Introduction

Many perfectly good mounts come without motors. At the cheaper end of the market, an unmotorised mount is the manufactruers 'loss leader' .. they know that for serious use the purchaser has to pay up hundreds of $$'s for their 'customised' motors and electronics needed to drive the head.

However it is at the lower end that motorised star tracking is most vital. Everyone will know that at high magnifications the slightest disturbance will kick the object out of the field of view. The cheaper the mount, the less sturdy it is, and the more prone it is to wobble. So attempting to manually track stars with a low end mount is all but impossible.

Retro-fit kits are available as 'after market' option, however these can also be expensive and often consist of little more than a simple geared servo motor with an 'open loop' manual speed controller (which you could build for yourself at half the cost). The other thing wrong with such kits is that they will not be optomised for your mount, can be difficut to adjust and may have unacceptable 'drift' .. something you will not discover untill after purchase ..

The two basic types are 'EQ' (Equatorial) and Alt-Az (). To track the stars, the EQ type needs one motor, the Alt-Az two. Putting together a 2 motor system is almost impossible without computer control and is best left to the experts, so I will consider only the 1 motor system. Fortunatly there is a way to conver an Alt-Az head into an EQ head .. this is with the so called 'Wedge'.

Setting up your EQ mount.

Before you can expect your mount to track the stars it has to meet 3 vital criteria (which must be achieved in the order shown) :-

1) The 'polar axis' of the head has to be set to the correct ANGLE for your Lattitude. For example, in London, England you would set it to 52 degrees .. but in Aberdeen, Scotland you would set it to 57degrees. This only has to be done once (unless you move around a lot :-) ). If you have an Alt-Az head, you set the Wedge to the correct angle instead.

2) The base of the mount head (or the Wedge) must be totally flat to the ground. If the base is not flat, not only will the angle be 'off' but your tracking will go up hill or down hill. Every time you move the telescope, the tripod legs must be adjusted untill the base of the head, as measued with a spirit level, is as flat as possible.

3) The polar axis of the head (i.e. the bit that's now inclined at an angle of 52 degrees or whatever) must be 'pointed' at the North Celestrial Pole (NCP). This is 'a tiny bit off Polaris'.  For MOST purposes, pointing at Polaris is 'good enough'.

Having achieved all 3 of the above, you can start to think about mounting a telescope on the head, selecting an object to view and turning on the motor drive. 

The drive has to 'oppose' the rotation of the Earth. i.e. it has to turn the mount head all the way around (make 1 complete revolution) exactly ONCE per 'Sidereal day'.

There are many ways to achive this, however 2 fundemental requirements must be met :-

1) the gearing must be such that the motor can be run at a 'normal' speed during tracking - running too slow will drain your batteries faster and may even burn out the motor.

2) the movement of the head must be controlled by a 'closed loop' system that compares the head movement against some highly accurate time source. This allows any mechanical errors (eg. imperfect gearing, motor speed variation induced by changes in drive resistance etc) to be 'fed back' and to adjust the speed.