I had made this little 6" travelscope (3.2 kgs total) early this year, and tried it in Gran Canaria and the Azores. I figured a small, portable equatorial platform would come in handy - not least for showing people planets and other high-magnification objects without having to adjust the aim every minute. I had the electronics already, and just about all I needed in my junk collection...
A Poncet style platform would be fine for south Sweden (55 - 56 deg N)and very simple to make - unfortunately, it won't do well if I travel far south. But at least I hope tho use it at the Mars/Sagittarius meeting in Öland later this Aug. 2003.
The COG of the telescope was appr. 250 mm above the bottom of the rocker box, and I wanted it reasonably balanced - though the small load shouldn't tax things.So after a minimum of calculations, I started to cut plywood...
The stepper motor is from a 5.25” floppy disk drive, 200 steps/turn, 70 ohms windings, meaning appr 200 mA total current consumption from 12V. I may put holders for 8 AA batteries on the baseboard, but so far I can use a 12V gel cell or the 12 v battery pack of an electric screwdriver.
The threaded rod is M5, 0.8 mm pitch, driven at about 0.84 turns per minute (not quite 3 full steps/second – impossible with full- or half-stepping!). I made a coupler from soft PE plastic, drilling a 4.5 mm hole threaded at one end for the rod and widened to 4.8 mm to press-fit on the 5 mm motor shaft. The other end of the rod is held in a ball bearing with 5 mm inner dia. On the shaft is a slider made from a piece of Teflon – wide and long enough not to turn while moving, and a hole threaded for the rod. Also on the rod is a small spring and washer. On the photo you see 2 end switches actuated by the slider to toggle between tracking and rewind mode – after about 1 hour of tracking, it will automatically rewind in less than 1 minute.
The inclined surface is angled about 56 deg from vertical for use in south Sweden.The south bearing is a commercial ball joint. The 2 north roller bearings are perhaps the trickiest parts to make – each 13 mm o.d. bearing (from a junked hard disk) is mounted on a piece of aluminium angle, in turn screwed to another angle that is screwed to the baseboard. I couldn’t try to calculate the angles In 3 dimensions, but as it turned out, they could be adjusted and placed on the baseboard without too much trouble, and adjusted to run true against the inclined board (fortified with iron-on laminate) at the right height.
On top of the platform proper are the 3 Teflon pieces for the rocker box - it is held down to the platform with a small screw.
Looking from the underside, you see on the inclined board a screw and a piece of aluminium profile. The side of the rounded screw head is pushed by the slider (it travels very close to the threaded rod, as seen on the last of the images below) against the Teflon slider, and the alu profile piece holds the washer+spring that presses the slider against the screw. To assemble, the platform is moved to engage the washer, the spring is compressed and the screw head is pushed down (and later held only by gravity). Addendum: I have since made a bracket that keeps the platform from lifting more than a few mm.
So far, I have tried tracking the sun and moon, and after adjusting the stepper speed, the tracking accuracy is quite sufficient for visual use, and there are no vibrations to be seen. Polar adjustment is no big deal – there is no adjusting the angle. Putting the platform on flat ground and pointing the north end towards Polaris seems to be quite enough, but if an object near the meridian drifts downward, I can “turn up” the whole platform by rotating it clockwise.
Nils Olof Carlin Aug 8th, 2003 - minor revisions Feb 1, 2004