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The original design of Dobsonians use wood rotating bases. The base mount is called
an AZ-EL telescope mount. Many telescopes these days use the Azimuth rotation and Elevation rotation type mounts.
The AZ-EL mount has many limitations in Astronomy. The AZ-EL is ok for just viewing by using your eyeball for short
periods of time. If you want to do long period tracking of a stellar objects with the Dob AZ-EL mount you have to learn to
push and pull and rotate in three dimensions all at the same time.
Astronomy viewing requires following an object as the Earth rotates and the sky moves westward. The rotation of
the Earth is around a single axis, the North Polar axis which runs through the center of the Earth. You are standing
on the surface moving some 600 mph eastward. When you look up at the stars you want to just point a telescope at that spot
and view. You now have to compensate for the Earths rotation eastward. The star motion wastward is in an arc across
the sky. So, why not make a telescope that just tracks the arc across the sky and does not have to do gyrations to follow
objects.
To make the AZ-EL mount work like an equatorial polar axis mount all we need to do is tilt the AZ-EL box up so the center
line of the AZ box points at the North Polar angle (your latitude on the Earth surface).
The AZ-EL box and be tipped up by using a wood wedge. The wedge front plate is tipped up to the angle needed to
point it at the North Polar Angle. Below is a picture of the original AZ-EL type mount. Next to it is the tilted wedge type
mount.
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| Figure 1: Left side a Dob Wedge, to the right is the AZ-EL Dob mount |
The next set of pictures shows the wood wedge assembled pieces. Each of the wood pieces is hinged by a 12" long hinge.
The side wood pieces can be folded down, and the front wedge plate will fold down flat. When folded down flat you have an
AZ-EL mount box. When you unfold all the wood parts it makes a equatorial wood wedge. Just insert the Dob box support
for the telescope into the rotator PVC cups.
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| Figure 2: Dob wood wedge folded down. |
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| Figure 3: Dob wood wedge being unfolded. |
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| Figure 4: Dob wood wedge opened and secured in equatorial mode. |
We can now look at the details of making the wood wedge front plate. Cut a piece of plywood wider than the 18" round
AZ circle that holds up the Dob box.
At the center of the Dob box support screw the 4" PVC pipe rotator nipple flange to the bottom of the 18" wood
circle. Be sure to get it centered on the circle.
At the center of the wood wedge front plate cut a hole so the 5" PVC pipe nipple will fit throught the wood and stick
out the front side. Screw the PVC pipe flange to the wood with wood screws.
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| Figure 7: The PVC pipe nipples screwed to the 18" circle and installed on the wedge front plate. |
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| Figure 8: This shows the 4" PVC nipple sticking throught the 5" with a retainer ring installed. |
When you finish the wood wedge and install the main Dob box support you may find that it rotates ok, but it wiggles up
and down and does not roll on the 3 wheel rollers very well.
You need to install a spacer on the wedge front plate around the 5" PVC pipe so that the FOR_RENT plastic sign can be
held a little bit higher above the wedge front plate. Then the 3 roller wheels should touch the FOR_RENT sign and roll
smoothly.
Go around the the back of the wood front plate and where the 4" PVC pipe plastic sticks through the 5" PVC nipple install
a PVC ring of plastic over the PVC pipe. Then install a pipe hose clamp over the PVC ring and tighten it so that the Dob box
is helt tightly in place.
This is a prototype so we may find we need to change something in the future.
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