Start here to assemble the Dob Telescope Azimuth rotation base circle,
and Dob box support circle.
The simplest Azimuth (AZ) base rotation system for the Dobsonian
(Dob) telescope is just two round wood disks and a smooth spacer between them. The
round wood circles need to be large enough to handle the telescope size and weight that you are building. There are several ideas for the spacer material that goes between the two circles and allows the telescope
to rotate in azimuth (AZ) in a smooth motion.
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| Figure 1: The Dob base azimuth wood circles |
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| Figure 2: Wood base that warped due to water contact |
Circles are used to allow the telescope to rotate in the horizontal plane (azimuth). The upper circle of 18”
rotates on top of the lower circle of 24”. The upper 18” circle will hold the Dob plywood box support for the
telescope tube assembly. The lower 24" circle will sit flat on the ground and be stationary.
The wood circles are precut 1.25" thick purchased from a hardware store.
The circles need to be varnished so that they don’t get wet and warp. If they get wet
they can warp badly. See Figure 2, the first 18" wood circle delaminated or warped during fabrication and had to be replaced. Warped circles will not rotate smoothly.
Preparing the wood circles
Starting with the 18” smaller
wood circle, mark the center of the circle. Center the 18" circle on top of the 24" circle and drill a ½” hole vertically through the center of both circles. A 1/2" all thread bolt
will pass through both wood circles from the bottom Dob base.
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| Figure3: Mark the center of the base circles and drill a hole through both |
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| Figure 4: Drill a larger hole to counter sink the 1/2" bolt head into the bottom circle |
Choose
one side of the 24" base wood circle as the bottom. The bottom will sit on the ground when the telescope is in use.
The head of the 1/2" bolt will need to be counter sunk into the bottom of the
24” wood circle. Use the side of the wood that will be the bottom and drill a larger hole around the ½” bolt
hole so the bolt head will counter sink flush with the surface of the wood.
Choose
one side of the 18" wood circle to be the top. If the wood circle is warped use the convex side as the top or buy another
circle. Go read the page on the fabrication and cutting of the plywood to make the Dob box support. Then, mark
the top of the 18” circle for the Dob plywood box support pieces. You
can pre-drill screw holes between the marked lines. The screws used to assemble this Dob will be simple drywall screws of
about 2" long.
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| Figure 5: Mark the top AZ circle where the Dob box tube support plywood will attach |
The Spacer Material for the AZ rotation
Now you are ready to consider a material to use for the spacer between the 18” and the
24” circles. Several materials will be tried.
If you turn the two wood circles against each other you will see there is quite a bit of resistance
or friction. It is difficult to rotate the Dob box support in the horizontal azimuth direction. You need a slippery
spacer between the wood circles that has smooth surface properties.
Place the ½” bolt through the 24” circle, through a spacer material, then through
the 18” wood circle. Don’t tighten the ½” bolt too tight. In
fact you should make the ½” bolt long enough so you can put a washer, a short spring, another washer, and then the final
nut on the bolt. Tighten the spring tension of the bolt so it holds the circles together firmly but not tightly.
Below is a picture of the installed bolt with washer, spring, and nut.
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| Figure 6: The center bolt for AZ rotation of the Dob box and tube |
First Spacer
Material Test
One
of the materials you can use as a smooth spacer is a large 33 rpm record. Drill a hole through the center of the record and
place between the wood circles. Try the rotation for smoothness. If you think the rotation in AZ is still not smooth then try another material. The record did not work because
it is too small in diameter. The spacer needs to cover the whole 18" wood surface.
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| Figure 7: 33 rpm old plastic record as an AZ spacer. This material did not work well. |
Try a spacer of clear plastic
The
next spacer to try is a plain piece of smooth clear plastic. Purchase a piece at the hardware store and cut and 18" circle.
Drill the 1/2" hole in the center and assemble the AZ rotation circles. Try the rotation again of the upper circle on the
base circle of wood. Again we did not think this was very smooth. We need an even smoother surface.
Now try a piece of teflon plastic
The
next thing to try is a large FOR_RENT sign made out of Teflon or PVC plastic. The
surface on each side is very smooth. Cut the sign into an 18” circle and
place it between the circles. Tighten the nut and spring just hand tight. Rotate
the 18" wood disk and see if you think the spacer allows the wood circles to rotate in AZ smoothly. This material seemed to work the best so far.
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| Figure 8: Plastic For-Rent Sign material as spacer |
A last spacer material to try!
Here is another spacer material you can try, a thin sheet of aluminum. Purchase a smooth thin sheet
of aluminum and cut an 18" circle. Assemble the AZ base circles and try the rotation test. What do you think? Here is a picture
of a Dob that uses a metal sheet spacer. When you look at this picture notice that the Dob bases are square and
do not need to be round, but round is nice. Also notice that the metal sheet is on the Dob box support side and that the base
wood has some plastic teflon spacers, 3 to be exact at 120 degrees around the base. The Dob box metal spacer rides on
these 3 spacers to minimize the contact area between the wood plates. We will discuss using teflon pads later in this paper.
The aluminum spacer has some good qualities.
Maybe
you can come up with some other materials from you local hardware store to try.
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| Figure 9: Dob base metal spacer test |
The FOR_RENT sign worked well, but it still depended on the wood circles not
being warped. After the wood circles were painted they started to absorb the
paint and this may have caused both of the wood circles to warp. Now the circles
did not make flat contact with the FOR_RENT spacer and rotation was impaired.
The flat area of the 18” circle and the spacer material touching each
other still caused a lot of friction. A modification is needed to decrease the
18” circle contact with the spacer material.
Try Three Chair Leg Teflon Pads
Install 3 teflon slider
chair leg pads on the bottom of the 18" circle so that they contact with the spacer material three places each 120 degrees
apart. This will reduce the contact area and the friction.
Obtain 3 Teflon chair
floor sliders from the hardware store. The pad sliders seen in figure 10 come in various diameter sizes. The larger size would work better. Nail the sliders to the
bottom of the 18” wood circle. Place them about 1” in from the outer edge and use only 3 sliders, one each 120
degrees apart. Here is a picture marking the base every 120 degrees. In the picture
shows the 3 small plastic chair sliders nailed down to the 18" circle bottom.
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| Figure 10: Marking where tto place the 3 chiar slider buttons at 120 degree angles |
The chair sliders helped to lesson the contact area between the 18” wood and the spacer material. But smooth rotation in AZ was still not obtained.
Another modification is still need to fix this problem. Remove the 3 chair slider pads and try the next modification.
Install 3 wheels to roll on!
Purchase 3 metal sliding door wheels (bearing in the center type) of the 1.5” to 2” diameter size. Do not purchase the plastic wheels, as they will deteriorate
over time. Roller wheels can be found in the hardware store in the sliding door
department.
Also purchase 3 bolts about 3” long and ¼”
diameter. These bolts will slide through the center hub of the metal wheels to
allow the wheels to roll. The wheels will be installed in the 18" wood base so they roll around on top of
the FOR_RENT plastic sheet.
Here is how to install the wheels. Mark the bottom side of the
18” wood circle using a compass to determin 120 degree angles. Make 3 lines from the center to the edge that are 120
degrees in angle from each other. See figure 10 above.
Now lay a wheel flat on the 18" wood circle so that the wheel center hole is about 1” from the
perimeter edge of the 18” circle. See figure 11 below. Using a pencil mark
around the outer diameter of the roller wheel. Lift up the roller wheel and hold
it vertically centered on the circle you just drew. Now mark the left and right
sides of the wheel width. Draw parallel lines.
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| Figure 11: Mark where the wheel roller slot will be cut at the edge of the wood base circle |
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| Figure 11a: Drill 1/4" hole about 3" deep into the perimeter of the 18" wood circle for the wheel. |
Obtain a ½” wood drill and drill out
the wood rectangle slot where you marked the width and length of the roller wheel. You
will be inserting the roller wheel into this slot vertically. To clean up the
slot use a jig saw or wood file and square up the wood edges. Cut all the way through the 18” circle wood.
The metal roller wheel must fit into this
slot all the way through the 18” wood and stick out the bottom about ¼” or more.
Repeat this for the other two roller wheel slots 120 degrees apart.
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| Figure 12: Cut the slot to insert the roller wheel vertically |
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| Figure 12a: Roller wheel in slot, held by 1/4" bolt. |
This next part is a bit tricky. Using your drill with a ¼” bit,
drill a hole in the outer edge (perimeter) of the 18” wood circle in line with the center of the slot you just
cut into the wood. See figure 12. You will be inserting the 3” by ¼”
bolt into this hole, passing it through the center of the metal wheel hub, and then another 2” into the wood. So drill
the ¼” hole at least 3” deep. Make the bolt head of the ¼” bolt flush with the perimeter of the 18”
circle.
Once all three wheels are slotted and installed you can put some glue on the
end of the 3” bolt and push the bolt all theway into position. The glue
will keep the bolt from falling out.
Remove the 3 slider pad pieces if
they are still nailed to the base. These are not required now, the rollers will hold the circles apart. You want the rollers
to roll on the plastic FOR-RENT sign for smooth operation.
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| Figure 13: Add 3 standoff feet so the base will sit level on the ground |
Final Dob base and Dob box support work
Be sure to paint all of the wood before you assembly the parts for the final time.
The Dob base that sits on the ground needs a bit more work. Cut from the plywood 3 large wood pads about 6"x3"x3".
Paint them and then screw them to the base wood circle. When the telescope is placed on the ground these 3 pads will help
stablize the platform.
Another nice modification!
Let us modify the 18” wood circle once more. The ½”
bolt going though the center of both wood circles is just too small. The bolt
wobbles around and does not make a firm center point for rotation of the two circles.
Here is a solution that worked.
Purchase from the hardware store a PVC plastic pipe end cap.
The PVC end cap should be about 2” in outer diameter. The PVC end
cap has a solid closed end. Drill a ½” hole through the center of the end
cap. Place the end cap over the center of the 18” wood circle. Draw a line around the outer perimeter using a pencil. Drill a ½” hole just inside the pencil circle,
and then using a jig saw cutter cut the circle out all the way through the wood. You want to cut the hole large enough for
the PVC end cup to press through the 18” wood circle all the way. Once
the hole is cut, push the PVC end cap from the top of the 18” wood circle through the hole. Insert the ½” bolt
through the 24" base circle and apply a washer and nut to tighten the bolt straight. Now, place the 18" circle with PVC
cap over the bolt and replace the washer and spring and nut. Tighten hand tight
only.
You now have a larger rotation center hole. This will help the 18” circle to rotate smoothly over all
azimuth angles.
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| Figure 14: Pvc pipe end cap used as larger AZ rotator |
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| Figure 15: PVC cap installed into the Dob 18" wood |
Finding The North Celestial Pole
If you have trouble finding the North Celestial Pole when you go out at night to do some Astronomy viewing, you can glue
a Boyscout compass to the Dobsonian base. The compass needle points in the general direction of Magnetic North Pole.
To find the North Celestial Pole you have to know the magnetic variation of the site you are at.
Here in Lancaster; California the magnetic variation is -14 degrees easterly. Rotate the AZ mount tell the
compass needle points at the magnetic North indicator. Use your head and rotate your viewing LEFT or
Westward 14 degrees and look up about 35 degrees and you will find the North Celestial Pole Star, the Little Dipper area.
Now turn your body around 180 degrees and look due south. Hold the star chart up over your head with the North
direction on the chart over you head. Now find the constellation you would like to see that night.
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| FIgure 16: Add a compass to the telescope base circle to help you find the North Magnetic Pole |
This section has covered the two Dob base rotator AZ wood assemblies.
The other web pages will cover the assembly of the Dob wood mirror cell
parts, the Dob tube and its parts, the spider and secondary mirror and the focuser.
In final assembly you will attach the round PVC plastic pipe pieces to the
telescope tube so it can rotate in Elevation (EL) on the Dob support box.
The final picture in this section is the Dob circle bases with the Dob tube box
support plywood assembled.
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| FIgure 17: The Dobsonian wood base AZ rotators assembled with the Dob telescope tube support box |
The paint group of class then chose a paint theme for the wood and tube pieces. Remember to not get paint on the optics.
GOOD VIEWING.
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