Finished Telescope
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A 8 or 10 inch Dobsonian
by The Bolton Group
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Finished Telescope
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IntroductionThis classical Dobsonian was designed to withstand the hard treatment of Bolton Astronomical Society members and follows John Dobson's original ideas very closely. Dobsonians of this size are best built with mirrors of longer focal lengths (ie f/8) otherwise the eyepiece can get very low. It was actually built using a 9 inch mirror we were given but due to it having a turned down edge, it was stopped down to 8 inch (200mm) by simply painting a matt black rim. The design here is suitable for up to a 10 inch mirror and has been successfully used down to a 6 inch mirror. |
 3-d Perspective
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ConstructionThe telescope was designed using CAD (Computer Aided Design) in 1984, long before most people knew it existed! All that is needed is an 8x4 ft sheet of plywood and a few offcuts and despite its simplicity results in a sturdy telescope that will blow any Tasco away. Points to note are the tube does not go completely through the cradle box but stops about 3 or 4 inches short to clear the mirror. A lightweight PVC tube is ideal - these are generally too flimsy but in this design trapping it in the cradle box and providing an external ring at the top stiffens it. Alternatively a cardboard tube could be used. Aim for a tube at 1 inch larger than the mirror - two inches bigger is OK. |
 Cutting Plan from a single sheet
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It is optimised to be made from a single 8 x 4 foot standard sheet of 9mm plywood. To cut out the parts a jigsaw is ideal, especially for the curved bits. You will also need some 1 ins square timber for bracing the joints. Ours was glued with Cascamite, a traditional water proof adhesive. It is still the best in my opinion but has to be mixed with water and applied by brush. The only other additional major requirements being a thick ground board (worktop offcut) and a plastic or cardboard tube. PTFE pads provide the bearings - only 7 pads about 15mm square are needed - 4 for the altitude bearings and 3 under the rocker box. Don't fix the altitude bearings on the side of the cradle box until you've checked the balance point. You will need about 3 ins of 6 inch PVC pipe for the altitude bearing - or alternatively a parallel sided aluminium pan will do. Check with its owner before you cut it up! |
 Mirror Box detail
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The tail-board is hinged and drops down to provide easy access to the mirror - note the foam draught excluder around the edge. The mirror support system is very simple comprising three bolts at 120 degrees located at 0.7 of the radius. The bolts should be filed round on the top or alternatively small felt or rubber pads can be stuck on their ends. They also provide the collimation adjustment. To locate the mirror laterally three blocks are provided, again felt lined. They will need tabs on top which are to stop the mirror titling forward - bent aluminium was used. These tabs don't actually touch the mirror but are about 3mm above it. |
 Plywood ring around tube end
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To stiffen the tube and make it look better a plywood ring was cut for the top of the tube. The spider was made from thin aluminium vanes with a wooden central boss cut to 45 degrees. Modern practice is to silicone adhesive the secondary mirror in place. A cheap rack and pinion focuser was added - knowing what we know now a crayford would have been better. Again a conventional finder was used but I would recommend a Telrad instead.
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