THE FLYING DOBSONIAN:
VOYAGER TO THE HEART OF DARKNESS
© Jack Gelfand 1999
My case of aperture fever started back in the early 70s, when I was working on new spectroscopic instrumentation for planetary atmosphere studies. I was at the McMath Solar telescope at Kitt Peak which is sometimes used at night for extended instrument development runs. The McMath is a 60" f/60 instrument with a number of fixed focal positions. A fresnel lens focal reducer in conjunction with a 2" diameter eyepiece is used to provide a large field of view for finding objects. We were recording spectra of Jupiter and Neptune and had a break in the middle of the night between the rising and setting times of the two planets. The spare time was used to calibrate our spectrograph. This left the telescope briefly available to cruise the summer night sky using the widefield finder. Looking through a large instrument from a premier observatory site is, of course, every amateur astronomer's dream. Needless to say the views were spectacular, even if the color correction of the fresnel lens was poor.
When the Age of the Giant Dobsonians arrived, I purchased a 17" Coulter Odyssey. On the day it arrived, I awaited sunset in anticipation of the splendors to come. But even the majestic Andromeda galaxy was just a bright spot in the middle of a wisp of nebulosity. It became apparent to me that giant aperture alone was not enough, because my east coast megalopolis location was bathed in constant twilight. A challenge began to develop in my mind. Could I both cure my aperture fever and travel to darker skies? I already knew that the best locations were on mountain top sites in the southwest and midwest, so this scope would have to capable of being shipped by air as luggage.
My first portable scope was a Dobsonian, that fit into a cube-shaped shipping container 24" on a side. The poles were carried in a ski bag. I carried the mirror on the plane in a box 18" square and 4" high that fit under my seat. The scope worked quite well but the airlines took one look at my 2 cube weighing 90 Lbs. and charged me $50 for oversized luggage on each leg of my trip. This brings me to a very important law of portable telescope design: The airlines will take anything for free, just so long as it looks like a suitcase. I spent some time in many luggage shops measuring suitcases and I purchased one of the larger ones which was 27"x21"x10". My challenge was to fit as large a telescope as I could inside.
My initial design was influenced heavily by the lightweight giant telescopes built by Steven Overholt described in his monograph, Lightweight Giants, which was briefly published by Owl Books. His use of composite construction and very lightweight, compact diagonal cages was my greatest inspiration. Second was the picture of the portable 13" scope built by Dennis DiCicco in Sky and Telescope of March 1988. Third was the picture of Tom Noe's motorcycle portable 10" that appeared in 1992 in both Sky and Telescope and Astronomy Magazines. He was the first one that I had seen use collapsible poles in a Dobsonian design. These instruments convinced me that it might be possible to construct a moderate sized scope in a sufficiently portable configuration. Many of the construction details of the scope described here were patterned after designs in Dave Krieg and Richard Berry's book, The Dobsonian Telescope: A Practical Manual for Building Large Apeture Telescopes.
The rest of this article is a description of my portable suitcase Dobsonian. In addition to describing the scope, I will outline the design tradeoffs that you will need to consider in order to build one of your own. My expectation was to be able to build a 14"-18" portable scope. Even the most careful lightweight construction of a 14"-18" Dob results in a 60-80 Lb. instrument. Attempting to put the entire scope in one piece of luggage would make it too heavy for one person to handle easily. Therefore, I decided to carry the mirror separately as I had in my prototype design. I made a number of measurements and concluded that an 18" square by 6" high mirror box could be carried on and placed under the seat on most airplanes. I used a 16" diameter, 2" thick, f/4.8 Galaxy mirror permanently mounted in a floatation cell in the mirror box. Carrying an unwieldy ski bag along with the other luggage through numerous airports convinced me that I would have to find a way to put the truss tubes in the suitcase as well. In this way I would have a total of two pieces of luggage, and my backpack for clothes.
Many observing runs consist of multiple trips over dusty roads to a remote observing site. This favors a nested design where the optics can be closed off from the outside world, either in the suitcase or when partially assembled. Additionally, the box structures used in nested designs are very rigid and protect the optics. This is especially useful for keeping alignment despite rough handling. The mirror box is divided into two parts, each 6" high. The bottom is the carry-on mirror box with the floatation cell. The top part is stored in the suitcase.
The suitcase contains the rocker box, top half of the mirror box and the diagonal cage nested into each other. The height of the space inside a suitcase limits the height of the telescope components dictating a low profile design for the scope. The suitcase also contains the side bearings, eight telescoping truss poles and a black nylon shroud. Through careful design and construction, this suitcase weighs only 45 Lbs. The carry-on mirror box contains the mirror and the 18-point floatation cell. The top cover of the mirror box is made of 1/2" birch plywood and serves as the ground board. The mirror box weighs 40 Lbs.
I believe that careful design of the diagonal cage is the secret to successfully building a functional portable scope. There are a number of factors to consider. First, as mentioned above, the height of the diagonal cage is limited. The cage is 5" high in my scope. With a 2.6" diagonal mirror, this does not leave much room for the vanes and diagonal holder. Second, the diagonal mirror must be mounted securely to protect it from rough handling. Typical diagonal mountings consist of a diagonal holder on the end of a screw that provides forward and back axial adjustment for centering the diagonal in the eyepiece tube. There is also some adjustment of the diagonal holder for returning the beam from the center of the mirror to the axis of the eyepiece. This whole structure hangs from the vanes making a typical diagonal cage in a Dob from 9-12" high. Besides taking a lot of room, my experience with standard diagonal holders is that the adjustment screws tend work their way loose in transit.
I modified a Novak diagonal holder to create a 3-vane framework where the vanes pass through the cylindrical part of the holder. All of the degrees of freedom for adjustment are an integral part of the structure. The adjustable components consist of flat surfaces which rotate or slide against each other, making it possible to tighten them securely so that they do not vibrate loose. This also leads to a compact structure that does not vibrate while observing. I glued the mirror in place with a bead of silicone sealant around the back. The silicone and the front rim on the holder prevent the diagonal mirror from falling out. More details of the diagonal holder are given here.
The diagonal cage is an octagonal box structure 18" across. It weighs just under 3 Lbs. without the eyepiece and finder. The diagonal cage must be light because it is a major determinant of the balance point of the scope. If the balance point is too far forward, the mirror box and the rocker box would then have to be too high in order to have a surface to attach the side bearing. This makes them too big to fit in the suitcase. One can compensate to some degree by making the side bearings of a larger diameter. But they too are limited in size by the suitcase. Thus, the height and weight of diagonal cage essentially determines the design of whole instrument.
The mirror box is 18" x 18" x 6" made of 1/2" birch plywood. It contains an 18-point floatation cell with the mirror supported by 2" wide seatbelt webbing fastened to the sides of the box. The design of floatation cells is covered extensively in Krieg and Berry's book. I found that maintaining the centering of the mirror in the box during transport was a big factor in reproducing alignment when reassembling the scope for observing. I considered gluing the mirror to the mounting to guarantee alignment but wanted the advantages of the floatation cell. So I designed a clamping arrangement using a V-shaped block which holds the mirror in the centered position while it is being carried. The mirror is released when I am ready to observe. The clamp is fastened to the inside top of the box and pushes the mirror against the webbing by the use of thumbscrews. Additionally, there is a clamp at the bottom of the mirror box that is pushed against the front rim of the mirror during transit.
A major feature of the telescope is the use of telescoping truss tubes. Because of the low profile mirror box and the thin diagonal cage, the truss tubes must be very long. In my scope they are 58" overall length, but must collapse to 20" in order to fit in the suitcase. Photographic tripod legs are a good choice for this application but their extended length is typically too short. I used Bogen #3006 monopods that are light, rigid and relatively inexpensive. A number of other alternatives could be used. Just remember that the poles must be of the telescoping type. Otherwise, if each pole is broken into 3 pieces, the bundle of 24 tubes requires too much storage space. The poles were modified by flattening the top ends in a vice. I found that it helps to heat the tube with a torch before flattening. I used captive, 1/4 turn electronic panel fasteners to connect the poles to the diagonal cage. These are light and can be operated easily, even with gloves on. The bottom ends of the monopod legs were modified by turning the plastic camera mount fitting down on a lathe to fit the Tectron clamp blocks in the mirror box.
Extensive use was made of composite construction. Though some exotic materials were used, construction techniques were quite simple. I constructed the side bearings and rocker box of thin plywood skin covering a styrofoam core. These were glued together with ordinary wood glue. I found it best to cut the component pieces 1/2" larger all around, and then trim the finished piece to correct size after gluing. The sides of the diagonal cage, the components of the 18-point floatation cell and the diagonal vanes were fabricated from carbon fiber composite panels available from Aerospace Composite Products. They supply fiberglass and carbon fiber prefabricated panels with foam, Nomex or balsa cores. These are available in sheets in many sizes and thicknesses up to 1/2". These sheets can be cut and drilled with ordinary tools and can be used for the fabrication of telescope components that are both very light and stiff. One important tip: If you are using screws to fasten anything to the composite panels, you must reinforce the holes because the balsa or foam easily crushes and the screws become loose. After drilling holes in the panels, I put a few drops of a very thin epoxy on the edges of the hole. The epoxy soaks into the core material surrounding the hole making it into a harder material that can support the compression of the screw.
Captive hardware is used throughout so that assembly proceeds very quickly. My personal record is 6 minutes from suitcase to sky - in the dark. Some minor collimation adjustment is usually required. The use of telescoping truss tubes simplifies this greatly. I carefully collimate the scope before leaving for an observing run. The mirror is clamped in place and the telescope is packed away in the suitcase. The dust cover on the mirror box has an easily visible cross centered on the optical axis. After the telescope is assembled, I loosen the top clamp of each of the telescoping tubes. Sighting on the cross with a collimating eyepiece, I just push in the appropriate truss tubes to align the axis of the eyepiece with the cross. I then remove the dust cover, unclamp the mirror, and touch up the alignment with the mirror mount screws.
I still consider this scope a prototype and many improvements are certainly possible. The mirror box is a bit heavy for carry-on luggage. Constructing the box and cover from foam core composite and using a thinner mirror would make it much more manageable. I have experimented with using only 6 truss tubes. There are Dob designs with as few as 2 larger diameter truss tubes which could be explored. Other telescope configurations are also conceivable. By using two suitcases instead of a suitcase and carry-on, I think that a portable 20" thin mirror Dob or a 13" Dob binocular could be constructed.
My travels to remote locations have convinced me that a truly dark site must be at least 125 miles from the nearest metropolis, 50 miles from the nearest town or settlement and 10 miles to the nearest outdoor illumination of any kind. There arent many locations of that kind left in the U.S., but the travel time is worth the work. Galaxies and nebulae in my 16" scope at really dark observing sites appear to the naked eye just as they do in photographs. One can see all of the detail in M-31, spectacular dust lanes, H-II regions and 16th magnitude globular clusters. Difficult objects like the Horsehead Nebula are easily visible. The Veil nebula contains infinite detail with strands of nebulosity twisting around each other like stretched cotton candy. Most spectacularly, the Virgo/Coma cluster of galaxies swarms with deep sky objects. Each field in my 32 mm wide-field eyepiece is filled with galaxies. I have printed a set of large-scale finder charts and have spent whole nights just sorting through the NGC and Messier objects. Is your mouth watering yet? Build a portable scope and I will see you in the heart of darkness!
I would like to acknowledge considerable help from my friend Jim Watson whose excellent advice and mechanical skills made this scope possible. I also want to thank Kirk Alexander and Roger Thorpe for all of their technical advice and encouragement and Bob Matthews for his photographic expertise.
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