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245mm f4.8 photographic Newtonian telescope This is the second rebuild of this telescope. Originally the optics were in an Orion Europa OTA that I bought off Peter Grego (it was featured in his book on the Moon). I disliked the original tube and tube rings (thin and wobbly) and focuser (1.25 inch plastic) and the spider assembly (clunky and wouldn't collimate properly). The original tube was used to cast a concrete pier on which the telescope now stands (I also use my C11 on this pier).
The first rebuild used a plastic tube and a 2" metal R&P focuser and a re-designed spider/secondary assembly, made from a bent aluminium ruler and other bits. This was an improvement, but the plastic tube was still too flexible and collimation too variable (this is the telescope shown in my book).
Having found that what I wanted could not be bought in the UK, I bought a 12" diameter, 48" long, 0.1" thick aluminium tube with aluminium rotating tube rings machined precisely for it from Parallax Instruments in the States. These were very expensive to import (far more than I paid for the original scope), but well worth it. Joe Nastasi at Parallax is a great guy. He painted the outside of the tube metallic grey, and the inside the blackest matt black I have ever seen.
I fitted a JMI Next Generation motorised low-profile Crayford focuser.
I adopted a spider design I saw on the Royce optical website, consisting of three unsymmetrical curved vanes, and I made this out of a strip of thin springy steel I found in a car park. I screwed this to a simplified version of the first rebuild secondary collimation system, and installed three M8 thumbscrews with nylon tips to allow adjustment without tools.
I basically incorporated every kind of adjustment I could think of in this rebuild, to try to create my ideal, completely adjustable Newtonian. The tube is rotatable (easily and smoothly), the focuser is rotatable and collimatable, the secondary can be rotated, angled and translated in all planes.
The original primary cell worked OK but was too small for the new tube, so I had to devise an arrangement of mounting bolts to take up the space. I also cut holes in the solid back of the cell and installed a fan (this was in the earlier rebuild).
I incorporated an exceptionally large amount of back-focus. This was accidental and due to a miscalculation. However, it allows the possibility of using a radial guider, which is normally impossible (or difficult) with Newtonians. It also places the secondary far enough down the tube that it is unlikely to dew (which used to be a problem). However, the secondary now doesn't fully image the primary and really needs replacing with a bigger one. The telescope can be used visually, but it required two stacked 2" extensions plus eyepiece to achieve focus.
On first assembly and bench-testing with a laser, the scope proved to be in almost perfect collimation already, which shows that I built it quite accurately.
In my opinion, the major triumph of this project has been the Royce spider. I have experimented with lots of spider designs before, but this one combines strength with non-obstructiveness better than any other I have tried.
The completed telescope is heavy, but the Astro-physics 900 will cope with it easily. It will live in my run-off shed, where I intend to alternate it with the C-11.
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