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Thursday: I braved the rain and wind and went over to the observatory. Where I mounted the 7" OTA without its objective lens. Which saved me lifting another 10lbs at the far end of a 7' tube an awkward 8" in diameter.
Somewhat surprisingly I have managed to center the mounting's axis crossing point exactly in the middle of the dome at 145cm from the north and south support ring timbers. The dome is slightly larger but doesn't count for focuser clearance. Not unless the tube is pointed below horizontal.
With the lower tube mounting ring butted against the tube counterweight bracket there now seems to be plenty of room for me. I'd have no trouble fitting my head between the dome and the eyepiece now. Pushed another 10cm upwards through the rings would bring the OTA central on the saddle. But I'd need to modify the tube's own sliding weight rail. Or mount it on taller standoffs. Or just take it off. The stumpy dewshield would still clear the dome up high but not by very much. I can't take any new pictures because the constant rain prevents me from opening the shutters to get enough light.
Friday is warmer and brighter with only a risk of a shower. I need to mount the 7" objective and tackle the problem of fitting the 6" f/8 alongside the bigger refractor. A second pair of 235mm tube rings, to carry the 6", would need plywood packing arcs to match the 8" tube diameter. The stumpy 6" refractor doesn't suit the spacing of the rings mounted at each end of the cradle.
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