Twin 20” Dobsonians at ScopeX 2011 - by Johann Swanepoel - May 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 - but with a number of differences and improvements to some of the components. For instance, the slots in the side bearings for the truss tubes are not cut all the way through - thus not requiring an additional strengthening piece.
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-wraps through a hole in the lead cylinder end.
The telescopes are motorized using the controllers and associated motors and shaft encoders from Sidereal Technology http://www.siderealtechnology.com/
Gears and drive belts were obtained from http://www.sdp-si.com/
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-annealed and diamond pre-ground to F/4.5. This latter step meant the removal of a large amount of glass to obtain a sagitta of 7mm and was worth the marginal increase in cost.
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-mode (be able to emulate various other machines, like Hindle, Zeiss, Ritchie, Draper and Mirror-O-Matic). It had to make provision for modifications, adaptations and experimentation
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-size tools, a crocodile frame was implemented to hold the mirror in the manner and for reasons given by John Hindle [Amateur Telescope Making volume 3 – Chapter A.2, Hindle – Machine Polishing Mirrors].
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-table and one each for the arms. Motor speeds are controlled by varying the motor voltages. When required, an electronic timer module can be switched in to provide pulsed rotation of the central table to simulate ‘walking around the barrel’ as for manual polishing. An important aspect was ensuring regular and even rotation of the mirror during all grinding and polishing processes, to avoid astigmatism.
As very little sensible information was available in the literature or on the web (especially that dealing with machine figuring - typical manual type polishing and figuring strokes do not always work as desired), the machine provided the opportunity to research some of these issues, especially the use of various sub-diameter pitch laps in parabolising and controlling the outer edge of the mirror
Testing of the mirrors was done using mainly the Foucault test but using an adaptation of Suiter’s method [Harold Suiter – “Digital knife-edge test reduction” - ATM Journal volume 2 chapter 13]
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/