A Low Profile Equatorial Table
By David Shouldice, DAS
planetary observer, and having been accustomed to tracking, the
purchase of a Dob left me wanting.
goals for tracking were:
profile possible - my scope did not need me to climb up to the
eyepiece, and I didn't want to start. My goal was a 2" max.
increase in eyepiece height.
- Primarily For visual astronomy, i.e. only a 1 axis drive
component that I could put the scope on if I wished, not a
permanent part of the scope.
to use my digital setting circles
- A one
person operation, i.e. easy to use.
my ability to build with available tools and parts.
electronics, hopefully all off the shelf.
I ended up with:
- A pure
D'Autumn design table
- All of
was < $100
No visible bending or flexing
it barely extends beyond the Dob base.
- Easy to
reset at the end of the tracking (45 minutes)
electronics (except motor) (I use an inverter in the field to
convert 12 VDC to 115 VAC)
- I can
view at 1000X and image barely drifts.
- Although designed for my big scope I can set smaller ones on the
base for solar viewing.
In my research I found 3 articles of note:
article in Sky and Telescope,
Sept '88 p 303 - This tells of the
pro's and cons of table designs and has pictures of his
and http://www.atmpage.com/platform.html - An article by Chuck
Shaw on the
ATM page, gives many construction details that I borrowed from.
a good article by Warren Peters with calculations and graphics.
I performed all the calculations, and referred to the drawings
Also I looked at the best platform on the market made by
The key to a low profile table was to put the feet and the bearings
directly below the scope’s Teflon pads. This minimizes the need for
a top or bottom structural baseboard. Most equatorial tables that I
reviewed seemed justly concerned with the cantilevered loads, and
opted for ¾" or 1" plywood for the top (baseboard) and bottom
(ground board). Due to my eyepiece height issue, I tried to use
structure only where needed.
The 3 Teflon pads that compose the azimuth bearing on the dob carry
all the weight. The desire was to place 2 feet to the North of the
table with one to the South.
The 2 North
support tracks attach to the bottom of the baseboard. Per D’Autumn,
this baseboard rotates around a virtual axis that points to the
North pole, but passes through the center of Mass of the telescope
(centered between the 2 altitude bearing axis). This is important.
If you don’t rotate the table around both your center of mass and
the pole, your motor torque, and price will greatly increase. The
added power is needed as the motor will have to lift your scope.
Also, the imbalance will make your scope tippy.
tracks attached to the baseboard mark D’Autumn’s conic sections. As
there is only 15 degrees (1 hour) worth of tracking, you can make
the N. bearing track from a vertical board, like the one the
"Equatorial Platform" design. The weight is directly supported. My
design has a maximum a 2.5" horizontal distance between the Teflon
pad and the roller bearing that supports the curved track. I added a
brace behind the tracks on the baseboard to minimize the droop of
the baseboard when it is at the end of travel. I lined the tracks
with a piece of rolled stainless to reduce friction and prevent
denting of the track.
A pair of cam
rollers is attached to the ground board with an angle bracket that
hold the threaded insert for the adjustable feet. As the force at
the roller bearing is almost vertical, only minimum support is
needed from the ground board for these feet.
The South pad sits over a conventional cylindrical bearing section
bolted to the South end of the baseboard. This is held up by 3
bearings, 2 supporting the radial force, with 1 supporting the
axial. The axial keeps the baseboard from moving N. or S. of the
resting position. Gravity keeps it from moving E or W.
The South foot of the table is held by a threaded insert near the
S. bearing holder (angle bracket). The rotation of the table shifts
the weight (from the S. Teflon pad) E. Or W. of the S. foot. To deal
with this you either need a ground board that doesn’t twist (I added
a ¾" brace), or you could add 2 South feet.
Forces from below:
The first time I set up on my lawn, I learned of another force, the
grass, that pushed up on the center, between the feet, bowing my
base. Most of the weight (hopefully) is supported by the feet, but
short of making the feet higher than the grass (and raising my
eyepiece) I reinforced the bottom board with a ¾" plywood with holes
to decrease weight. This also stopped the twisting of the S.
This too took a
lot of research. I decided that I was only interested in a single
axis tracking drive, not a hand controller for fine tuning, or slow
motion axes. To add these features would have driven the design to
stepper motors with controllers, computers etc. adding complexity,
failure modes stress etc.
was to make a synchronous worm gear drive like Shaw chose. Designs
like those made by D’Autumn and Ken Florentino of CSAS show how to
use threaded rod for the worm and rack. The motor and worm are
located on a bracket mounted on a spring loaded hinge. When the
hinge is moved to disengage the gears, the table can be reset to the
beginning of travel.
took me 4 months of obsessing to design and build, with most of the
time in calculations and drawings. I assume you will only take a
fraction of this. On first use I saw the central star in the Cat’s
eye and have tracked the planets for 45 minutes.
I still use
my DSC (digital setting circles), with the DSC aligned when the
table is reset (at the beginning of travel). To find an object by
the DSC, I first reset the drive. If it is a struggle to locate, or
at setup of the DSC, I turn off the drive. Actually I tend to run
the drive mainly when I am with the public, when using high power,
or for long stares.
(download all the pictures, graphs, and perform
Measure the vertical distance from the bottom of your scope to
the center of the altitude bearing pivot. Measure the distance
center to center between your Teflon foot pads. Use these in the
Peters article to calculate the dimensions of the radius of
curvatures of the N and S bearings. I made the south bearing 6"
long arbitrarily. Long enough for E / W support, but not so long
to cause bowing of the baseboard. This resulted in a kite shaped
baseboard. Note that the motor drive bearing rack will mount to
the N. end of the baseboard, so leave room. Do the calculations
in Peters’ article to determine the radius needed dependent on
the pitch chosen. Make sure you will have room for the rack and
motor between the N. bearings.
The base and ground boards are made of 3/8" plywood with 3 coats
of urethane. I mark the center point (azimuth pivot point) of
the scope on the baseboard, as well as the location of the
scope’s Teflon pads. The bearing surfaces will be directly below
Make a polar axis fixture board. This is used to allow you to
rotate the baseboard around the polar axis of the table, and is
used to grind your bearing surfaces. I bolted a ¾" plywood
triangle of wood to the top center N/S axis of the baseboard
with a 4" shelf bracket so that it’s top edge would point along
the polar axis of the table. I bolted hinges to it (see picture)
so that I could rotate the baseboard around this pivot point.
The fixture needs to be quite solid, but easily removable from
the baseboard. You will have the fixture on and off the
baseboard several times before and after you are done. When
done, to check it out, attach it to the baseboard. Take them
outside at night, level the top surface of the baseboard, point
its N/S axis North, look through the center of the hinges, and
see if your polar axis points to Polaris. Fix, cut or shim it if
it does not. (see afterthoughts)
4. Make the South bearing like Peters’ or Shaw’s. I used
a 6" X 2" X ½" solid wood board, cut roughly to the radius,
added a 1X1 brace to the N side, glued them together, then cut
the assembly to tilt to my latitude. Bolt the bearing to the top
board with countersunk wood screws (Nothing should extend above
the top surface, as this would increase the height of the
Drill holes for countersunk mounting screws through the center
of the location of the 2 North Teflon pads. These mark the
resting locations of the bearing on the bearing support. To make
the N. bearing supports I cut two triangular 1/2" boards. The
length of the board should be a bit greater than the length of
the 15 degree (1 hour tracking) circumference. (N. Radius
*2*PI*15/360). As I made the base a bit longer on the inside, my
2 N. bearing supports (triangular boards) measured 7", 6" and
1". Screw the middle of the long side to the countersunk hole.
The roller bearing will almost touch the baseboard at the side.
Draw a vertical line on the support block below the screw to
mark the future resting location of the roller. This will be
used later when attaching the roller bracket to the baseboard.
We have the N. bearing supports held by the middle, but now need
to rotate them to be tangent to the rotational axis. Rather than
lined up E/W, they will be pointed to the location where the
polar axis crosses the plane of the baseboard. Now, to figure
out the angle, bolt the baseboard to the fixture, and bolt the
hinges to a solid wall. (see pic above) I used a planter. Take a
pointer or pencil and have it gently touch the location of the
bottom of the above line (the resting roller position). I piled
up some concrete blocks to support the pencil. The pencil marks
the location of the roller as the table moves.
As you rotate the table on the hinges, note that the pointer
will at some point touch the bottom of the baseboard. Mark this
point on the bottom of the table, and rotate the N. bearing
support so it touches this point. This has placed the N. bearing
supports so that as the table rotates, the roller will be
directly below it, and not wander N/S.
8. When it has been located, put another 2 screws (each) to secure
the N. Feet to the baseboard.
Now grind the bearing supports. Bolt the hinges to a wall,
support your grinder (I used a power drill with a sanding disk).
Slowly grind your N. Bearings with the grinder parallel to the
baseboard, grind the S. Bearings with the grinder parallel to
the pivot axis.
Measure the radius (distance from the polar pivot axis) and
length of the completed bearing surfaces. Order 1/8" stainless
steel rolled to this radius and width for the S. Bearing, verify
that they can also roll a cone with the radius you measured, to
your latitude. A sheet metal company did it for me for $35. My N
bearing was a 82 degree cone of 1" wide 5" long 1/8" thick with
a radius of 19.5". My S. bearing surface was ½" wide, 6" long
and 9.4" radius.
Glue these to the ground bearings with a flexible glue. (I used
liquid nails). I also glued (thin glue) a piece of brass sheet
metal to cover the S. face of the S bearing rolling surface.
Cut a ground board much the same size as the top, it needs some
extra room on the N. end for the motor drive. So much for the
woodworking, now the drive.
Measure (using the fixture) and calculate the position and
radius range for the motor gear rack. With this radius
determine, from Peters’ calculations, the pitch needed for the
threaded rod. Except, note that the # of teeth for a full
revolution at 1 RPM is 60 minutes * 24 hours * 364/365 = 1436. I
chose a 1 RPM 115 VAC synchronous motor (MMC $21) with a 12
pitch rack (and rod). The calculation defined the radius of the
rack to be 19". Wider pitches give better contact with the
threaded rod. If the position is off, you can move it N or S. to
compensate. (But the motor will need to move too). For a minimum
height table, the big challenge is to make the motor centered
between the baseboards. It cannot extend above the baseboard or
the scope will hit it, below the ground board you hit the ground
and you wont be able to uncouple the gears to allow the table to
The motor will need a gear to drive a gear on the threaded rod.
This keeps the motor from hitting the rack. The distance of the
motor from the rack, the centering of the motor between the base
and ground board, and the calculated radius of the rack forces
the location of the rack. It is advisable to make the hinged
motor support and rack before attaching the rack. I cut a hole
in my ground board to allow for more space. At end of travel,
ensure that the rail does not hit the motor. Mine clears by ¼".
Also, to avoid need for an end of travel limit switch, I offset
my motor W so that at end of travel it runs off the end of the
Make a bracket (1/8’ aluminum to hold the motor and
threaded rod (held in place by sealed bearings (MMC) (McMaster
Carr). The choice of gears should be done early in case it
impacts the pitch. There is a small selection at MMC. Note that
if you use gears, and a clockwise motor (viewed from the end of
the shaft), your motor will be on the E. Side of the rod, not
the W. like mine. The rigidity of this plate is important as it
will define the stiffness of the drive. Bolt the bracket to a
solid hinge. I used a gate hinge that I had taken apart and
hammered on the hinge to take out the slop. I also inserted a
washer in series with the hinge to take out lateral movement.
Don’t bolt this to the ground board yet.
15. Make (fabricate and grind) a surface like the S.
bearing surface with the calculated radius (- ½ thickness of the
threaded rod) to support the motor rack. Note that the height of
this surface is critical to the height of the table, and needs
to be coordinated with the motor bracket to center the motor
between the base and ground boards.
Buy threaded rod of calculated pitch. Cut a piece for the rack
(>15 degrees). Grind one side flat until it can be smoothly
bent, and rigidly attach around it’s needed radius. D’Autumn
cast his rack with epoxy using his threaded rod as the mold.
17. Now that you can figure the separation of the base and ground
boards, you can figure the needed height of the bearing
supports. For the N. bearing support on the ground board, I
bought a 1 ½" X 1/8" thick large gate handle and cut it into 2
right angle brackets. I drilled and tapped holes for the Cam
follower bearing (MMC $4.40) 2 screw holes to bolt it to the
baseboard, 1 hole to insert the 3/8" #16 threaded insert
(inserts from below). I put double stick tape on these to hold
them in place on the ground board before I drilled it. The
distance above the threaded hole (cam bearing mount) cannot be
too high, as the top board of the table can hit this when it
tilts at end of travel. Note that the bearings are on a cone and
point to the centerline. I had to shim these to make sure they
were in full contact with the bearing surfaces.
18. For the S. Bearing support I followed D’Autumn, Peters and Shaw
and used an aluminum angle with screws to hold the bearings.
This is bolted to a spacer board to raise the S. end, and make
the table level. I had to bend and shim these too to make sure
all 3 bearings are in full contact.
Attach the hinge to the ground board so that it makes full
contact with the rack.
Add a spring on a screw between the hinge and the ground board.
I used a lock nut to adjust the "stop" of the spring so that it
would just engage but not force the rack.
Then, without the motor attached, I took a power drill, with a
shaft extension on the motor shaft, and ran with fine grinding
powder on the rack and shafts. (You might try toothpaste). I
ground the assembly until it ran smooth when clean. I lubricated
the gears and rack with a bike chain dry lube, then, finally I
attached the motor and make sure it works.
22. I added a level bubble to the ground board, and a N/S line to
align a compass.
I made the ground feet from a round headed bolt with 2 ½"
washers and fender washers bolted together. I made them as long
as I could without hitting the baseboard when fully retracted. I
added a bolt to snug them in place if they were too sloppy after
adjustment. Just rotate them to adjust to E/W level, after that
the S foot. Check the operation for good bearing and gear
contact and motor function.
I added a stiffener board to the ground board and end of travel
stops to the N. rails to improve function. It does not interfere
with the baseboard. I also added a brace to the baseboard behind
the N. rails to stiffen from bowing of the baseboard when off
center. I added end of travel stops (aluminum bracket) to keep
the N rollers on the tracks.
Time for a daylight test. Set up on concrete in the daytime, put
your scope on the base with the feet on the marks on the
baseboard. Make sure nothing flexes as you rotate the scope over
it’s range of travel. Make sure bearings stay in full contact.
If so shim or add support. You can also redo the test of step 3
with the fixture to see if the polar axis points to the pole
when the ground board is N/S and level, and stays pointing at
the pole as you rotate through the travel.
To test the tracking, I leveled the base, used a compass to
point it N/S, put a small scope with a solar filter on the table
and tracked the sun for as long as I could. I used a large FOV
eyepiece and watched how the image drifted. If it drifts E/W it
is because the radius of the rack is not exact. Mine is ¼" too
far from the polar axis, so mine tracked too slow. I unbolted my
rack and moved it ¼" S. This is a little late to find this out,
but I couldn’t figure out how to know it sooner. I have an old "Digitrack",
a product for old scopes that all used 115 V synchronous motors.
It generates 115 V 60 Hz from 12 VDC or 115 VAC. It allows
trimming the frequency for free, so it gives a single axis
control to your scope. I use this in the field. I could use a
115 V inverter that you can get from RV camping stores.
When it looked like I had a good base, I remade my scope’s
ground board. It supported the Teflon pads and was ¾" plywood,
with ¾" feet. I made it from ¼" aluminum plate so it would sit
on my table without adding height. I drilled a larger hole at
the pivot point on the table’s baseboard for the pivot bolt, and
used the scope’s new N. feet to align the 2 baseboards together.
All together with the adjustable feet to minimum, it is less
than 2.5" additional to the eyepiece height.
- By the
way, and also, not a casual note I am at 40 degrees latitude, so
the polar axis splits the loads between radial and axial forces.
Also, my scope weighs 100 +lbs. ,there is 14.5" between my
scope’s Teflon foot pads, the height from the Teflon pads to the
center of the horizontal scope bearings is 18".
- When I
calibrated the table with full weight on top, it was rotating
about a point 2.5 degrees above the pole. I believe this to be
the result of the remaining torsion noted in the discussion of
the S. bearing, but I can not see any deflection.
- When I
went to a star party in a neighboring state, I was forced to
realize that the pole shifts 1 degree for about every 350 miles
N/S. The table needs to be tilted to accommodate. I have
recently added to the ground board a small mirror with a target
on it and a small eyebolt to align the table to Polaris. After a
normal alignment, I look through the eyebolt at the reflection
and move the table till Polaris is on target.
power usage of the table is so slight that I am contemplating
getting a 12 VAC synchronous motor instead of the 115 VAC so
that I can run off a small battery pack.
love the table, and use it all the time. I have to believe, soon,
most Dobs will have them built in.