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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
The bearings are made of 2 pieces of 1/2" inch plywood glued
together. The friction surface is Teflon against Formica.
- A telescope will only behave well if
the center of gravity of the telescope matches the center of the
- 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
- 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
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.