Friday, April 2, 2010
Under darker skies...and more refinements
Bill mentioned (and I noticed it too) the scope becomes stiffer at higher altitudes. After talking it over with him, and reading the section on stiction in Dave Kriege's The Dobsonian Telescope , I calculated out what the teflon pad sizes should be.
I weighed the upper assembly (mirror box, trusses, secondary ring, and accessories), and it came to 60 pounds. Divide the weight of the upper assembly by 4 to get the pounds per pad, which came to 15. Dave recommends 15 pounds per square inch for the teflon pads. Divide 15 by 15 and you get 1 square inch which is what I had on the altitude bearings already.
On to azimuth bearing, the total weight of the scope minus the ground board is 105 lbs. Divide that by 3 (because I use 3 pads) and you get 35. Again, using 15 lbs per square inch, divide 35 by 15 and you get 2.33 square inches. Take the square root of that, and that comes out to about 1.5 inches per side for the bottom teflon pads.
I cut out 3 1.5" by 1.5" pads and mounted them to the ground board. The scope moves noticeably smoother at the higher altitudes.