A Ponchet styled Dual Cylindrical Bearing Equatorial Platform
This project was a more ambitious one. I used one of these on a 15" Dobson at a star gathering in the summer of 2002, and decided I could build one. I had no real idea how much trouble it was, but thanks to several other internet websites, (see below), I got it done. It took about a week of continuous fiddling until it came together. Even yet, I have some finish details I would like to do and some drive changes I plan for the future. But for now, it works great.
A Ponchet platform usually has one main cylindrical arc on the northern end, and a rear axis bearing at the southern end, but the more accurate way is to design a dual cylindrical bearing . That way the table top has a true cone-like movement around the polar axis. This was my choice when making this mount. Another thing to consider is the latitude where you will use it. It can only be used within 1° north or south of the latitude it was designed for. This mount is for visual use only. I have taken a few shots at prime focus of the moon and planets, but it is not accurate or steady enough for long exposure photography.
Here are a few shots of the mount. Click on the thumbnails to enlarge them.
This shows the platform top (table)
lifted off the base drive. The table is bolted down with a threaded rod when
transported, but otherwise just rides on the rollers when in use. Notice the
circular bubble level at the rear of the base platform. It is the black and
white piece between the rear bearing supports. All the wood is Ash and Pine, and
the cylindrical bearings (arcs on the table) are made of 3/8" thick aircraft
aluminum. They were designed on a computer aided design program (CAD) and cut
using a CNC waterjet machine. They are very accurate and smooth. I don't see any
wear ever occurring that could age this mount. The total drive and support
accuracy is important so as to not transfer vibration to the eyepiece. At 200x,
there is no noticeable shaking or drifting.
The drive is shown here. I have yet to secure the switch box, battery pack and
cables. I am planning to do this soon so I can take it to my dark site with a
little less worry about it coming apart. The northern bearing supports are
shown. The bearings are nylon shower door bearings found at a local hardware
store and only cost a few cents apiece. The vertical bearings are flat on the
outside diameter so to make full contact with the edge of the cylindrical
bearing segments, and the flank bearings on top are rounded at the outside
diameter so they only make minimal contact in case there is a slight error in
the Alpha angle of the wood supports. This insures there is no drag. The drive
yoke and channel are galvanized steel. The drive pin (not shown) that is
attached to the table top is steel as well. The screw is a 1/4"-20 threaded rod
mounted between steel ball bearings. This requires a speed of 1-3/4 RPM. The
motor is a 4-1/2 RPM 12 volt DC gearmotor. I fine tuned the drive using a DC to
DC converter circuit contained in the black switch box. There is a limit switch
at the final end of the channel that the yoke trips to stop the motor.
Another close up of the drive. You can see the motor take-off and clutch. Also,
notice the rubber band I have around the drive pin and yoke to keep the slop
going one way as it drives. This is another thing I am working to eliminate. The
motor take-off is actually two plugs of pine cut with a 7/8" hole saw! They are
exactly the same size for a 1:1 ratio. The "belt" is an O-ring. It is crude, but
effective. I simply put a screw at the table end of the motor mount, which is a
piece of angle iron, and hinged the whole thing so the belt could be removed
easily for rewinding. The angle iron bracket is wedged under the larger screw at
the north end of the base. I rewind the screw drive by hand in about 15 seconds.
It drives for over an hour before having to be reset. Notice the wooden mounting
braces for the aluminum cylindrical segments. These are made of pine and are
glued and screwed to the table. The south bearing is the same. The leveling
screws are 3" carriage bolts and nylon insert wing nuts. There are threaded
inserts in the wooden base so the bolts can be screwed up and down for leveling.
The bolt-down rod is laying on the base by the right support block. This holds
the table top down for transport.
Well, there it is. I don't consider myself to be all that skilled and I am terrible with woodworking, but the job got done. If you decide to make one for yourself, there are a few changes you could probably think of. I spent more than I had planned at $300 US. The aluminum segments were almost $60 by the time I got them cut, and the motor drive was nearly $100 in parts, although the motor was only $39 at Grainger. Still, the mount is considerably less than a commercially bought product. Though not as elegant, it does track very accurately and smooth. As long as you can "sight-in" the celestial pole pretty good and have it leveled, the image stays in the eyepiece for over thirty minutes before any noticeable drift occurs. Plenty of time to say, draw a picture, do a variable star report, or even watch a Galilean moon transit Jupiter without any hands on the telescope.
Here are a few links to some of the websites I used to get the design details for my mount.
http://members.aol.com/mauair/EQPlat.html
http://www.atmpage.com/platform.html
http://home.hiwaay.net/~warrenca/Astronomy/Projects/Tracker/Manual.html
http://home.wanadoo.nl/jhm.vangastel/Astronomy/50cmscope/newplatform.htm
http://www.jlc.net/~force5/Astro/ATM/Poncet/Intro.html
Email me at davidr@wf.net for further details