300mm f 8 Dobsonian telescope adventures - by Wim Filmalter - May 2011
I live in Riversdale in the South Cape region of South Africa. My first exposure to telescopes and their ability to open up a whole new way of life came during a visit to Sutherland, the site of SALT - the South African Large Telescope. However, after the very short view through the visitor telescopes during a presentation, I decided to build my own.
12" Dob: I got hold of an old home-built telescope that was rusting away in a garden in town. The 300mm mirror had to be re-aluminized and it seemed a waste of time to salvage the rest of the crude construction only to be limited to a fixed position again. With no previous experience of telescopes, I read a lot and found the Dobsonian to be the most transportable. The f/8 mirror did not make the design any easier, leaving me with a focal length of 2450mm, and the need to transport a step ladder as well.
The end result was a very satisfactory telescope with everything folding away into a box of which the lid doubles as an accessory table. Only the twin set of truss tubes go into a separate bag, while the ground board (ex washing machine drum mounting) and counterweights are taken along separately, saving some weight in the box which weighs some 40 kg on its own.
Equatorial stand: The 300mm f/8 telescope has a long optical tube which is quite vulnerable to vibration. Apart from that, the high power magnification and relatively small field of view of a long-focus mirror causes any celestial object to race through the view and requires regular catch-up manoeuvres by the operator. During public viewing sessions this can become extremely disrupting and valuable observing time is lost. So I was looking for a serious piece of equipment to facilitate tracking.
After looking at a few designs of conventional equatorial platforms, I decided to build my own design of a swing-type stand that could accommodate the big scope as well as smaller ones. It also had to be foldable and transportable. The frame was constructed using 38 X 38mm square tubing, with sturdier pipe as a beam for the scope carrier. Hinges for the folding stand are of the ordinary “bullet” type, while the bearings for the swing are made of stainless steel round-bar on Teflon pads. Instead of a straight rotating rod in a nut moving the swing from side to side, I used a threaded rod curved to the radius of the swing turning circle and a rotating nylon nut-wheel driven by a small motor through a well-reduced set of gears. Power supply is from an ordinary twin-cell torch, modified to accept wires from the motor. A slot on the vertical part of the swing provides the capability to fine-tune the tracking speed. A camera tripod head can be mounted on the swing for long exposure photography.
THE PORTABLE ‘BIKE-SCOPE’, a foldable 8” Dobsonian: The 12” Dobsonian is well suited for travelling in a car or a pick-up truck but our group of friends love travelling the Karoo area of South Africa on our dual-purpose motorbikes. This brings us to places with clear, dark sites - but I needed a telescope that can be transported on a bike. I got hold of an 8” mirror and started designing with the sole aim of lightweight transportability in mind. Eventually the ideal rocker-box proved to be the bottom part of a Hart casserole, a household brand of alumin(i)um cookware in South Africa. The lid doubles as the ideal mirror dust cover. The truss tubes are from old TV antenna struts. The first altitude bearings were considerably smaller than the final design, requiring some serious counterweights. It would beat the object of portability if an 8kg telescope needed 5kg of counterweight. By increasing the diameter of the altitude bearings, the need for counterweights was completely eliminated and the scope balances perfectly with a range of different size eye-pieces. A weight is needed only when mounting a camera to the scope, and in the bush even a stone can be used!
In the end, the best scope is the one that you have with you wherever you go!