Remote Fine Control
for Focus and Elevation
by John Ball
Many amateur
astronomers are now using webcams. Operating the focus control whilst
looking at the laptop is often awkward and any vibration imparted to the
telescope will cause the image to move significantly on the small CCD chip
making focusing very difficult.
I had for some time considered buying a cheap radio controlled model with a view
to using the pieces to make a focus controller. I then became aware of a
servo tester kit available from Dr Mike Roberts (Custom Electronics) and for
little more than £20 (kit option) one can make a very effective remote control.
This image shows the general arrangement
I used a servo that had a ‘cross head’ output but a ‘disc’ would also be suitable. One needs to couple the servo output to the focuser knob in such a way that it grips the focus knob tight enough to turn it but will slip when the focuser is operated manually by the knob the other side to the servo. I got the cap off a squash bottle (Oasis citrus punch bottle cap was suitable for my focuser) and honed out the threads and cut some slots down the side so it would grip the focuser knob but not too tightly. It is important to fix the cap central on the servo crosshead/disc. The body of the servo needs to be fixed with a suitable bracket to the body of the telescope. The servo tester gives about 120 degrees of travel. The accuracy of ‘follow’ is extremely good with the ‘error’ between the input and output only a fraction of a degree (full torque is given for a difference of about ½ degree!).
The tester kit (probably best to call it a servo driver for this application) is easy to construct by anyone competent at soldering components to a printed circuit board. All components (less than 20) are supplied with the kit. As the kit is intended for model makers who usually have some knowledge of component markings you may need to seek some advice for identification of components but I’m sure Mike Roberts will be happy to answer any queries you may have by e-mail: mike@customelectronics.co.uk (Web www.customelectronics.co.uk).
Laptop, Servo Testers and Batteries
The servo tester is quite small and two are shown at the front of the table. The setup will work on a dc supply between 4.5v-6v. I found a 6v lantern battery is ideal (two such batteries are at back of the table). I have used plug/socket connectors, as available from model shops, for connecting the testers to the servos (see picture below).
It is very important the units are connected correctly to the battery as a reverse connection will damage them. To protect the units one can insert a diode in one of the supply lines to both the servo and the driver. This then means if the battery connections or the plug/sockets are connected incorrectly the system will just not work until connections are made correctly but no damage will be done.
Plugs/sockets
Make sure the diodes are in the lines adjacent to the circuit they are protecting. (Note you need diodes capable of carrying sufficient current for the servo - ‘signal’ diodes are not suitable).
My equatorial telescope is now about 10 years old and the mount has sunk slightly affecting the polar alignment. I use a threaded elevation support rod and decided to include another servo to give fine control of elevation.
Elevation rod control (blue thumbwheel)
Good model shops sell ‘square’ brass rod in sizes that will slide into each other. If cleaned with some wire wool a very smooth telescopic section can be made. It is possible to fix a servo to one section with a slightly off centre disc (cam) on its output disc and fit a cam follower on the other brass section. A rubber band or spring can be used to keep the cam follower tight against the disc. (Note a small ball race is ideal as a cam follower). The threaded rod can be made to fit in one end of the assembly and held to it with a captive nut. Coarse control can then be made by turning the blue ‘thumb wheel’ (see diagram). The top end of the rod is attached to the telescope via a ‘split’ nut held in a crocodile clip for ease of insertion. The bottom end of the assembly needs to be ‘fixed’ to the elevation support in a way that it can easily be fitted/removed and will also allow angular movement for various elevations. I have included this note on elevation adjustment just as an idea. Anyone interested in making something similar for their telescope will need to tailor it to their scope - I realise there will not be another telescope out there identical to mine!!
The picture below shows the whole set up. The other black box on the table is my equatorial drive control. (If you notice the large ball races hanging down near the focuser that is just to give a bit of additional force to keep the elevation cam follower tight against the cam!).
Complete telescope setup