The K-20 Project

The K-20 Project

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6/23/2007 - Rocker Box and Ground Board are Completed, First Light is getting close!

The rocker box has been painted flat black on the inside and the glassboard bearing material added.  The Ground Board feet and teflon pads are installed and the Azimuth bolt (complete with the mounting hole for the azimuth encoder) is centered in the board through the Rocker Box bottom.  The last task was to install the teflon pads for the altitude bearings.  Not only is balance important for that "buttery smooth" motion Obsessions are known for but the right amount of friction on the various bearing materials is just as crucial.  The altitude bearings are of textured Ebony Star formica riding on Virgin Teflon.  When the balance is correct the pads must be sized so that they receive 15 pounds per square inch.  This meant the weight the Secondary Cage, truss tubes and fully loaded Mirror Box had to be determined.  We then divided that weight by 15 and then again by 4 (4 pads).  This gave us the square inch area required for each pad.  The actual width of the bearing surface on the altitude bearings is about 1 inch.  Knowing one dimension we worked out the length of each pad and cut them to size.  The Azimuth bearing is of textured glassboard on Virgin Teflon.  On smaller scopes Ebony Star is used but Obsession found that with an increase in weight above 18 inches of apeture, glassboard made a better bearing surface.  These two materials require 12 pound per square inch on the pads for the right friction.  This meant the weight the Secondary Cage, truss tubes, fully loaded Mirror Box and Rocker Box had to be determined.  We then divided that weight by 12 and then again by 3 (3 pads).  This gave us the square inch area required for each pad.  A little more math allowed us to figure out (using the square root function) that each pad would be about 2 inches square.  To eliminate any unneeded flexure in the ground board, the teflon pads were mounted directly OVER the feet, thus transferring the telescope's weight from the altitude bearings directly down through the Rocker Box sides to the teflon pads and straight down through the feet to the ground.  This design also causes the diameters of the Azimuth and Altitude bearings to be about the SAME size and thus the effort required to move the scope in either axis is the SAME!  Pretty neat huh?

ENTRY PICTURES


Flat black finish in the Rocker Box.	Installing the teflon pads on the ground board. They are sized so they carry 12 pounds per square inch for the correct friction. Also note (in the bottom picture) that they are mounted directly over the feet (lower pad in picture) for direct load transfer to the ground. They are mounted with counter-sunk brads.

Glassboard gets glued with contact cement to the rocker bottom and trimmed with a router.	Azimuth bolt installed, the rocker is now ready for the altitude bearing teflon pads and it's ready to carry the scope!!!