EQ Platform Finished

Thought I'd share the finished project.

Here's an assessment of the pros and cons of this design, as I've found so far:

Pros:

- Very powerful, weight/imbalance tolerant drive system
- Strong, backlash free linkage
- Resets instantly by disengaging drive rod from a half-nut

Cons:

- Some periodic error unless leadscrew and connection to motor shaft are perfectly straight
- Tangential error common to all tangent driven platforms

After straightening the threaded rod and making the best rod to motor shaft connector I could, the platform has a small amount of periodic error--which admittedly caught me by surprise. It will move an object around approximately 1/3 of the apparent field of my highest power eyepiece (4mm ortho--305x) in my 8" f/6 newtonian every 45 or so seconds. So, it's not a photography-capable platform as currently driven.

But... I insisted on instant reset and weight/center-of-gravity tolerance (and design simplicity)--these objectives it achieves. Traditional tangent drives are cumbersome to reset and direct drives are sensitive to weight and center-of-gravity issues. In short, I don't like periodic error, but I'd take a small amount over lengthy reset procedures or a slipping table.

I am considering eventually driving the platform using a variation of the linear conversion drive shown via the link below. I imagine instead a timing belt doing a continuous loop while forcefully engaging/disengaging the belt via a clutch where the current pivot (brass thumbscrew) is located. I would also insist that any drive system employed retain fast reset and variable load tolerance.

http://faculty.washington.edu/quarn/stpmtr.html

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Center knob, to right of carry handle, is a long screw that holds the platform together during transport. It stores away conveniently during use.

Top-side feet help raise my scope close to eye-level at zenith (my preference). Protruding brass screws fit into divots in my telescope's feet to prevent the scope from sliding off.

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Behind the motor, a circuit breaking plunge switch mechanically cuts power when the table reaches the end of its run--didn't want to worry about the table pulling itself apart if I left it running accidentally.

Power to the table comes from the telescope above via a 12v 7ah battery. I made a brush-type power transfer system to get power from the battery housed in the rocker box to the telescope's ground board, where a short cable crosses down to the platform--no dangling wires and unlimited rotation.

Motor speed control is via a potentiometer in a control box at the end of a cable velcroed near my eyepiece (not shown). It is connected to the telescope mount, not the platform, so it wouldn't be a dangling cable liability, and to keep the control at the eyepiece where it would serve its purpose best.

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Details of the ball bearing assemblies. South bearings were conveniently and inexpensively purchased for $5/pair as screen door hardware--rubber O-rings fit perfectly around the bearings. Half-nut, tensioning spring, bronze bearings also shown. The hole in the maple block (foot) is where I slide the long "transport screw" when the table is actually being operated.

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Top/side view. I included a compass (only place it wasn't affected by magnetically charged metal strips, motors, screws, etc.), bubble-level (the center section of a $4 Stanley torpedo level), top-side carry handle, top-side scope feet matched to my telescope--I have been trying to raise my scope to eye-level with this project as I prefer to stand when observing, especially when it's cold. I seem to notice the cold less when I move around more.

North feet are very adjustable: for leveling, to raise the platform as discussed above, and to permit use of the platform a good distance N & S of my Salt Lake City home.

Aluminum channel is decorative, protective and structural--I wanted it to look sort of finished until I finalized a drive option (and the current one might just be it). Then I'll finish the wood black to match my telescope.

I'm guessing this platform would work for a scope well over a hundred pounds and with virtually any center of gravity, so it ought to (hopefully) work for that minimalist 18" to 20" scope I have planned ;^).

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I chose a 3-point bearing support system over four--it's just easier to align three points.

The rear bearing block here at the South end of the table is also the only part that required some wittling and re-making--until the table was level, until the N & S bearings attached to the block had full contact with both sides of the cylindrical bearing, etc. I'd do it this way again, no question.

So... this is how I did it. I've been wanting to make one for years and am glad I finally did. I can't recommend this ATM project highly enough--it was really fun!

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Very nice looking Lee!
Floyd

Very nice, indeed - a work of love. It sure deserves less of a tracking error. For near-perfect tracking, a point on the circle section needs to make full circle (360 degrees) in 24 hours. At present, it seems to be some 70 degrees behind or ahead of it.