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My Equatorial Platform

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Equatorial Platform (EQ) - Polar Aligned Tracking Base

Major Hardware Purchase Links:
  • High Torque Drive Motor - 10 RPM (12VDC) - $24.99
  • Hub Gear (x1) - 3.167-inch OD (48 pitch) Acetal Hub Gear - $9.00. I salvaged one from a dead ink jet printer's gear system. Saved the $9.00.
  • Pinion Gear (x1) - 48 Pitch Gearmotor Pinion Gears - $12.99. I salvaged one from a dead ink jet printer's gear system and saved $13.00
  • Clamping Hub (x1) - Gear mounting hub for the Hub gear above - $7.99
    NOTE: Larger gear and hub should be sized to fit a 3/8 inch threaded rod. I epoxied mine directly to the threaded rod instead of using hubs. Saved $16.
  • Bearings (x2) - Ultra Precision Ball Bearings - $7.99 each I salvaged these from an old 70mm film projector. Saved another $16.
  • Bushings (x2) - Bronze Bushings - $1.45 each. (to eliminate the "slop" between the threaded rod outer diameter and the inner diameter of the bearings).
  • Threaded Rod (x1) - 3/8 inch, 16 pitch, 12 inch long, Zinc Plated (to prevent rust). Can be found at any hardware store (i.e. Lowes, Home Depot, etc)
  • Precision potentiometer (trimpot) - 200 Ω (two HUNDRED ohm) Potentiometer Trimpot - $0.21 Do not make the mistake and get a 200K Ω (kilo-ohm) potentiometer! The 25 turns give extremely fine tuning (good!) for the motor. There's a minimum order of $5 with a $1.60 shipping charge from this site. (I still have several left over to give to friends building their own platforms)
  • Inline Skate Wheels and bearings (x8 as a set) - approx $20 on Amazon.com new. Used as the sector bearings. Will also dampen vibration and smooth out the travel of the upper section of the platform. Avoid USED wheels at thrift stores or on ebay due to uneven bearing and wheel wear and tear. [size - no smaller than 76mm due to mounting and spacing issues (80mm good!) / hardness - 78A is fine]. The harder the wheels the better and will reduce the vibration and bounce that softer wheels will produce.
Things I learned from this build:
  • Screw Drive is easier to build than Stepper Motor method. It just has a couple more moving parts to build. Easy peasy though.
  • Screw Drive is very accurate if you take your time during the platform build phase.
  • Screw Drive is very smooth and has no vibration in the viewed image.
  • I get about 50 minutes of tracking before having to reset (rewind) the system.
  • Screw Drive is cheaper and simpler to build than Stepper Motor (motor type is cheaper and no controller circuit needed).
  • Other friends who have attempted to build direct drive platforms (stepper motor method) found that if you don't get the bearings (skate wheels) EXACTLY aligned right, the drive wheel can lose traction as the weight of the device shifts. This can result in the platform slipping against the driven sector (rocker that the drive wheel is touching) when trying to move the platform. A screw drive has contact garanteed at all times via the drive pin, regardless of whether your bearing alignment is perfect or not. There is even an EQ platform website with one individual who suggests letting the rear sector bearings "float", i.e. not attached to the base plate at all, in order to prevent this from happening. This leaves the rear sector bearings loose for you to have to carry around or secure when moving the platform. In other words the screw drive method is much more forgiving and works regardless of any slight manufacturing errors.
  • I used two (x2) 6 volt lantern batteries to power the motor to its full 12v capacity. I have checked using a voltmeter the drain on these batteries and it's virtually nill. Since the motor is converting rotational motion to linear motion between the gears and the screw, there is almost no load (torque) on the motor at all. So running this motor at 12V with no load and the miniscule drain on the batteries I'm using, I should get several months if not a year's worth of use before I have any drop in power from this set of batteries Update 12/03/12 - Been using same batteries for over a year now and still fully charged, 12+ volts reading!)! Sweet! (UPDATE: I have since mounted a small digital voltmeter ($7.95) in the motor/battery circuit under the platform with a temporary-on switch that I can press to get an instant voltage reading on my batteries. This allows me to monitor and note the voltage on my batteries before each observing session, giving me a heads-up when they start to drop enough to effect my tracking.)
  • There are only three "required" electronic/electrical components in the entire drive "circuit": motor, potentiometer, batteries. Anything else (main power switch, run switch, limit switch, voltmeter, digital temp meter $9.95 - yes I put on of those on my system too!) is icing. The poteniometer is used to adjust the battery power to vary the motor speed in order to fine tune it to track the sky accurately. That's it - 3 simple parts in the motor drive circuit! Anyone can do this circuit for under $40 (a stepper control circuit costs at least that for a kit you have to solder together yourself!). I also put a master "RUN" switch in that circuit just for convenience, but you could just as well hook the wire to the battery and unhook the wire to it. The options are limitless, but the basic circuit is still only 3 items.
  • This thing is so well balanced that you can disengage the motor (it's on a hinge and compression spring) and manually "rewind" the platform with just your finger tips with little effort in about 20 seconds. I cut a straight groove into the threaded rod on the opposite end from the motor to accomodate a flat-head screwdriver bit. I salvaged a DC motor from a dead inkjet printer and fitted it with a hardwood dowel that I beveled on two sides at one end to make a homemade flat-head screw driver bit (metal bits are usually hardened steel and very hard to drill into with conventional drill bits - again I opted for cheap but effective as my build method). I use this device to fit into the slot on the threaded rod which rewinds the entire platform to the start position in about 4 seconds. (much like a handheld R/C plane propeller starter)
  • The cone sectors are the most critical build part and the angle of the bearings. These are calculated for you on a spreadsheet eqplatform_calc2012.xls which is a modified version of a spreadsheet (created by Tom Hole at his EQ Platform build page.) that gives you all the other dimensions.
  • I'm getting a slightly perceptible drift across about a quarter of the field of view using a 4mm Plössl eyepiece (which I could never really use before building my platform, due to how fast everything moved through the FoV) with 0.16 ° FoV after at least 15 minutes of tracking! WOW! (0.16 ° = 0° 9' 36") That's only a 0° 2' 24" drift in 15 minutes! This can further be reduced by trimming the poteniometer more using an ohmmeter to monitor the resistance while dialing in a more precise speed.
  • I've added a Limiter Switch that I savaged out of a dead fax machine to automatically stop the motor once it reaches the full extent of travel on the threaded rod. It works perfectly and halts the platform automatically after 53 minutes of observing. That way I won't have to worry about stripped gears or a damaged motor if I get engrossed during an observing session and forget to stop and rewind.

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My Equatorial Platform
Figure 1-1

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Let me start off by saying that this EQ platform project is not meant to be used to build a commercial or research grade tracking system. If you are a metal worker (I'm not) and have lots of money and tooling equipment (I don't) and wanting to build an uber precise platform then this is not the project for you. If you have basic woodworking skills and are a little mechanically inclined (scavenging skills will help keep the costs down too) then this should be a relatively easy project to accomplish. You should, by the end of this project, have a reasonably precise tracking platform on which to place your alt-az dobsonian telescope (or even a camera and tripod at the same height?) and have it track a single object for approximately 45-55 minutes without having to reposition your telescope.

Now when I say that this platform will not be "uber" precise, that is not to say that this will not be a "quite" precise tracking system. I am able to get very high tracking precision with my scope and platform (see the "Things I learned from this build" section for precision claims), without having to be space-shuttle precise in my build techniques. Granted there are a few things you do have to make as precisely as possible (such as the "sectors" aka the rocker segments), but with a little care and patience these are nothing that a weekend woodworker or a high school teenager can't build in a few sessions.

I have found from my own research and experience in building one, that 90% of the websites out there on how to build an EQ platform are either WAY too caught up in the math of this thing, or they are WAY too caught up in how much precision they can get out of their tools and jigs and stepper motor aspects of this project. Some are high art, and some are high science. This webpage is all about getting you into building a platform and how to do it and what you REALLY need to know in order to accomplish that task. You can take these instructions and make them as precise or artistic as your skill and talent can take you.

I'm just here to sift out all the superfluous information that you don't need from those other sites (which I often found contradictory or confusing) and to help you get on the road to building and completing a practical equatorial platform. Now I'm not saying the information on those other sites isn't valid or correct or even worth using. They are. But they all seem to be working against each other in the simple goal of telling other people how to make an EQ platform. They often are written by telling you how THEY did it or how they calculated this or that, without realizing you don't know how to do it to begin with and might not understand what they are talking about! You went to their site to find out how to build one - not take a course on EQ building theory 101 or stepper motor programming, or maybe you don't have a full wood working shop. Or worse, they show you a couple of photos of their platform, and then textually describe how they build it with no other instructional photos to explain what they did. They are great as a group, but terribly hard to get to the nitty gritty of the How-To. This is the How-To. I hope it helps you.

Scavenge for Parts = Free!

I want to first tell you about how you can save some significant money on this project if you take the time and make the effort.

Ask your friends if they have old computer printers (dot-matrix, ink jet, laser) or fax machines laying around in their attic, garage or office, that don't work or they just want to throw out. See if they will give them to you for free if you will haul them off for them. "Free" is the key. You are trying to save money. If they want you to pay for it, then you might as well buy the parts new. You were going to scavenge the parts you needed out of those dead devices to save the money. Most of the parts needed for this project are under $10 or less if you buy them new so the hassle of paying for scavenged equipment and then having to strip it yourself isn't worth it. Just buy new in that case. Most parts sellers (especially online) will require that you buy at least a certain minimal amount (usually $5.00 or so) in order to place an order. So if you only need one component but have to buy 5 or 10, then you're buying more than you need, so scavenge any chance you get. Save money where you can. The condition of the control parts aren't nearly as important as the condition of the platform itself.

(Motors, gears, switches, buttons, magnets, encoders, etc. - ALL FREE, ALL SCAVENGED!)
Figure 2-1

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Figure 2-2

Printers and fax machines have tons of gears, optical encoders, limit switches, buttons, etc that are perfectly good even if the device itself is dead and useless. With a screw driver and some time, you can have a pile of useful parts that will save you a good chunk of money.

Check for people throwing out shelving, or other wood that can be used for the platform base and bottom. Make sure it's flat. Plywood is ideal. Stay away from particle or press board. 3/4 of an inch thick is also required. I lucked out and ran across several wooden carts that my work was about to throw away in the dumpster. It was cabinet grade plywood in the correct thicknesses and in really nice condition. I acquired them all and then pried them apart and pulled out the brads joining the pieces. In the end I had a pile of wood worth about $300 already cut into workable sizes for FREE! It pays to ask around.

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Figure 2-3

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Building the Platform

Entering Data into the Spreadsheet

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Figure 3-1

Before you begin, download the MS Excel spreadsheet eqplatform_calc2012.xls. You will need it to calculate the dimensions for the top plate and the sectors of the cone which will be the "rockers" which the platform will ride on. The numbers in RED are the data you enter, and the results that tell you how to cut the wood and place the parts are in BLUE. It will also calculate the angles of the bearing wheel mounts (inline skate wheels) and the spacing on the bottom board. It basically does it all. This file is the foundation of this project (thanks Tom Hole for the foundational spreadsheet and diagrams that this modification is built upon!)

Don't pay too much attention to the motor calculations in the spreadsheet at this point though.

1 - Finding your Center (A)

FIRST, determine the rough center of gravity (CG) of your telescope's rocker body with the base plate (the part that touches the ground) removed. The base plate will not be used when the telescope is sitting on the EQ platform.

In order to find the rough CG of a standard dobsonian telescope, remove the base plate. Set the telescope tube into the rocker body and angle it to 90 degrees (parallel to the horizon). You will take two readings for this measurement - one from the front and one from the side of the telescope.
(NOTE: For larger truss Dobs (see image right), the tube assembly can be positioned upright at 0 degrees (zenith) and secured. This is due to the lighter weight of the truss tube style and the lower center of gravity in the mirror box.)

Figure 3-2


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Figure 3-3

1 - Mentally (or physically) draw a center line on the front and side of the rocker base. (green line) A strip of masking tape with the line drawn on it would work. Just make sure the line is centered on the side.

2 - Gently tip the entire telescope onto one edge of the rocker base, font-to-back. Tilt it until the telescope essentially balances on that edge. Then tilt the scope structure again side-to-side to balance it. This might take more than one person if your telescope is large.

3 - While balancing the scope on its edge (pivoting just to the tipping point), have someone eyeball the point where the telescope is touching the ground (blue dot) and have them mentally draw an axis line (dotted line) vertically through the telescope from that point upward. The point where the verticle axis line crosses the previous marked center lines on the telescope's rocker body, after both measurements, is the center of gravity. (red dot)

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Figure 3-4

4 - Set the telescope back down on the flat ground like normal. Measure the height of this newly deteremined CG point (red dot) from the GROUND up and write down the number you get. This is the height of your telescope's CG (A). You will need it for the spreadsheet.

2 - Measuring the Baseboard (C)

The measurements of the plywood panel upon which the telescope rocker body will actually sit (baseboard) are really straightforward. It just needs to be at least as wide as your telescope rocker body base. It can be a little wider (an inch or so more on each side), but it doesn't need to be.

The length of the baseboard is important in that it will determine how far apart the sectors are placed underneath it. How far apart they will be effects the curve of each sector since they are actually sectors of a cone. (See Figure 3-5)
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Figure 3-5

The cone is "laying" on one side with the point being just behind the platform, and the open end pointing toward the front of the telescope with the center line (height line) of the cone pointing to Polaris (the North Star). This is very well illustrated in Figure 3-6. (image source: Reiner Vogel - Backyard Voyager - One of the better EQ platform sites) However the design on my page will use two sectors instead of the single sector show in the image, so the back sector will consequently be a smaller cross section of the cone than the front sector.

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Figure 3-6

3 - Your Latitude is your Attitude

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Figure 3-7

Your Latitude Angle is the geographical latitude where you will predominantly be using your scope in relation to Polaris. My location is North Texas which is approximately 32° latitude. This works fine for most places I will travel to using my scope. It will be fine for approximately 3° north and south of my primary location. But the accuracy goes down the further you get from the designed latitude angle.

You can also find your latitude of course by using GPS on any smartphone (Android/iPhone/etc). But you only need to know your latitude to the whole degree. Anything else is beyond the precision of this device.

4 - Drive Shaft

The main drive shaft of this project is a threaded (screw) rod approximately 12 inches in length but it can be up to 16 inches long. You won't get much more than six (6) inches worth of glide from this system on the threaded rod. Anything longer and it is just wasted materials for this project. You also don't want to use too thin of a rod or it could flex and bow under any load. Too thin of a rod can also get accidentally bent if it is compressed while being moved of shipped in your car. Then you'd have a warped drive shaft that would make your platform move in a pronounced wave motion. Not good. 3/8 inch thick rod is as thin as you want to go on this. And a 3/8 diameter X 12-inch long rod is what I used in my design.

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Figure 3-8

The threads per inch (or pitch) is usually going to be something like the standard 16 threads per inch (16 pitch). Just make sure you know what the pitch # is so you can enter it into the spreadsheet. If you bought one and don't know what the pitch number is, just get a standard ruler and count how many threads there are in one inch - that's your pitch #.

5 - Plywood Thickness (B)

Self explanitory. Measure it yourself! Don't take the word of the hardware store sign. It makes a difference if it says it's 3/4 inch but it's really 5/8 or something else. The top baseboard should not "give" under the weight of your telescope at all. You need it to stay as flat as possible. 3/4 inch is as thin as you should go on this part.

6 - Latitude Baseline to Bottom of Topboard (D)

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Figure 3-9

This is the point where the imaginary cone's bottom side (the side laying on the ground) is measured to the bottom side of the topboard (the board your telescope will be sitting upon). The baseline is also the exact point where the inline skate wheels will touch the sectors (aka runners). This distance can be kind of tricky to figure out. It's not readily apparent.

Just enter some reasonable number you want that height to be (about half the total height you want between the topboard and bottom board is a good place to start) into the spreadsheet and see how it changes the "Runner Dimensions" (J) number. This number is how deep the UNTILTED runners will be when you cut them. So depending on what you put in for the height between the latitude baseline and the bottom of the topboard, that will determine the depth of your runners.

Once the runners are tilted to the right latitude angle this will reduce the distance a small amount between the two main boards, so factor in a tiny bit extra when entering in the (D) amount. You do not have to have a large gap between the two main boards. Three inches is about as much as you'd probably want to go. My distance was 1.3 inches for the (D) amount. Yours may need to be more if you have a larger telescope.

Enter the number into the spreadsheet and then place two scrap boards that are twice that (D) number in width, apart from each other (use soup cans or scrap wood as spacers). Inbetween the two boards place your inline skate wheels, temporarily leaned them in a mocked up manner as they might be tilted in the finished design so you can get a better idea of how much space you will have. This doesn't have to be exact. You're just getting a visual feel for how much space you will eventually have under there. Remember you may want enough space to have batteries (I used 6V lantern batteries which are about 2.5 inches tall on their side) and other parts (switches, motors, dials, wires, etc) stored under the top board out of sight, so you may want to figure these into your mock up too. Remember also that your topboard will be easily removable when not in use so you will have easy access when it's not in use.

All of this needs to be hashed out before you start cutting wood. It's the most time consuming part of the project but it's worth doing these mental exercises and "measuring twice and cutting once" in order to get a precise platform made especially for your telescope. This is where the spreadsheet REALLY becomes your friend in figuring all of this out ahead of time.

Also note, that the lower the number you put into the (D) field, the wider your inline skate wheel placement gets from the center line. BUT, it also reduces the amount of travel you will get because you will run out of runner length sooner at the edges. If you make the number too high, the skate wheel placement gets very narrow and can make the whole platform "tippy". On average you'll get around five inches worth of travel on the northern sector give or take some length depending on your calculations for your scope. Mine gets right at five inches for a 50 minute run.

7 - Bearing Placement (E) & (F)

These are calculated for you in this spreadsheet based on the other measurement given.

8 - Placement of the Guide Pin (G)

This dimension of where to place this pin is going to need to be in the thickest part of the front sector (rocker) base. I basically just added 1/4 inch to my topboard thickness for this measurement. This put the guide pin (a 1/4 lag bolt) right into the thickest part of the wood.

9 - Guide Plate Distance (H)

The Guide Plate is what rides along the threaded rod and pulls/pushes the Topboard via the Guide Bolt. The distance from the front of the Topboard to the point of contact between the Guide Pin and Guide Plate is (H).

This is a guess-timation before the build. So you will have to make sure to place your Guide Plate contact point at this position when building the drive system. 1.5 or 2 inches is reasonable if you use a 1/4 bolt or thicker for your Guide Pin.

10 - Rotational Angle

This is how much tilt (in degrees) your telescope platform will travel, from beginning to end, through each session (half of this amount will be how far maximum the telescope will tilt to the right or to the left). The general amount of rotation from one side of the platform to the other during one session is usually around 10-13 degrees. That's about 35-50 minutes of observing time per session before you will have to stop and rewind the platform. You can enter 11.00 for the Rotational Angle into the spreadsheet as a starting point.

Building the Platform

Top and Bottom boards

Cut your top and bottom boards according to the sizes you entered into the spreadsheet. You might want to cut the bottom board about 6 to 10 inches longer than the top board to allow mounting space for the motor and gears. This can always be cut shorter later if you need.

The Cone Sectors (Rockers)

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Figure 4-1
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Figure 4-2

These are THE most important part of the build. If these are sloppy and rough then your scope will not track accurately and you'll be disappointed in your results. That being said, it's relatively easy to get accurate circular cuts using a router and a simple jig to cut radii. If you don't have a router you can use the same jig setup but use a jigsaw instead. You'll just have to be more careful to make sure the cut is smooth.

Since we were building several platforms for different sized scopes, we settled on building a simple jig that consisted of a sheet of plywood to temporarily mount the hardwood board onto for each sector using brand nails. You could also use screws to hold it in place, but I didn't want the screw holes in the sector board. This simply was used to keep the board in place while we routed it.

The top board in our jig had the radius of each person's north and south sector rocker drilled into it and their initials written next to it with the (n) or (s) to tell which sector was which.

We then proceeded to route the rockers out of 1x4 hard (cabinent grade) plywood board. (And proceeded to break the router bit off from the stress of cutting 12 sectors like this.) If we had it to do again we'd rough cut the sector about 1/8 inch outside the desired edge using a bandsaw, THEN mount the board and route the extra off the end. Live and learn.
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Figure 4-3

I've seen websites that created jigs that mounted on the wall and swung to and frow with the sectors mounted on them as they swung past the cutting/grinding power tool. Kewl, but unneccessary. A radius arm (long straigt board) and a rounter are all you need. You could even do this with a jigsaw on a radius, but the edge won't be nearly as clean. And clean and straight as you can get is what you are shooting for!

Once you have your sector rockers cut, you will have to cut two strips of wood at specific angles in order to mount them to the underside of the top board (the board your telescope will ride on). These strips must create a flat bottom to screw and glue to the top board. The angles will be in relationship to whatever your altitude angles are. (see animated gif LEFT and diagram RIGHT: red, green, blue).

Do this same thing for the south sector rocker as well.

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Figure 4-4

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Figure 4-5

The next step is to secure the wedged sector rockers to the underside of the top board. This is where your (E) and (F) measurements come into play. Take these measurements and using the CENTERLINE of the WHEELS measure these distances and place the north and south sector rockers in place and mark them. Then clamp these in place and drill screw holes and screw and glue these pieces down to the top board.

Sector Bearing Mounts

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Figure 4-6

The mounts that hold the sector bearings (skate wheels) that the top board will ride on are at compound angles. They tilt along the latitude angle AND angle upward to fit along the curve of the sector rocker. This can be difficult to cut accurately entirely out of wood. However, wood can be used for the mounting bracket supports. See drawing above. These can be made from any type of wood as long as they are wide enough to hold two screws that will hold the angle bracket securely to those supports.

The simple solution we came up with for the main bracket was to use a 2-inch wide piece of aluminum angle bracket and some wooden blocks to create the latitude angle across the width of the platform.

Then we used another set of shorter cut 2-inch aluminum angle bracket for the "risers" to get the rocker angles (Alpha n / Alpha s).

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Figure 4-7

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Figure 4-8

Once you have measured the Ds and Dn distances from the centerline and then tilted the shorter brackets to the correct Alpha s and Alpha n angles, clamp the pieces together (Figure 4-8) and drill one screw hole close to the pivot point of the shorter bracket (upper left hole - Figure 4-7). Then fit a bolt with a lock washer into that hole to lock the short angle bracket into place on the larger bracket. Measure again the Alpha angle of each piece and adjust it now! You won't be able to correct this once you drill the other holes. Next drill two more holes at the other two corners (see Figure 4-7) and mount bolts in them to secure the whole bracket.

Your next step is to drill a hole at the Dn/Ds center point for the front wheel axel to fit a 1/4 bolt (large hole on small bracket - see Figure 4-7). Drill this through both brackets while it's secured with the bolts and lock washers.

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Figure 4-9

Drill the axel hole for the top wheel as well. Then remove the bolts and bore out the hole in the larger bracket for the top wheel to allow a wrench to fit around a 1/4 nut up against the top bracket (shorter angled bracket) from the bottom side. (See Figure 4-9)

Repeat this process for all eight wheels on the north and south wheel mounts. Then mount all of your wheels with a nut and a lock washer on both sides of the bracket. This will act as a spacer between the bracket and the wheel.

The next step is to secure these sector bearing blocks to the bottom board. These MUST be perfectly square to the sector rockers you mounted earlier to the top board. I would suggest applying glue to these first then, while the glue is wet, set the top plate onto the wheels while the glue dries. Roll the top board back and forth over the bearing wheels a few times to make sure they center and will stay in the same position as it rocks. Clamp these all together or put the scope on the top plate while the glue dries. This should square the bearing mounts to the top board and sectors.

If they don't and the bearing mount blocks move slightly back and forth when you rock the top board, that means some of your angles are off. That's not necessarily a huge problem if you are using the screw drive method as we are using.

This is where the screw drive method shows its forgiving nature of any misconfigurations in the angles. A misaligned angle only means that any given wheel may slightly lose "contact" or traction against the sector for a few moments during the platform run. In a direct drive system any loss of traction (especially on the drive wheel) will result in slipage and mistracking of the top board and therefore bad tracking on the telescope. Screw drives eliminate this possible problem since the wheels are only being used as the rolling bearings and not for driving the platform itself.

Once the glue is dry, you can drill and screw the bearing mounts down. I do it this way instead of screwing it down first because any difference in the angles will be mostly wiggled out by the weight and then set instead of drilling a hole and finding out the angle is way off and having to drill another hole to correct it.

Building the Drive Train

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Figure 5-1

The Motor and Speed Control

The motor on this system is a 12V, 10 RPM, 300:1 geared, DC motor that I purchased new from ServoCity.com. The 10 RPM is with a full 12VDC load on the motor, lower voltages would produce a lower speed obviously. After doing some experimenting with various voltages on this motor, it was determined that 12 volts produced the closest speed that is required of the turning drive shaft (threaded rod).

The motor speed at this voltage, with the drive system gears in place, is actually slightly faster than the rotation of the earth. If run at this top speed, the platform will overtake and move an object out of the field of view opposite from what it would do if there were no motor at all. So we must slow down the rotation of the motor slightly to synchronize with the earth's rotation obviously. We do this by introducing a precision variable resistor (a.k.a. a trimpot) into the motor/battery "circuit". This trimpot, which I provide a purchase link above on this page, is very precise and takes 25 turns to go from fully opened to fully closed. This allows you to fine tune the speed on the motor with very small changes in the speed - which is what you want.

The Gears and Drive Shaft

After attaching the pinion gear to the motor drive shaft and mounting the clamping hub to the larger gear. You can now clamp the large gear to the drive shaft (threaded rod). Verify that the gears are as perpendicular to the drive shaft as possible. If they aren't then it may induce a wave motion in the drive system or the gears may ride off of each other and damage the gear teeth or your motor.

The Shaft Bearing and Bushings

At this point you should build two "bearing mounts" out of wood for the drive shaft, bearings and bushings to fit into. They should be tall enough for the main gear to clear the bottom board of the platform, but short enough so that the platform doesn't collide with the top board when it is fully tilted on that side.

I used a forstner bit to cut a hole in two blocks of wood that were the same thickness as the bearings that the shaft mounts into. Then I cut the top of the holed part off of the blocks so the bearings fit down into them like a "U" shaped saddle. The bearings can be secured to the wooden bearing blocks with metal brackets screwed down or blocks of wood screwed down. Use your own discretion. It's not functionally important how you secure them as long as they are secure.

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Figure 5-2

To secure the drive shaft into the bearings I used two nuts at each end of the drive shaft positioned inside in relation to the bearings to use as lock nuts. For a total of only 4 nuts (see Figure 5-2 enlarged). When I had them into position I simply turned them counter to each other to lock them in place. This locked the shaft in between the shaft bearings and held the main drive gear from moving side to side. You don't need to secure it in any other way to the actual bearings. The shaft just rides inside the bearing holes.

The metal bushings take up any slop in the inner/outer diameter of the bearings to the drive shaft fitting into the bearings.

The Drive Plate and Drive Pin

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Figure 5-3

The Drive Plate is the upright metal "fork" that holds on to the Drive Pin sticking out of the Top Board. It is made from sheet aluminum. This is secured to a threaded rod coupler (looks like a long nut) using u-bolts. This all rides along the threaded rod pulling or pushing the drive pin back and forth.

The Drive Plate's bottom edge fits into a simple track mounted to the bottom board. The track is made from two strips of 1/4 inch aluminum angle bracket butted back to back to each other. There is a gap between them slightly wider than the width of thickness of the Drive Plate. This track keeps the Drive Plate upright and centered along the length of the drive shaft (threaded rod).

click to enlarge
Figure 5-4

Testing the Tracking

Once the motor and drive train are mounted, and the batteries are installed and connected, it's time to test the tracking to see whether the motor needs to be sped up or slowed down to track. More than likely you will have to slow it down.

To test this, first, aim your telescope at a bright star (Sirius, Rigel, Spica, Vega, etc). Center it in the field of view using a medium power eyepiece such as a 25mm Plossl. Without the motor turned on, note which direction the star moved to leave the field of view. FYI: This direction is West in your eyepiece.

Now re-center the star in the FoV and turn the motor on (full power - without any resistance on the circuit i.e. no potentiometer). Note which direction the star now moves toward. It should move in the opposite direction as before. This indicates that you have to slow the motor down to get it synchronized with the Earth's rotation.

If the star still moves to the West when the motor is on and at full power, then your motor is moving too slow. You will either have to replace the main gear on the drive shaft with a smaller one (i.e. reduce the gear ratio between the main gear and the motor pinion gear), or you will need to buy a faster motor. Either way, you will have to speed up the rotation some how, so you can then back the speed off gradually using the trimpot (potentiometer).

Using the 12VDC 10RPM motor, the 200 Ohm trimpot, and the gear sizes listed on this site, and using an Ohmmeter between one of the leads of the motor and the battery, you should be able to read about 100-115 Ohms of resistance on the motor to get it in fine-tuning range to match the Earth's rotation (Remember, these numbers are for 32 degrees latitude. Your numbers may be different due to your location). You will then have to fine tune the trimpot further to get the star to stay in the center of the FoV continually. Once you have this accomplished, test it using a higher power eyepiece, such as 5mm or 4mm. Then fine tune the speed some more. If you can get a minimum drift of about 1/4 of the FoV in about 15-30 minutes of one full run of the drive plate, you are about as good as this thing is going to get.

You should be ready to use this now for any observing session. Once the trimpot is set you shouldn't have to mess with it at all. The motor pulls almost no load on the two 12V lantern batteries so they should last you for several months worth of hours long observing. The rest left to do is all extras.


  • Run Switch -

    A push-on/push-off switch to turn the motor on and off manually. A red LED with a resistor in series with the circuit can be used to indicate the "ON" state.

  • Master Swtich -

    A single switch which enables all of the other devices to get power. This is a redundant switch if only using it for power to the motor.

  • Limit Switch -

    A three-pronged pressure switch that is mounted at one end of the Drive Plate Rail. The switch lever is "pressed" by the Drive Plate when it reaches the end of the session run and is attached to the "open" pole of the switch. This shuts off the motor (opens the circuit) automatically when it is pressed. A red LED with a resistor in series with the circuit and attached to the "closed" pole of the switch can be used to indicate the "END" state. The light will come on (closes the circuit) when the switch is pressed.

  • Return Motor -

    A second motor mounted to the drive shaft to quickly return (rewind) the platform to the intitial start position. This can be a handheld unit or a system mounted to the platform. It is disengaged or unpowered to allow free turning of this motor's shaft to prevent interfering with the main drive motor.

  • Volt Meter -

    Cheap digital display that can have a momentary on switch to read the voltage coming from the batteries. This can be an early warning when the batteries begin to drop in voltage. To be used before every observing session.

  • Analog Amp Meter -

    Cheap analog ammeter to monitor the speed of the motor. The 10RPM motor I used and the gears I have required the motor to turn with a resistance of about 111ohms (ohms measured after dialing in the platform tracking under visual testing). Using Ohms' Law: Amps = Voltage/Resistance. So 12VDC/111ohms = 108mA. These are proportional numbers, meaning if you change one the others change too. If the Voltage doesn't change (12VDC) then the only other numbers that can change are the amps or the resistance. And, since cheap digital panel mounted ohm meters are hard to come by, and cheap panel mounted ammeters (~$6.00) are all over the place, using an ammeter to monitor motor speed makes sense.

  • Tempurature Meter -

    Cheap digital display that reads ambient temperature (or temperature of equipment if taped to the component). This comes in handy on those cold winter observing nights. Helps guard against staying out too long and getting too cold (it's easy to lose track of time and discount how cold it can get at night in the winter). Linked example displays in celsius only.

    To easily convert from Celsius (C°) to Fahrenheit( F°) you simply multiply the Celsius temp by 1.8, then add 32.

    (1.8*C°)+32 = F°

  • Digital Hygrometer Humidity Meter -

    Dew Point can cause finderscopes, eyepieces and mirrors to fog up and become unusable. The Dew Point is based on the Relative Humidity of the air. As the DP temp gets closer to the ambient temp, condensation can occur on optics. This little meter (along with the Celsius temp) can help you determine how close the conditions at your location are to producing fog on your optics.

    To get a simple approximation of the Dew Point (DP) from the Relative Humidity (RH), you simply subtract 100 minus the RH divided by 5 from the Celsius temp.

    DP = C°-((100-RH)/5)

  • Final Comments

    Check the numbers after entering all of the required data. Draw out a basic mock up of what the numbers say. I used MS Visio to give me dimensioned drawing so I could put all the numbers in and everything was to scale. This gave me a better idea of how things were going to be spaced and how much the rotational angle would affect the scope sitting on it. Visualize and experiment with the numbers before you cut any wood. Then....dive in and do it. This is a hands-on project. Build it and you will be surprised at how much easier it actually is to build than what some websites or news groups made it seem to be.

    My EQ platform has literally changed the way I use my telescope. It has breathed new life into it and opened up a whole new usefulness in areas I didn't think I'd be able to do with my alt-az scope.

    • I can to very high power planetary observations. This opens up finding moons around Uranus. Fainter moons around Saturn. Planetary features on Mars, etc
    • I can begin to experiment with astrophotography in a limited way.
    • I can now dabble in sketching what I see.
    • I no longer have to fiddle with my scope every other person during public star parties, freeing me up to talk more with the people and get them interested in the hobby.

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