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Making an equatorial platform

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#1 Ed Jones

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Posted 26 July 2005 - 09:34 PM

I'm in the midst of making an equatorial platform for my 12 inch Dob and was thinking of posting how I designed and build it. These things are so simple that I'm surprised that every Dob owner doesn't have one. Is this of any interest or just wasting space?

#2 rnabholz

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Posted 26 July 2005 - 10:02 PM

As you may have already guessed, I would be very interested in your project.

I have been trying to get my arms around some of the different approaches, looking for the easiest build.

By all means, Please post your project!!

Thanks

#3 MrLunch

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Posted 26 July 2005 - 11:17 PM

What Rod said. :)

I have been collecting notes for a while, and with your permission I'll chime in with what I have found out here and there, but I'm interested in your timing motor approach!

#4 JamesBaud

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Posted 27 July 2005 - 02:46 AM

Me three!

#5 Chriske

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Posted 27 July 2005 - 05:03 AM

Hi,

I made my first eq. platform in 1983(wrote an artikel about it in S&T). But after a while I made me a lower one because my first experiment was much to high(also changed the tracking system). The platform I'm using now is about 6" high and support a 90 lbs Newt. Using just 1 AA battery my platform/DC motor tracks stars for about 6 hours. I have to rewind the system every 2.5 hours. Almost no force is needed because the complete system is perfectly in balance due to the airpumps pushing force in the base of it.
Yes indeed Ed, to few ATM's use these systems.

#6 Stefan Rostyne

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Posted 27 July 2005 - 05:29 AM

Please post! I wish to build one, too!

#7 Ed Jones

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Posted 27 July 2005 - 07:49 AM

OK so be it. I'm about 2/3 done so far and this my second one, my first pictured in my avatar works super. I rebuilt my scope since I made this one so the center of gravity is a lot lower than it was. I'll keep the old one for my 8 inch. So I'm fairly sure this thing will work (wouldn't that be embarassing if it didn't!) I'm making it out of walnut because I had some pieces I got for free at a sawmill otherwise I would have used baltic birch. I'm not set up like Norm on "This Old House", I have a cheap table saw, belt sander, drill press (not necessary) and clamps and hand tools and work out on my picnic table.
So my first step was determining the center of gravity of the whole telescope and platform. My OTA weighs 41 lbs, rocker box 21 lbs and I estimated the platform will weigh 7 lbs. Each of these components have their own center of gravity and distance apart so the calculation is done exactly the same as balancing a Newtonian and using an Excel spread sheet I computed the center of gravity to be 13.5 inches above the bottom of the platform. Do I need to elaborate on this, it's pretty straightforward?

#8 Stefan Rostyne

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Posted 27 July 2005 - 08:22 AM

In most cases, you never know to much. Personally, I feel most interested for an easy to make gear system. Have to do some experiments with a freshly obtained router. Soon, the real fun can begin. Others will benefit the complete saga, being first time builders or so. And if not so, even the skilled die-hard platform maker allways appreciates the personal tricks of a friend-ATM...

#9 MMICKELS

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Posted 27 July 2005 - 08:31 AM

Ed, could you pay special attention to the electronics aspect? I'm a little electrically challenged and in need of the "for dummies" version. Thanks.

#10 sixela

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Posted 27 July 2005 - 08:51 AM

Please post! I wish to build one, too!


You can come and have a look at mine (not self-built, one of Brian Reed's platforms).

You can even try it out on one of your scope next time we observe together - at least until I make it the ground board of my scope ;).

#11 Stefan Rostyne

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Posted 27 July 2005 - 09:14 AM

That's a nice idea, Alexis! I'm occupated during next weekend for work, though. Anyway, seems it will rain...

#12 Jim Svetlikov

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Posted 27 July 2005 - 09:27 AM

I agree with the others Ed, please do document the project here as I'd like to build one too.

#13 Guest_**DONOTDELETE**_*

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Posted 27 July 2005 - 04:06 PM

I'm also watching this post with anticipation Ed. I'd rather see you post too much than not enough info. Thanks in advance!

#14 Ed Jones

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Posted 27 July 2005 - 06:01 PM

Electronics? None. But this has to do with how I will drive it. I'm going to use a direct drive, a motor driving a knurled rod, very simple. I think some call this a capstan drive or something like that. To drive it I could use a DC gearmotor, stepper or timing motor. I'll need a motor with a speed of 1/10 to 1/30 rpm and I didn't find any DC motors this slow, I bought a stepper that had a 50 to 1 reduction but going through the calculations I would end up with 2.5 pulses a second and I am afraid that this isn't very fast and might cause vibration. I found a suitable stepper for $76 and also a timing motor for $18, that was a no brainer but then I found a 1/25 rpm motor in my junk box (guess which one I went with). Timing motors are not reversible normally so be sure you get one with clockwise rotation!! To drive it I have a inexpensive inverter to run off my car battery when I don't have power. With a timing motor you must change the line frequency for speed control.

I was wrong, my OTA weighs 48 lbs and is 20 inches above the ground board or platfrom. Also the rocker box center of gravity is 3 inches above the platform but combined with the weight of the platform should be 2 inches above the platform. This gives 18 inches between the OTA and rocker box/platform combo. This gives the equation:
48*X = 28*(18-X) Solving it X = 6.6 inches. (18 - X) + 2 inches = 13.4 inches is the height of the whole center of gravity above the platform. In any case it doen't need to be very exact at all and it will likely work for different scopes. Just be careful though, if you put a scope on it with too high a center of gravity from this it could tip over!

#15 Ed Jones

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Posted 28 July 2005 - 07:12 AM

After determining the center of gravity I calculated the distance from the rocker box pivot to the platform pivot point. The lattitude of Cincinnati is 39 degrees so this distance is 13.5/tan 39 degrees or 16.67 inches.

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#16 Ed Jones

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Posted 28 July 2005 - 07:26 AM

I then drew on a piece of construction paper an equilateral triangle with 9 inch radii which is the diameter of the ebony star on my rocker box. I drew the pivot point 16.67 inches from the center and then added some distance around this and came up with these dimensions for the top of the platform. You could do all this on CAD but I just did it on paper.

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#17 Ed Jones

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Posted 28 July 2005 - 07:47 AM

One of the Teflon pads falls between the two pivot points. The higher your center of gravity the larger this distance and at some point it would be better to use a second roller under this pad for more stiffness. Using a pivot instead of a second roller makes things simpler if you can keep it sturdy. The distance to the front of the platform is 5.5 inches so now I calculate the radius of the north roller. With the front edge to the pivot point 22.17 inches the distance to the axis 22.17*sin 39deg. or 13.95. Looking down the polar axis the front edge of my platform is 17.5 inches and the distance from the front edge to the axis is 16.47 inches. However I can't have a zero thickness roller so I need to add about 2 inches to this radius to allow for stiffness of the roller, the thickness of the platform and clearance.

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#18 Ed Jones

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Posted 28 July 2005 - 05:18 PM

I planned to use a 1/15 rpm motor to drive a knurled shaft. I'd like to use a standard size steel rod to avoid extra machining. If I use a .375 inch rod I would need a 35.9 inch polar diameter or 17.95 which is close enough to 18.47. At least that was the plan. Then I discovered my 1/25 rpm motor so now I need a .625 inch shaft.

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#19 Ed Jones

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Posted 28 July 2005 - 06:01 PM

So now that I know the radius I'm ready to make sawdust. The north bearing is first and actually the most fun part. The top edge of a 6 inch board is beveled first by tipping the saw blade 39 degrees. I need the sag of the chord to be 4 inches deep (17.95-13.95). I temporarily tacked with brads a scrap piece of plywood to the back side of this piece of wood. With a compass I drew a radius an inch longer and 5 inches deep on the sag. I roughed this out with a saber saw but a band saw would have beesn faster. I fastened a 3/4 inch board to the bed of my table saw with a finishing nail on it for a pivot and drilled a hole at the center of curvature of my bearing. I wanted to bevel this radius so that the motor would be parallel to the ground. I could have cut it at 90 degrees but the the motor would need to be parallel to the polar axis. To do this I used a sanding disk on the table saw but first i need to rough in the bevel. I tilted the blade to 39 degrees and lowered it all the way down. I raised the blade 1/8 inch at a time after each pass rotating it around this pivot point. Always push the woood into the blade!! Eventually the bevel is roughed in. I then switched to a sanding disk in the saw and again pushed it into the disk. To take of more stock I tapped the board with a hammer to move it a little closer to the disk (crude I know). Measure the radius to the center of the bevel from the nail. I switch to a finer abrasive (flip the disk) and finish smoothing it. This is so cool to do and it makes a really smooth perfect bearing surface! I love it.

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#20 Ed Jones

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Posted 28 July 2005 - 06:52 PM

Next I ripped and glued together two boards for each side of the platform A-frame. After smoothing they measured 1.5X1.75X28 inches. These attach to the bearing at a compound angle. I used a miter saw tilted at 39 degrees and rotated at 18 degrees for both a left and right side cut. Afterwards I ripped an angle on the bottom side of this rail so that the bottom edge was parallel to the bearing where it attaches to the bearing. One side measured 1.25 inches.

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#21 Ed Jones

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Posted 28 July 2005 - 07:16 PM

Next I cut a 72 degree angle on the other end of these rails to fit together at a 36 degree angle. Next I fit a triangle 8.5 inches long to go between the two rails. This will be the support for the far Teflon pad. It was also glued in at a slight angle ~6 degrees with the two rails.

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#22 Ed Jones

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Posted 30 July 2005 - 08:13 AM

Next I marked the center of the north bearing and screwed it to the rails with stainless screws (predrilling first) and making sure the perpendicular to this mark intersected the polar pivot point. There was nothing for the rocker box to pivot around so I glued and nailed a 1.375 wide wooden crosspiece 10.5 inches from the front edge to center.
Next I glued two triangles shown here which form a channel for a wedge to slide in. This will be a slow motion adjustment driven by an AC reversible motor driving a 1/4-20 brass rod. This channel is lined with ebony star bottom and sides and the wedge made from maple plywood will have Teflon on both the bottom and top with Teflon tape on the sides.

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#23 Ed Jones

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Posted 30 July 2005 - 08:20 AM

Here's a closeup without the motor or Teflon. Note I had to grind down the top of the triangles so that nothing is above the top of the platform.

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#24 Ed Jones

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Posted 30 July 2005 - 08:38 AM

Next I marked off 22.7 inches from the front bearing edge. I made a 39 degree cut about 1/2 farther using the miter saw. This was a compound angle cut. This surface should be perpendicular to the polar axis. I cut a piece of 3/16 in. stainless plate to fit the end as shown, secured it with screws and drilled a 1/4 in. hole where the axis intersects this plate.

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#25 Ed Jones

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Posted 30 July 2005 - 09:10 AM

Here is the nearly finished platform still without the motor installed. Note the eds of the north bearing were rounded off. I could just see myself kicking my ankle on the sharp ends!

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