Dave Shouldice's Low Profile Equatorial Table

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Being a planetary observer, and having been accustomed to tracking, the purchase of a Dob left me wanting.

My goals for tracking:

• Lowest 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, IE only a 1 axis drive 
• A component that I could put the scope on if I wished, not a permanent part of the scope. 
• Ability to use my digital setting circles 
• A one person operation.. IE easy to use. 
• Within my ability to build with available tools and parts. 
• Minimal electronics, hopefully all off the shelf.

• A pure D'Autumn design table
• All of the above
• Cost was less than $100
• Weight 8 Lbs.
• Strong, No visible bending or flexing
• Small, it barely extends beyond the dob base.
• Easy to reset at the end of the tracking (45 minutes)
• No 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.

References:

• D'Autumn's 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 creation.
• http://www.atmpage.com/platform.html - An article by Chuck Shaw on the ATM page, gives many construction details that I borrowed from.
• http://www.airnet.net/warrencami/Astronomy/ASTRONOMY.HTML - a good article by Warren Peters with calculations and graphics. I performed all the calculations, and referred to the drawings many times.
• http://astronomy-mall.com/regular/products/eq_platforms  - Also I looked at the best platform on the market by “Equatorial Platforms”

The key to a low profile table was to try and make all the forces (weight) from the scope supported as directly as possible by the feet and structure, not the baseboard. IE keep the horizontal distance from the bottom Teflon pads to the feet to a minimum. This will minimize 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 and bottom boards. Due to my eyepiece height issue, I tried to use structure only where needed..

The forces:

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.

North Feet:

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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 needs will greatly increase, and it will be much harder to purchase. The power is needed as the motor will have to lift your scope. Also, the imbalance will make your scope tippy.

The curved 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 bearing track from a vertical board, like the one the "Equatorial Platform" design. The weight is directly supported. My tracks have, at the farthest ends of its rotation, a 2.5" distance between the Teflon pad and the roller that supports the curved track. Only after it was functional, and knew where the ground board moved, 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.

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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 forces at the roller bearing is almost vertical, only minimum support is needed from the ground board for these feet.

South Foot:

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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. This holds the base board from moving N. or S. of the design position. Gravity holds it from moving E or W.

As the 3 bearings attach to the ground board support the S. load, and as the table moves E. or W. shifts East or West of centerline. The South foot of the table is held by a threaded insert near the S. bearing holder (angle bracket). To deal with the force that moves on either side of center, you either need a bottom plate that doesn't twist (I added a ¾" brace), or you could add 2 South feet.

Forces from below:

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The Drive

This too took a lot of research. I decided that I was only interested in a tracking drive, not a hand controller for fine tuning, or slow motion axes. These would have driven the design to steppers with controllers, computers etc. To add complexity, failure modes stress etc. The result, however 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, the table can be reset to the beginning of travel.

This project 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 with the table 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.

Construction

You should download all the pictures, graphs, and perform calculations from Peters' and Shaw's article mentioned above in the references.

• 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 pads on the feet. 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 ended with 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 base board, as well as the location of the scope's Teflon pads. The bearing surfaces will be directly below these points.
• Make a polar axis fixture board. I bolted a ¾" plywood triangle of wood down the center North South axis so that it's top edge (through the centerline of the hinges) would point to the polar axis of the table. I bolted hinges to it (see picture) so that I could rotate the top board around this pivot point.

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This needs to be quite solid, but easily removable. You will have this on and off the top board several times before and after you are done. After making this, take it outside at night, level the top surface, point it North, and see if your polar axis points to Polaris. Fix (re cut or shim it if it doesn't.)

• 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 table).
• Drill holes for countersunk mounting screws through the center of the location of the 2 North Teflon pads. This is the resting location of the bottom plate support rollers. Screw a triangular ¾" board cut in a triangle. 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, at finish, my 2 tracks sides (triangular blocks) measured 7", 6" and 1 ½". Screw the middle of the long side to the top hole. Make a line the screw hole on the bottom of the block. This marks the resting location of the roller.
• Now, to figure out the angle of the N. feet, bolt the base board by the hinges to a solid wall. (see pic) 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.
• 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 block so the front touches this point. This has placed the block(s) so that as the table rotates, the roller will be directly below it.
• Put another 2 screws (each) to secure the N. Feet to the base board.
• Now grind the bearings. Bolt the hinges to the wall, Support your grinder (I used a power drill with a sanding disk). Slowly grind your N. Bearings so they are parallel to the table, grind the S. Bearings 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 base board 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 be reset.

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The motor will need a gear to drive a gear on the threaded rod. This keeps the motor from hitting the rack. This drives 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 rack.

• 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.

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• 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.
• 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.
• 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 base 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.
• 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 base board 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 base board. I also added a brace to the base board behind the N. rails to stiffen from bowing of the base board 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 base board. 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 base 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 base board for the pivot bolt, and used the scope's new N. feet to align the 2 base boards together. All together with the adjustable feet to minimum, it is less than 2.5" additional to the eyepiece height.

Important details to consider: 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".

If you are building for a different lattitude, or other dimensions of your telescope are very different, you should carefully consider the changes you must make for this mount to work for you.

Clear skies -