How To...
Build an equatorial platform, part two.
In part two of our project, we show you how to complete and motorise the Dobsonian equatorial platform. We have documented our build with downloadable photographs in this month’s Bonus Content so you can replicate the parts, but because your choice of drive motor and power supply could differ according to circumstances and availability you may have to adapt the design a little to suit your kit. Our spreadsheet calculator (also in the Bonus Content) once again comes to the rescue when it comes to working out gear ratios so all you need to do is input your values and experiment with the numbers until the ratios match your needs.
Commercial motor drives for telescope mounts are available for projects like this, but they can be expensive. They use sophisticated stepper motors, which move by very small, predictable increments. Short pulses are sent to these motors by a special controller to precisely control the motion of the scope. One of these would be ideal but it is possible to build a cheaper (albeit slightly noisier) alternative. Our drive is based on a more straightforward DC motor. When the power is on, these motors turn fairly constantly, but quickly. We used a home-made gearbox along with the gearbox built into the motor, and an off-the-shelf controller to modify the output speed to suit our needs.
Aim for sidereal
The principle behind our motor drive is simple: the telescope’s platform should rotate very slowly at the same rate as the Earth is spinning – the sidereal rate – but in the opposite direction. In 24 hours this is only slightly more than one full revolution (360°), so in one hour the platform should turn 360/24 = 15°. This is both a useful movement for
observing and a sensible amount for our design to accommodate.
Since we know the radius of our north segment (from Part 1) we can work out the length of a 15° section of the circumference. After measuring the diameter of our output gear, we can work out how many times per minute it must turn to create the required movement. This is the necessary output speed for our gearbox system.
To prevent slipping, the output gear drives a toothed section of the north segment. We created this ‘rack’ by making a mould from modelling clay (rolling the output gear along a strip of clay to create
the profile) then filling the mould with glue from a hot glue gun. If you find this too fiddly, you can buy toothed belts with matching small gears instead. Screw some stops onto the bearing strip to prevent the platform rotating too far either way.
The DC motor you need to buy should come with a built-in gearbox with a published speed of 10rpm or less. A controller can also slow this down, so our homemade gearbox will only need to reduce the speed a little. A worm gear with a wheel (ours has 57 teeth) will do the job nicely and these can be bought online.
Grab your offcuts
Offcuts of aluminium angle saved from Part 1 are great for building the gearbox, although careful measuring and some fine tuning may be required before you get it running perfectly. Once completed, the gearbox is mounted on a pivoting arm. A rubber band pulls on this so the output gear presses against the rack and drives the platform. To reset the platform, simply disengage the pivoting arm.
The controller is connected between the motor and power supply (we used a 12V powertank for our 12V motor). You can fine-tune the speed during use by observing an object at the eyepiece and making adjustments with the knob. This may also become necessary if the battery level drops during the night.
To set up, align the baseboard northsouth and get it as level as possible. Place the scope on top and switch the board on. You can fine-tune your alignment using a technique included in the Bonus Content.
Mark Parrish is a consummate craftsman. See more of his work at buttondesign.co.uk