On his quite informative web page, German ATM demigod Stahis Kafalis states the prime objective of the amateur stargazer: to get as big an aperture under as dark a sky as possible. On a number of trips we had taken our 102/500 rich field telescope along and had gotten quite a lot out of it, both visually and photographically. Now we lusted for more aperture to take along, but a quick survey of the market showed that nothing like the kind of telescope we wanted was available for purchase. If you wanted a compact and lightweight telescope you could take on an airplane or motorbike trip, you had to build it yourself. And there we were...
In autumn 2003, the Munich public observatory (Volkssternwarte München) offered a mirror making workshop for amateurs. We had heard quite a few horror stories about mirror making, but we were curious. For that workshop, we needed an attainable goal that we would actually be able to achieve within reasonable time, so a plan was hatched to make a 20cm mirror later to be placed in an airplane travel compatible Dobsonian telescope.
Most of the mirror making went smoothly with only a few glitches. Getting the parabola into the glass while avoiding turned-down egdes on the other hand proved to be a lenghty and unnerving process. Nevertheless we succeeded in the end, which left us with the task of building the scope around the now finished mirror.
Our goal was to build a 203/1270mm Newtonian telescope on a Dobson style altazimuth mount that should be compact and lightweight enough to be taken on all kinds of journeys.
Our main sources of inspiration were the telescopes built by Stathis Kafalis, Achim Strnad, Greg Babcock and Rüdiger Heins. Initially we had intended to use a two-truss design, yet none of the scopes realised in this way quite appealed to us and whichever way we tried to tweak things we could never unite our two goals of a diagonal viewing position and a triangular truss framework. So, after several weeks of worrying, we dropped the two-truss design and decided to go for six trusses instead. Some search through Google showed that no one on the Internet had built a small six-truss travel scope before, so our work would even have some pioneering character. Most of the mirror and rocker box design was bluntly copied from Stathis Kafalis' Travel Archimedes scope, no revolutionary ideas there.
Using six trusses also meant we could use a monoring upper tube. This, even though being slightly heavier than the typical two-truss contraptions, in our eyes had a number of advantages, like the possibility of using a solar filter, the feasibility of a standard spider design and a stray light baffle moved out of the optical path.
Transportability was to be achieved by a compact rocker box/mirror box design, where the upper tube and the altitude wheels should be stowed inside the mirror box (along with all sorts of small parts like the finder and the screws needed for assembly), which itself would be strapped into the rocker box. The trusses were to disassemble into two parts of equal length and would travel in a separate bag.
In the meanwhile we have come to understand that designing a travel Dobsonian resembles lying in bed with too small a cover. Whenever you tuck it in one direction to cover some spot, some other part will unavoidably be left out in the cold. Building a travel scope will present the telescope maker with a multitude of conflicting design constraints. Just as an example consider the issue of the telescope's weight. For transport, limited baggage allowances require the weight to be kept as small as possible, whereas for observing a larger weight will provide better stiffness and stability as well as better ease of use (particularly when using a Dobsonian near the zenith, the telescope must be able to offer some resistance to the push and shove of the user). The focal ratio of f/6.3 was chosen to allow for a more comfortable viewing position (though this did not turn out quite in the way we wanted) but later presented us with some problems regarding the height of the center of gravity.
Upper tube, spider and secondary
The upper tube is made of a simple plywood monoring of 18x20mm. In order to save some weight, the ring has been drilled with 10mm holes. The focuser is a Kine Optics HC-1 helical Crayford model fixed in a 4mm aluminum L-profile. The HC-1 accepts 1.25" eyepieces only, but for a travel scope we decided we did not need 2" eyepieces and were rewarded by a focuser weight of only 70g. A Celestron Star Pointer serves as the scope's finder and a stray light baffle cut from a cheap sleeping mat is buttoned to the ring opposite the focuser.
The spider is an excentric aluminum construction, a near identical copy of the ones used in Stathis Kafalis' 6-inch twins. The 38mm secondary from Antares Optics has been glued to the beechwood holder with silicon.
The six 10x1mm trusses each come apart into two parts. The individual pieces are connected via M5x25mm threaded inserts, cut in half and one half glued into each truss part (the inserts have a convenient 8mm outer diameter and snugly fit into the trusses). The lower parts then had a piece of M5 threaded rod glued into them. The biggest problem with this kind of connection is that you need perfectly straight and evenly cut truss tubes. As this is very hard to do with your average home DIY equipment I used the straight edges provided by my aluminum supplier (I bought six individual pieces of tube, so two even edges came with every piece) and used the not so perfectly cut egdes for the outer ends of the connected trusses.
A simple coding scheme (not exactly two bit, the zero is used additionally once but not explicitly coded) is employed to make clear which truss parts are to be connected (this system also works in the dark).
As a consequence of the long focal ratio of f/6.3 the trusses become quite long and the center of gravity rises to an undesired height. Since the altitude wheels are to be connected to a mirror box that itself has to be as flat as possible and rest on altitude bearings that themselves must be as low as possible, too, to reduce the torque and flex induced into the rocker box, the center of gravity has to be lowered to a point where the altitude wheels can be attached to the mirror box in an adequate way. In our case, this did not work out entirely and so we resorted to attaching a 400g counter weight made from a steel ruler to the underside of the mirror box with velcro. This counter weight is flat enough as not to encumber the movement of the mirror box.
The downside of a lightweight construction in general and a lightweight upper tube in particular is that the minor weight changes that occur when changing the eyepiece will already unsettle the scope. To counter this we installed a simple altitude brake whose design was also copied from Stathis Kafalis.
The trusses are attached to the upper tube by aluminum L-profiles and to the mirror box via beechwood braces drilled with 10mm holes at the appropriate angles.
The six-truss design caused a number of problems regarding the construction of the mirror box, the arrangement and positioning of the trusses as well as the attachment of the trusses to the upper tube.
When using eight trusses, all the trusses can be conveniently attached to the corners of the mirror box far away from the mirror and the optical path. With six trusses on the other hand, some of the attachments have to be fixed in the middle of the box very close to the mirror. Also, these trusses have be tilted in two dimensions which makes the manufacture of the braces much more difficult. It also means that a six-truss design requires a larger mirror box than an eight-truss design. Also great care has to be taken to move the trusses out of the optical path. These and certain other constraints require a complicated 'angled' upper tube attachment for four of the six trusses.
The rocker box
In its original state, the rocker box made out of 12mm plywood altitude bearings and a 15mm base plate was too heavy by far. By applying generous cutouts in all parts we could reduce the rocker box weight by more than 1 kilogram. The style of the cutouts was designed to maintain maximum stability.
The mirror box
The mirror box was glued and screwed together from 6 and 9mm birch plywood built to accomodate the upper tube and the altitude wheels. For transportation a protective lid is placed on the mirror box and secured by safety pins.
For journeys the mirror box is packed with the upper tube, the altitude wheels and all the small bits. The mirror box is then strapped into the rocker box and carried around with an additional shoulder strap. The disconnected trusses, the counter weight and the stray light baffle travel inside a 60cm plastic tube of the kind usually used by architects. The outer dimensions of the rocker/mirror box assembly are 32x32x24 centimeters.
The scope was finished in early September 2004, the evening before we went on holiday to the Canary island of La Palma. In this post-9/11 world, bringing such a strange contraption to an airport certainly means some hassle, so come a few minutes earlier to allow for an extensive examination. The trusses went into the main luggage, the box turned out to be compact enough to easily fit into most overhead storage bins. One exception was the small turboprop from Tenerife to La Palma, but in these cases a different solution is easily found. The scope appears to be rugged enough and lived well through the trip. The entire assembled scope weighs around 7 kilograms (Update: In the meanwhile, the assembled scope has been weighed at 7.6 kilograms, which is a bit more than expected. Obviously I never took the counter weight into account) and the box could be carried around without problems for the distances to be covered in your average airport.
Once arrived, we immediately assembled the scope from its parts, after all we had not been able to test it at home. The assembly went reasonably well even though following a fixed order is necessary for inserting the trusses into the lower braces and in one instance it seemed we had not been as precise in sawing off a truss as we would have liked. Nevertheless, the scope was erected quickly without tools and collimation also went smoothly (this time with a 2mm hex key). The first nights after that were quite unnerving as neither the transparency of the skies nor the seeing were up to our expectations. But finally a 6m-plus sky opened above us and we could seriously test our new scope.
Our new Dobsonian telescope works better than we had ever hoped in our wildest dreams. The self-made mirror is better than decent, the low linear obstruction of 18% also helps and the focuser works a treat (we had hitherto experienced the 2" version only, but the 1.25" works every bit as well). Properly focusing the eyepiece (No problems with focus positions either even with only 25mm of travel. The eyepieces we use are parfocal enough and a Speers Waler would be too heavy anyway) produces needle-sharp stars, contrast in faint nebulae is very good. Of course, 200mm aperture have their limits, but within these limits our Dobsonian performs quite well. The stiffness of the truss construction (we had been a bit anxious about this) is good, although we did not test the scope in high winds yet. The mechanical aptitude is sufficient as well, guiding works smoothly enough even at magnifications of more than 200 times. In fact, with a properly aligned star pointer we were able to properly position several objects in the 6mm eyepiece directly (211x magnification). Guiding becomes more difficult when observing objects near the zenith, the telescope's overall lightweight construction means that you easily drag the entire instrument around when trying to move around the azimuth axis. A bit more weight might be of help here.
The observing position is a difficult issue. With a focal length of 1270mm, the scope had been intended to be used without crawling on the ground. Observing from a chair, even objects barely above the horizon could be conveniently viewed. Observing near the zenith is a different story though. Sitting in a normal chair the eyepiece is located too high, and while standing taking up a fairly bent position is necessary (even though things are nearly perfect for my SO who is 20cm less of height). 1200mm focal length would have been preferable there at least for me. Placing the scope on a small stool or table might be an option we haven't tested yet.
The white paintwork had not been intended originally. But after a few days of use we already got used to it and leastways it means that there is no danger of stumbling over the scope in the dark.
The minor changes we intend to make to the instrument include better stray light protection by darkening the upper parts of all trusses and their ring attachments and improvements of the altitude brake, where cork from a wine bottle has shown to work much better than the original plywood. Also, the cottonwood eyepiece tray will be redone using stiffer birch plywood.
Already while building this telescope a large number of possible improvements became obvious.
Apart from a few minor glitches our first endeavour into the world of ATM was a thorough success. The relief we feel that after all the work our efforts were not in vain is enormous. It was a long way from the beginnings to the finished scope (almost 10 months) but it was worth while, especially as you still cannot buy our kind of scope in a shop. Optics and mechanical parts work and the whole thing can easily be brought under dark skies. The one major misgiving we have about the telescope is that there is no way of fixing this thing to an equatorial mount (what a waste!).
We would like to thank all the helpful people who aided us in building this telescope, giving information, comfort and moral support. Most prominent among these of course was Stathis Kafalis, also thanks to everybody else from the mirror making workshop, to Alois Ortner and to Martin Trittelvitz.