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The "Deep Space Scope" Design

Home | Astronomy | DS-3 | Bearing construction and mounting

Picture of the bearings mounted on the mirror box, and the bearings alone.

The bearings are made of 2 pieces of 1/2" inch plywood glued together. The friction surface is Teflon against Formica.

• Methodology:
  • A telescope will only behave well if the center of gravity of the telescope matches the center of the bearings.
  • One approach is to add weight to the top or bottom of the telescope.
  • Another way is to add springs in various combinations to the telescope.
  • The last, and preferred method, is to move the location of the center of the bearings to match the center of gravity.
  • The size of the bearings is not important for balance.
  • The size of the bearings is important for telescope size, bearing friction and total stability.
  • We want to make the bearings big enough that we minimize the size (and required strength) of the rocker box. Bigger bearings also slide better, giving the telescope that "buttery smooth" feeling.
  • We want to make the bearings small enough that they do not increase the total size of the collapsed telescope. Also, the smaller the bearings are, the less prone to vibration that they are.
  • We want to make the bearings a size that allows us to mount them directly to the primary mirror box.
  • We want to make the secondary and trusses as light as possible. The reason is simple. Any weight added to the secondary will move the center of gravity higher away from the primary. This in turn will force us to move the center of the bearings further from the floor of the rocker box. This forces us to have a bigger rocker box or larger bearings. Since this is bad, keep the total secondary weight to a minimum.
• Find the center of gravity. Put together the telescope so far. Take off the mirror cover. Add everything to the secondary cage of your scope that you will eventually need. This includes your heaviest eye pieces, finder, Barlow, filters and baffling. Since it is a lot easier to add weight to the secondary cage than the primary box, add 1/2 pound to the secondary cage. (Fishing weights work well for this.) Add 1 1/4 pounds to each side of the primary mirror box, to approximate the weight of the bearings. Lift up the telescope by a truss using one of your fingers.  Keep moving your finger back and forth on the truss until the truss is parallel. Mark and measure this point with a piece of tape. This is the center of gravity and the center of the bearings.
• The bearings can actually be quite a number of sizes and work.  Assuming that the columnation bolts stick out of the bottom of the mirror box 1", a good bearing radius is from the center of gravity to about 1/2" below the bottom of the primary mirror box. This allows the telescope to never sit on the bearings when off of the rocker box, gives the bearings a good surface next to the mirror box to attach to, and doesn't move the physical layout of the bearings TOO far back and up. You will notice that my bearings are larger than this by a bit, and that works also. Have these bearings cover 180 degrees around.  (We will trim them smaller in a moment.)  The bearings should be 2" from inside radius to outside radius. Glue the two plywood pieces together that makes the 1" thick raw bearings.  Cut these bearings using a router.  Glue a piece of Formica to the outside edge of these bearings, and trim up the Formica with the sander.
• Cut a 1" dowel 15 1/2" long. Drill pilot holes in these dowels.