Twin 20” Dobsonians at ScopeX 2011 -
The telescopes: For the last several years I have been working on completing two 20” F/4.3 Dobsonians; one myself, the other for a friend. One of the telescopes was displayed at ScopeX 2010, where it received an ATM award that year. This year, both of the 20” Dobsonians were displayed. Each time, the one telescope was transported all the way from George in the Southern Cape, a distance of some 1250 km. This year the second scope was already in the Johannesburg area with the friend, making it possible for both telescopes to be at the 2011 show. Refer to pictures below.
he design for the two telescopes was largely based on information in David Kriege and Richard Berry’s Practical Manual for Large Aperture Telescopes -
Also, the mounting brackets at the top of the truss tubes that attach to the connecting ring are simply made by appropriate drilling, cutting and rolling out of the tube end to form an integrated offset bracket without requiring elaborate and loose additional hardware. Easily removable cylindrical lead weights fit inside the top ends of the truss tubes and are stopped from dropping down the tubes by plastic tie-
The telescopes are motorized using the controllers and associated motors and shaft encoders from Sidereal Technology
Gears and drive belts were obtained from http://www.sdp-
Grinding and polishing machine: Two 20” (508 mm) diameter 48 mm thick Pyrex blanks, each weighing about 20 kg, were imported from Newport Glassworks in California. The blanks had been factory fine-
It was decided from the outset to build a machine that could perform the required fine grinding, polishing and figuring functions. The machine had to be modular (have interchangeable components) and be multi-
The machine was built from an old washing machine as basis, with two removable steel frames added to hold motors, gearboxes, bicycle sprockets and chains. As most of the grinding and polishing would be done with mirror on top (MOT), using full-
Not all the mode capabilities were implemented at the outset – the latter three evolved later when the need arose to correct recalcitrant zones and deviations near the outer edge of the mirror. Special but simple wooden arms were implemented for these modes.
The machine has three DC motors, one for the central turn-
As very little sensible information was available in the literature or on the web (especially that dealing with machine figuring -
Testing of the mirrors was done using mainly the Foucault test but using an adaptation of Suiter’s method [Harold Suiter – “Digital knife-
Further reading on the construction processes can be found at:
Further reading on the testing processes used can be found in an article in the December 2010 issue of MNASSA (Pushing the Limits of the Foucault Test): http://www.mnassa.org.za/