* Stop press..... a new 20" Dobsonian arrives at the Astronomy Centre- pictures and write up soon.*
The 30" Dobsonian
1) The 30" Dobsonian.
Dobsonian is a telescope mounting design popularised by the late legendary John Dobson in the USA. It is by far the most common method of mounting large amateur telescopes today.
I built the 30" Dobsonian in 1985/6 to view the return of Halley's Comet. At the time, to the best of my knowledge, it was the largest of its type in the world. The current 30" is a modified version of the original, during his visit to the UK in 2002 John Dobson himself signed the telescope as a guesture of approval.
The 30" F4.1 in 1 3/8" thick plate glass was made at Astronomical Equipment Ltd in Luton. The 6" secondary mirror was made by Oldham Optical.
Although outwardly a standard Dobsonian there are some novel features. Instead of the altitude and azimuth axes running on the usual Teflon pads, this mount runs on ballrace bearings, damping is by Teflon 'brake' pads which can be adjusted to control the ease of movement.
The primary mirror is located on a central boss passing through a 3" perforation in the mirror. There is no complicated support system to the back of the mirror, just a 30" disk of industrial grade plastic bubbles providing around 750 points of contact, so far this has proved satisfactory.
The whole Telescope is housed in a rectangular building and is wheeled out on rails for use. Set up time is just a few minutes by one person.