TAKAHASHI FOCUSSER

This project came about from CCD imaging again.I found it difficult to focus manually.The Meade has an electric focusser based on a DC motor and a gearbox so at first I looked at solving this problem that way.I turned to stepping motors in the end as there was better control and due to the slow speed of the stepping motor a smaller gearbox.Essentially the project comprised a stepping motor driving off the shelf gearboxes bought from RS components UK.The Takahashi focusser mechanism is a rack and pinion and because of its quality it does not need much force to rotate the foussing knobs.I had to turn from aluminiun an adapter to connect the drive to the focusser knobs.This has a locking screw so that for large focus movements you undo the locking screw and use the knobs as normal.I designed a PIC microprocessor based controller to run the motor that responded to two pushbuttons.When the buttons are both out and the motor is stationary all of the motor coils are off so the current drawn is very small.All of the components are held together using pieces of sheet aluminiun to make a crude box and the driver circuit is mounted on the back of one of these panels.The circuit for the driver is here In my case the pnp transistors were BC212L and the output NPN transistors were TIP33A types.However other transistors could be used.The stepping motor ran off twelve volts directly but a five volt version would need resistors in series with the coils.The PIC used was a 16c84 programmed by a ponyprogrammer using a simple ludipipo board connected to the serial port of the pc.For the compiler I used a copy of MPASWIN as part of the mplab IDE free from the manufacturers microchip.The code is here.There is no reason why this sort of approach could not be used for any rack and pinion focusser.The electronic side is the easy part choosing a gearbox for the motor requires care.A stepping motor can in one step usually rotate from as low as 1.8 deg per step to several degrees per step.A CCD needs to be focussed to an accuracy of a few thousandths of an inch so the aim is that one step should at least move the focusser that distance.In my case I rotated the focussing knob once and measured the travel of the eyepiece barrel.My stepping motor was 7.5 degrees per step so I ended up with a large gearbox ratio in the form of a 100:1 as well as a 10:1 gearbox that changed the drive shaft angle through 90 degrees so making it easier to fit the scope.A 1.8 degrees per step motor would have meant less gearbox ratio.The motor I used was out of the junk box from another project and large in size.