are identical, see Figure In this case an installed elliptical flat redirects the beam through the side of the tube, making it a photo-visual instrument. The F/D ratio is limited to about 4 or slower. This is caused by the need to keep the flat size no greater than 25 30% of the primary diameter, due to the diameter of the annulus which is situated at the entrance of the system.

11 Section 12.3: Making Large Thin Mirrors 11 Fig Wright Telescope Tube length - < 1 F/L Best field curvature - flat Primary figure - oblate ellipsoid Off-axis image quality - acceptable Annular ring Fig Lensless Schmidt Camera Tube length - 2 F/L Field curvature - convex Primary figure - sphere Off-axis image quality - good Annular ring Fig Lensless Schmidt Telescope Tube length - 2 <F/L Field curvature - convex Primary figure - sphere Off-axis image quality - good Summary In considering the differences that will affect your choice of a telescope for deep-sky and photographic work, be mindful of the need to choose an instrument with a wide angle and diffraction-limited field of view Making Large Thin Mirrors By Mel Bartels Astronomical objects, being very far away, are often very faint. It takes a great