With the completion of our first selfmade telescope, the 8" travel dobsonian, and the hubris that ensued, we thought it might be a very good idea to have a slightly larger dobsonian for visual use just in case we ever got the autoguiding to work properly. Slightly larger because it would still have to fit in the car together with two mounts and two sets of imaging optics. Looking at Martin's 12-inch we decided to get ourselves a scope of this size. It was around that time that the Taiwanese dobsonians began to get really cheap so we went out and bought a GSO 1080 12"-inch full metal tube scope. We intended to scrap the cumbersome tube and scavenge the rest of the parts for integration into a truss design. Reusing most of the parts this should be done in comparativley little time and still be a lot lighter and more compact than some GSO redesigns we had seen on the net.
Dragging the scope out of the shop I got my first shock. That was really a big telescope! It barely fit inside the car, which would then be unfit for carrying any other equipment. The base diameter of the rocker box was 63cm! The first test of the optics (after several hours of cool down time, the scope had been inside the car for an entire summer's day) indicated that the primary was better than OK. Unfortunately, this did not apply to the mechanics which ranged from idiosyncratic (elevation) to plain useless (azimuth). There'd be no way around a complete reconstruction.
After we had taken a closer look and done some thinking the list of reusable parts grew ever shorter. The secondary holder and spider would not fit in our design and were scrapped. The original focuser, even though not that bad at all, had to make way for a Kineoptics item. In the end, all that remained were the mirrors, the primary cell and the 8x50 finder. Starting from there, the reconstruction should be a mostly conventional one, no exotic materials were to be used and we wouldn't aim for ultimate light weight. Nevertheless some improvement regarding the telescope's weight was intended, hopefully without balancing problems this time courtesy of the bloody heavy f/5 primary.
However, we could not refrain from including at least some gimmicks: The mirror box should contain some lateral ventilation for the primary and the upper tube should get a filter slide. Unfortunately, both of these caused more than minor construction problems and were mostly responsible for the rather lengthy 21 month building period. Due to the filter slide, the focuser had to move outwards which created difficulties fitting the upper tube inside the mirror box. And how to support and lock the slide in position? Solving these problems took its time and for a while the GSO reconstruction all but disappeared from our list of priorities. But then I finally managed to finish the project just in time for the ITV 2006 star party (which we did not attend after all). See below for some details.
The upper tube is a conventional two ring design made out of aircraft plywood and 9mm birch plywood. Two finders are in use, the original 8x50 is attached to the upper ring via an aluminum angle plate. Next to it a star pointer is fitted exactly like we did for the travel dobsonian. The spider, once again, is copied from Stathis' 7-Inch-Twins, the secondary holder was lathed and milled at the Munich Observatory (see here and here, text in German). The filter slide was changed into a filter wheel and was made to order by Schaeffer in Berlin. The bearing was scavenged from an old hard disk, arresting the wheel is done by M3 scews in the wheel and small magnets in the focuser board. Stray light baffles are fixed to the upper tube by velcro.
The anodised 22mm diameter truss tubes came from Dieter Martini and had to be reworked to get the edges right angled. Bicycle bar end covers give good grip and are much more comfortable to the touch than the bare aluminum. The truss poles are attached to the upper tube using slit angle plates and quick release skewers. The mirror box features standard beech plywood locking blocks fastened again with quick clamps.
The run-of-the-mill mirror box design originates again from Stathis' Travel Archimedes. Sides and bottom are made of 9mm plywood and the lower front of the box has been chamfered at an angle of 30 degrees. The primary support is made of 15x15x2mm square rods, the back of the mirror box has two cutouts for 90mm battery powered PC ventilators that can be run on two speeds. The 500mm altitude wheels have also been made by Dieter Martini and are of good quality, they only had to be sandpapered and painted.
The rocker box has simply been scaled up from the travel dobsonian, but this time without pivoting screw. The sandwiched ground plate has a round cutout and rotates around three roller brearings. The supporting triangle had initially been made of 6mm plywood but had to be reinforced by aircraft plywood after getting damaged once. The rocker box turned out not to be stiff enough, a 6mm plywood board cured that and now also serves as transport rest for the mirror box.
For transportation the mirror box accomodates the upper tube (a really tight fit), the mirror box and baffles are stored within the rocker box and the altitude wheels are screwed onto the mirror box lid. The battery box, filter wheel and finders go into a small separate case, the truss tubes are carried around in a truss tube bag that was as cheap as it is practical. The entire pack is reasonably easy to carry around, yet for a scope any heavier than this one extra carrying handles would be a must. Our reconstructed GSO 12 inch totals 19.5kg with all accessories but without eyepiece. Not a super lightweight scope, but still much better than the original GSO contraption. Weight was nearly halved and measuring 45 x 44 x ca.35 cm it has approximately standard German beer crate size, it will fit into the car boot together with two complete equatorial mounts.
Our GSO 1080bis telescope turned out OK, even though not in the same league as superb pieces of craftsmanship like this one. The optics are very good and including a filter wheel was one of the best ideas we ever had. The mechanics are so so, some bits do not work as well as we had planned (such as the truss tube attachment to the upper tube). But the scope works and produces beautiful images, never mind the minor troubles. We have no idea whether the primary ventilation improves the image quality, but anyway the soft whirr has an enormously soothing effect on the stargazer.
Looking back the rebuilding of the GSO dobsonian was quite a dogged affair and we're not sure whether we'd do it again. If you do not want to grind your own mirror, just buy a mirror kit and a focuser and do everything else yourself. This is better than permanently compromising your design in order to integrate some half-good purchased parts (in the end, you'll throw them out anyway). In addition to our special gimmicks this became so annoying that temporarily we lost all interest in the project. In the end I finished the 12 inch mostly so I could get all the loose parts out of the way and regain some space in our flat.
But now the GSO is ready and usable and everything has changed. With this scope we rediscovered the fun of visual observing. It is quite tempting just to skip the astrophotography and instead go hunting for small planetaries (always accompanied by the clack-clack of the filter wheel) or faint fuzzies. We are really glad that all the effort paid off in the end. All's well as ends well!
I would like to thank the people who helped us build this telescope, in particular Stathis, Tassilo, Martin and Harald. Also, we want to thank everyone from the mirror making and ATM workshop at the Volkssternwarte München.