Ed Jones
Pooh-Bah
Reged: 04/06/04
Posts: 1084
Loc: Sin-sin-atti
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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?
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Ed Jones
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rnabholz
scholastic sledgehammer
Reged: 07/25/04
Posts: 764
Loc: Iowa
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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
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Rod Nabholz
Home Built Astronomy Projects
Wild Bird Photography
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MrLunch
professor emeritus
Reged: 07/11/04
Posts: 674
Loc: San Jose, CA
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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!
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JamesBaud
Pooh-Bah
Reged: 08/21/04
Posts: 1137
Loc: Granite Bay, CA
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Me three!
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Chriske
Kijkerbouw Urania
Reged: 08/15/04
Posts: 1963
Loc: Boechout, Belgium
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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.
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Chris
Edited by Chriske (07/27/05 07:09 AM)
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Stefan Rostyne
scholastic sledgehammer
Reged: 10/19/04
Posts: 940
Loc: Assenede, Belgium
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Please post! I wish to build one, too!
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Stefan Van de Rostijne
4.5" F4.5 newt 5°widefield/finderscope
8" f/5.6 travel dob
old 12.5" F5 dob (used to look better...)
30 cm f/30 Classic Cassegrain (polishing primary)
23" f/4 dob project
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Ed Jones
Pooh-Bah
Reged: 04/06/04
Posts: 1084
Loc: Sin-sin-atti
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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?
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Ed Jones
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Stefan Rostyne
scholastic sledgehammer
Reged: 10/19/04
Posts: 940
Loc: Assenede, Belgium
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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...
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Stefan Van de Rostijne
4.5" F4.5 newt 5°widefield/finderscope
8" f/5.6 travel dob
old 12.5" F5 dob (used to look better...)
30 cm f/30 Classic Cassegrain (polishing primary)
23" f/4 dob project
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MMICKELS
Aluminum Knight
Reged: 01/20/04
Posts: 25198
Loc: The Land of Shake and Bake
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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.
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Mark
"The only thing wrong with immortality is that it tends to go on forever."
Herb Caen
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sixela
Postmaster
Reged: 12/23/04
Posts: 9499
Loc: Boechout, Belgium
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Quote:
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  .
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400mm f/4.46 David Lukehurst truss Dobsonian on Tom Osypowski equatorial platform
Orion Starblast (114mm f/4 reflector, Alt/Az)
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Stefan Rostyne
scholastic sledgehammer
Reged: 10/19/04
Posts: 940
Loc: Assenede, Belgium
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That's a nice idea, Alexis! I'm occupated during next weekend for work, though. Anyway, seems it will rain...
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Stefan Van de Rostijne
4.5" F4.5 newt 5°widefield/finderscope
8" f/5.6 travel dob
old 12.5" F5 dob (used to look better...)
30 cm f/30 Classic Cassegrain (polishing primary)
23" f/4 dob project
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Jim Svetlikov
Carpal Tunnel
Reged: 10/06/03
Posts: 2033
Loc: Crescent Beach
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I agree with the others Ed, please do document the project here as I'd like to build one too.
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It is better to build a boy than try to mend a man.
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Anonymous
Unregistered
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I'm also watching this post with anticipation Ed. I'd rather see you post too much than not enough info. Thanks in advance!
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Ed Jones
Pooh-Bah
Reged: 04/06/04
Posts: 1084
Loc: Sin-sin-atti
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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!
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Ed Jones
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Ed Jones
Pooh-Bah
Reged: 04/06/04
Posts: 1084
Loc: Sin-sin-atti
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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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Ed Jones
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Ed Jones
Pooh-Bah
Reged: 04/06/04
Posts: 1084
Loc: Sin-sin-atti
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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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Ed Jones
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Ed Jones
Pooh-Bah
Reged: 04/06/04
Posts: 1084
Loc: Sin-sin-atti
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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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Ed Jones
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Ed Jones
Pooh-Bah
Reged: 04/06/04
Posts: 1084
Loc: Sin-sin-atti
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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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Ed Jones
Edited by Ed Jones (07/28/05 06:20 PM)
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Ed Jones
Pooh-Bah
Reged: 04/06/04
Posts: 1084
Loc: Sin-sin-atti
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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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Ed Jones
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Ed Jones
Pooh-Bah
Reged: 04/06/04
Posts: 1084
Loc: Sin-sin-atti
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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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Ed Jones
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