This construction project requires accuracy, resourcefulness, and perseverance. I hope to make it easier for others by sharing the issues I encountered and solutions that I found.

Start with Build a Dobsonian Equatorial Platform by DIY Astronomer Mark Parrish at BBC Sky at Night Magazine. I'll call this the "SaN" description. Scroll to "Download materials" and download the data files.


Then download the PDF file Building the Sky at Night Equatorial Platform for the Skywatcher 200 Dobsonian, by Julian (jooli_ash at Stargazers Lounge). I'll call this the "Skywatcher" instructions. This document provides more detailed instructions than the original SaN web page.



The SaN and Skywatcher documents provide excellent information that I need not repeat here. The following new information is supplemental:

• Parts and Supplies in U.S. Measurement Units
  
• Preview Design in Drawing Software
  
• Marked Dimensions in Sky at Night Diagram
  
• North Segment Height and Width
  
• South Segment Height and Width
  
• Rib Connecting North and South Segments
  
• Ball Bearing Support Assemblies
  
• Skinny Bolts on Ball Bearing Assemblies
  
• Place Bearing Assemblies on Bottom Board
  
• Make the Clay Mold and Rubber Strip
  
• Attach Aluminum Strip to Bearing Segment
  
• Lever to Apply Pinion Gear to Rack
  
  

Parts and Supplies in U.S. Measurement Units

See my parts list spreadsheet (.xlsx file) for the parts that I used, where I got them, and their cost. The total was $164, not counting items I hand on hand already and an extra plywood sheet that I needed after making some mistakes.


In the U.S., nominal 3/4-inch plywood sheets are 23/32 of an inch, very close to 18 mm called for in the plans.

Nominal 2-by-2 lumber is actually 1.5 by 1.5 inches, or about 38 by 38 mm, which is just right for the bearing supports. I used inexpensive 2-by-2 rough-cut redwood posts from the Home Depot garden center, measuring 36 by 36 mm.

Aluminum strips and angles are available in 1/16 inch thickness from Home Depot and Ace Hardware, or 1.5 mm, which worked fine for me.

Preview Design in Drawing Software

The carpenter's adage "measure twice, cut once" is highly applicable, with many chances for mistakes and little room for error. I found it very helpful to draw out the wood pieces in software and print out the plans at 100% scale to make templates for precise marking, cutting, and shaping of the wood.


I used LibreOffice Draw, which is a free download similar to Microsoft Visio. Here are my drawing files for 38.2° latitude:

Main drawing .odg format (reads directly into LibreOffice Draw)
Main drawing .svg format (opens in web browser)

Roller support .odg format (reads directly into LibreOffice Draw)
Roller support .svg format (print at 100% for paper templates)

Marked Dimensions in Sky at Night Diagram

In the SaN diagram, the dimensions J and H are marked incorrectly, which is important only if you are trying to understand the spreadsheet formulas. Here are the corrected markings.

North Segment Height and Width

In the SaN calculation spreadsheet, the north bearing segment height "L" is measured from the bottom tip to the top surface of the platform. This makes sense for the SaN design that has segment "wings" extending beyond the width of the platform, as the wings go all the way to the top surface.

The length (very long dimension) of the north segment "P" with wings is based on the mount base width "E" plus 22.5° (11.25° per side) plus another 30 mm (15 mm per side). This is apparently to add 1.5 hours of tracking time plus an allowance for the two screws at the ends of the track.

Skywatcher says that the wing extensions are not needed, as you can already do hours of uninterrupted tracking, and construction is easier without the wings. In that case, the top of the north segment ends on the underside of the base board, reducing the total height to "W" in the diagram above.

I added some cells to the SaN calculation spreadsheet to calculate the north segment dimensions without wings (download .xlsx file with added cells).

The SaN plan view of the north segment shows a notch height of 98 mm (the longest part the fits under the lower surface of the platform). I think this is an error; it should be 90 mm (.xlsx file with SaN data & added cells).

South Segment Height and Width

The south bearing segment has the same height as the north segment without wings. The SaN plan diagram of the south segment shows a height of 98 mm, but I think it should be 90 mm.

The south segment length only needs to match the same angular span of the north segment, so it is just the north segment length reduced by the ratio of the north and south segment radii. You can make it a little wider than that (say 30 mm) if you want to accommodate the screws at the ends.

Rib Connecting North and South Segments

A trapezoidal piece of wood on the underside of the platform joins the north and south bearing segments. This piece precisely sets the latitude angles of the two segments and the spacing between them.

So it's important to cut this piece precisely. The length of the long edge "Z" is the mount base width "E" reduced by the thickness of the tilted plywood, as calculated in my spreadsheet with added cells line 40.

Trim the trapezoid's corner where it meets the north segment to allow space for the bearing that will roll on the rear surface of that segment.

Ball Bearing Support Assemblies

I didn't know how far apart the left and right wood-and-metal ball bearing assemblies should be, so I made four separate wooden blocks for the four assemblies. I followed Skywatcher's suggestion to support the full length of each assembly with wood instead of having the metal overhang the end of the wood.

Remember to make left-and-right mirror-image versions of the assemblies.

It's important to cut the wooden support angles accurately, as they determine the whether the north and south bearing segments rest evenly on the metal ball bearings.
Using LibreOffice Draw or Visio, draw an accurate cross-section plan, print it out at 100% scale, and glue it on a cereal box with a glue stick. Cut out the shape with scissors, and you have a precise template for checking the profile of the finished wood.


To cut the angles on the wood, I started with a 2-by-2 (36 mm on a side) and marked the cut edges along its length. Then I used a jigsaw with the blade angled away from perpendicular by the amount of my latitude, 38°. After the first cut, I turned/rotated the 2-by-2 by 90° and then used the exact same blade angle for the second cut.

If your latitude is greater than 45°, set the blade angle at (90 minus latitude) away from perpendicular.

After cutting, use a wood plane or sanding block to flatten the cut surfaces and to make them reach the target angles and dimensions.

Skinny Bolts on Ball Bearing Assemblies

The SaN plans show a long skinny bolt on the angled part of the bearing support. The purpose is to freeze the angle of tilt while supporting the weight of the telescope on the angled ball bearing.


Adjust the tilt to match the curvature of the north or south bearing segment, tangent to the bearing segment curve (more tilt for the south bearings), then adjust the skinny bolt to reinforce this tilt setting.

Place Bearing Assemblies on Bottom Board

Mark the four bearing assemblies as left-north, right-north, left-south, and right-south. Put the bottom base board on a level surface, checking it with a bubble level. Place the bearing assemblies on the base board in the approximate expected positions. Support them with blocks of wood and weights as shown here.


Place the top board with its bearing segments resting on the ball bearings. This tends to push the assemblies outward, but the wood blocks and weights keep them in place.

Adjust the positions of the four bearing units (along with the blocks and weights) until:

• The centerlines of the top and bottom boards are aligned
• The top and bottom boards are level (check with bubble levels)
• Each ball bearing contacts the aluminum strip flat against the surface and centered
• The two north assemblies are equidistant from and perpendicular to the bottom board centerline
• The same for the two south assembles

To raise/lower the upper platform, move the bearing assemblies toward/away from the bottom board centerline.

With all criteria met as closely possible, carefully mark the locations each of the bearing units on the bottom base, marking all four sides of each block with a pencil.

Remove the top board and bearing assemblies. This is your last chance to adjust the tilt angles of the bearing assemblies if needed. You can disassemble the unit and sand/plane the angled surfaces, or alternatively, sand/plane the bottom of the bearing unit. Recheck the full assembly before proceeding.

Apply wood glue to the bottoms of the bearing assemblies and place them in their respective marked positions on the bottom board. Put back the four wood blocks and weights in their respective positions. Place the top board on the bearing assemblies.

While the glue is still wet, verify that everything still fits together well. Gently put weights on top of the top board to apply more pressure on the glue joints. Check one final time, then allow the glue to dry.

Make the Clay Mold and Rubber Strip

The rack is the grooved rubber strip that engages the driving gear, as in rack-and-pinion gear set. The pinion is the small gear that turns against the rack. My first attempts at making the rack came out poorly. After a day of experimenting, I developed the following procedure.

(I didn't take any photos of my work because I was rushing to complete the project before my nephew returned to school. He took the platform with him so I don't have the gear any more. The following re-enactment photos use a fake gear without teeth.)

1. Put the gear on the axle rod and tighten the set screws.
   
   
2. If you are working on a wood surface, cover it with a strip of packaging tape. The clay needs a smooth surface to stick to.
   
   
3. Find a flat wood moulding strip or similar material having a total thickness slightly larger than your gear radius minus the axle rod radius.
   
   
4. Make a right-angle guide like the following.
   
   
   
   
   
   
5. Tape or nail one flat wood strip to the smooth surface.
   
   
   
   
   
   
6. Roll some clay between your hands to make a rope thinner than a pencil. Lay the rope down on the smooth surface along the edge of the wood strip.
   
   
7. Press the clay against the wood to mold it into the approximate final shape.
   
   
   
   
   
   
8. Make sure the gear is clean. Moisten it lightly with lubricating oil to prevent sticking. Wipe away excess oil with a rag.
   
   
9. Lay the axle rod and gear at the end of the clay with the gear against the edge of the wood strip and the axle rod along the right-angle guide.
   
   
   
   
   
   
10. Roll the gear over the clay with the gear against the wood guide and with the axle rod rolling on top of the wood strip. Push the right-angle guide along with the rod to keep it perpendicular to the rolling direction.
    
    
    
    
    
    
11. Roll all the way to the end, then remove the wood strip.
    
    
12. Using a hobby knife, carefully cut away and remove the excess clay running alongside the grooved gear path, leaving only the track with the little grooves.
    
    
    
    
    
    

The clay mold is now ready to receive the hot glue. You need a helper for this step.

1. Stack wood and/or solid cardboard on both sides of the clay track, rising to a level about 2 mm above the clay track.
   
   
2. Apply the hot glue on top of the clay track. Be sure to cover the whole width of the track.
   
   
3. As you lay down the glue, have your helper follow behind and immediately smooth over the hot glue using a stiff square of cardboard spanning the tops of the two cardboard guides. Your helper should make a single slow pass as you apply hot glue over the whole length.
   
   
4. After the rubber strip hardens, peel it off of the clay.
   
   
5. Using a hobby knife or scissors, trim away the excess rubber on both sides, leaving only the part of the rubber strip with the tiny grooves.
   
   My nephew took the following photo of the rack rubber strip we made. The clay mold (broken after we finished using it) sits between two cardboard guides. The small cardboard rectangle on the lower-right was used to smooth over the hot glue. In the upper-left corner is a poor result of our early efforts.
   
   
   
   

At first I tried to make the rack long enough to span the entire length of the north segment, but that's overkill. A foot-long (300 mm) rack is good for about 2 hours of tracking. Use the .xlsx spreadsheet with added cells, lines 43-47, to calculate the tracking time.

The reason for using only one wooden guide for clay-rolling is to give the squeezed-out clay a place to go (namely, out to the side). When I tried rolling the gear between two wooden guides, the gear pushed the extra clay under the wooden guides, making the mold uneven.

Skywatcher suggests using hot melt glue to attach the rubber strip to the north bearing segment. However, I found that the hot glue melted and damaged the rubber strip. Use super glue (cyanoacrylate) or wood glue instead.

Attach Aluminum Strip to Bearing Segment

Skywatcher suggests pre-bending the south bearing segment aluminum strip to match the curve of the segment. That might be advisable at higher latitudes (smaller south bearing diameter) and thicker aluminum (like 2 mm). However, I found that this actually made the curve worse because I could not pre-bend the strip evenly into a smooth curve.

I started over with a new 1/16" (1.5 mm) thick aluminum strip and left it straight, bending it only when I actually applied it to the south segment. Then the curve came out a lot smoother.

The first time I put in the two screws, the aluminum strip did not tightly hug the wooden bearing segment; it was loose. I started over and marked two hole locations on the wood again. However, the this time I drilled the pilot holes in the wood an extra 2 mm apart. This stretched the aluminum strip tightly against the bearing segment.

Lever to Apply Pinion Gear to Rack

The motor and gear assembly must be attached to the bottom base board in a manner that applies the pinion gear against the rack (rubber strip) with the proper force. You can see how this was done in the photos by SaN, Skywatcher, and myself.


In each case, the gear assembly is tilted to match the tilt of the north bearing segment, and a lever swivels at a fulcrum to apply the pinion gear to the rack. A rubber band or spring applies the force.

I chose to put the fulcrum behind the lever. I cut out a triangle of wood left over from making the north-south rib, with the critical edge already cut at the proper latitude angle, and glued the triangle to the bottom base board. I drilled a pilot hole in the triangle to accept the fulcrum screw.

I took a long skinny piece of wood, also left over from making the rib, to serve as the lever. I drilled the fulcrum hole through the lever just barely larger than the screw threads. I screwed the lever to the triangle, loose enough to let the lever turn/swivel about the fixed screw, but tight enough to prevent the lever from wobbling.

You can use more rubber bands, or double them over, to increase the force. If the force is too small, the gear might slip and fail to turn the north segment. If the force is too big, the motor and gears might have trouble overcoming friction.

You need to apply more force with the full weight of the telescope on the platform than when you are testing it without the telescope. To test, run the motor and check for the north segment turning at the expected rate, 2 mm per minute in my case (see .xlsx file with added cells, line 45).

My Platform

I initially followed the SaN instructions but made a mistake in calculating the slant of the north bearing segment cut, so I started over with a new piece of plywood. At that point, I studied the spreadsheet cell equations to get a better understanding of the measurements. I made the north segment with wing extensions before I realized that I could save a lot of work by omitting them.

I misinterpreted the "desirable thickness of platform" F as the distance from bottom the of the bearing segment to the bottom of the top board rather than the top surface. As a result, I built the platform too high by 18 mm relative to the telescope center of gravity. I don't expect this to affect final performance.

My confusion about the height and width of the north and south bearing segments led me to create the new spreadsheet cells.

I worked on this project intensively every day for a full week together with my nephew, the telescope owner. He had to go back to school soon after that so there was a deadline. I don't think I'll ever build one again, but if I do, I think I could finish it in three or four days instead of seven, having learned what I did.

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