Introduction and concept
Introduction and concept
The idea for building a large telescope came from seeing the coulter optical companies telescopes back in the early 80's. I made my first telescope in 1981. It was a 8.75 f5.4 fullescopes mirror. I bought the optics, mirror cell and spider then made the tube, focuser and equatorial mount. I had many years use out of if and did lots of modifications to it over the years including modifying the mount, tube and adding an RA drive system and VFO to suit. I still use the optics today in a skeleton dobsonian arrangement to take to dark skies or on holiday. Parts of the mounting (RA bearings, part of the RA shaft and the RA wheel) are now used in my present observatory on the 18 inch telescope. This was started back in 1990 and is still in use today. This is an 18" F5 mirror, made of Suprax, mounted equatorial with a 6"f8 guide scope, car wiper motors for slewing with a differential gearbox in RA on the Synchronous motor for tracking. The system uses a SBIG ST4 CCD camera (electric focus and electric shutter to aid imaging also) to down load images to a 386SX notebook mounted in the run off shed observatory. The system operates quite well and many diffraction limited images have been taken (G1 in Andromeda, Comet Hale-bopp rotation, Martian surface details and very short exposures also. M13 in 5 seconds, M57 in 10 seconds).
Original plans to start back in January 1995 did not get underway because of other commitments and funding. But the costing exercise at that time proved that a large telescope could be made at a resonable cost. After much phoning around I found that 1" plate glass of 500mm diameter with a nipped edge could be purchased for around £50. A 1000mm (40") piece of glass could be purchased for around £150 with a nipped edge (Specialised glass of St Helens). Aluminising from Vacuum coatings (in London) estimated around £180 for a 1000mm mirror was within reach (I had expected it would cost a lot more). It seemed that a 40" dobsonian could be built, including the dobsonian mount, for around £650 all in!!
Original worries about a thin mirror were flexture of the mirror in its support. One idea was to make a blank from two discs of one inch plate glass separated by vertical sections of glass bonded to it to make a sandwich. This idea was tried on a 10 inch scaled down version of a 40 inch mirror. Different glues were tried for strength. Test pieces were left for 6 months to see if the bonds weakened. It was found that the bonds did weaken but that standard epoxy glue worked best. Roughing the surface of the test pieces prior to gluing helped a little to maintain strength but still the bonds were not good enough. It was decided that it would be too risky to build the blank in this way only to find it collapsing a few years later. The single blank idea was reverted to with ideas developing with how to support it sufficiently. The cost would now also be cheaper. (Future ideas for larger mirrors could be to sandwich the discs directly together rather than separate them by vertical strips). The contact surface area would then be much higher with the chance the bond may last. Differentials in temperature between discs and glue may be a problem though?)
The concept did not really get going until December 1996 when patience ran out and some Christmas funding just happened to be available. Geoff Regan and myself began to plan in earnest the size of mirror to go for. It was overall cost, logistics of physical size to get to the eyepiece and, as will be seen later, the length of my garage, that determined a size of 30" in aperture.
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