The full-thickness Duran 50 mirror was (as always) hand ground and figured, and has a 1515mm focal length. I don't normally like f/5 mirrors for visual telescopes as this usually means a large secondary, critical collimation, and generally small field of good definition, but I'll make an exception for this 'scope because it offers such benefits:-
The top end is as small a diameter as possible and the low-profile focuser has allowed me to use a 54mm (2·14-inch) secondary - the same size as my f/6·5 - thus eliminating my first objection. Attention to the mechanical details has generally alleviated the collimation complaint; only occasional minor tweaking is necessary after re-assembly. Unfortunately there is nothing that can be done with the small field and poor performance that such a fast mirror delivers. I use Nagler eyepieces and they perform adequately, but if you've never looked through an f/10 Newtonian with a Nagler then you don't know how good an image they can deliver.
The original ball was spun aluminium 600mm in diameter, with a 450mm diameter hole in the top. Unfortunately, it was only 1mm thick and it eventually buckled under the weight, so I covered it in several layers of fibreglass which resulted in a much better ball. A spun aluminium ball should work, but it needs to be thicker than the 1mm skin I used.
There are 6, ¾-inch diameter aluminium truss poles connecting the ball to the top end; they seat in split wooden blocks. The top end is plywood separated by 4 of the same aluminium poles. All the reports I'd read say that Kydex is best for light-tighting top ends; but I'd never seen Kydex before and wasn't even sure that it could be bought in Australia. I've now discovered what Kydex is often used for - computer print-out binders. I'd often thought that these computer binders would make a good substitute for Kydex, and after I used them around the top end I was told that it was in fact Kydex. Here is a close-up of the top end. You can see a short section of baffling on the top end opposite the focuser to make sure light can't get directly to the eyepiece and reduce contrast.
The top end has a removeable 8×50 finder, the only piece that needs to be removed to allow the top end to be placed inside the ball, sitting on the mirror cover. The short "tripod" upon which the ball sits is placed on top of the ball during transport, leaving only the truss poles and finder not in the one package. Here is a picture of the telescope folded into the ball for transport.
Like any alt-azimuth mounted telescope, it really needs an equatorial platform to perform at its best, and one day...
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Page last updated 1997/09/20