Backlash can occur when the direction of motion on an axis is reversed. The telescope may not immediately start moving in the reverse direction, even though the motor is turning. The usual cause is loose meshing of the gears in the drive train. In the case of worm gears this can often be largely adjusted out; unfortunately in some cases, attempts to adjust the gears can cause excessive friction resulting in motor stalls. If the backlash is inside a gearbox it generally cannot be fixed without redesigning the drive system.
Generally backlash is only a major problem on the Declination axis, because the Right Ascension axis is always moving forwards due to sidereal tracking. Backlash on the RA axis can become a problem, though, if the mount is poorly balanced, e.g. heavier on the west side.
Backlash can be compensated for – to a degree – in software. If the drive takes 1.2 seconds to reverse direction, the software could automatically add 1 second of drive time whenever the direction changes. A suitable backlash time can be entered into the Guide tab Settings dialog box for each axis. This number is used for both calibration and for guiding. You should always set this to less than the estimated backlash time; if it is too high, it will result in chronic overcorrection. If in doubt, set it to zero.
Note that some mount drive systems include built-in backlash compensation. It is strongly recommended that this be turned off during autoguiding. Usually backlash compensation works by adding some extra time to the guide correction. If the mount is still moving when the next autoguider image is taken, it will induce instability into the drive servo loop. This will cause the telescope to oscillate back and forth around the guide point.
The term ”stiction” comes from ”static friction.” Static friction occurs when two objects in contact are at rest or moving very slowly relative to each other. Dynamic friction occurs when the two objects are in motion. Static friction is always larger than dynamic friction, so when an object starts moving, the amount of friction present can change very rapidly. An example of this happens when you brake your car. Just before the car stops, you have to let up on the brake pedal; otherwise the car jolts to a stop (if you’ve been driving for years, you may not even be aware you are letting up on the brake).
Stiction occurs between the teeth in a gear system. Of particular interest here is the effect on worm drives, which are typically used in astronomical telescopes. When the drive is stationary, the gears ”stick together.” When the worm starts rotating, it pulls on the teeth of the worm wheel. In this situation, the wheel can bend, or the mounting or bearings can flex, at right angles to the normal rotation. The resulting motion is usually backwards – you try to move the mount North, and it goes South! This only happens for a brief period of time after reversing directions; eventually the worm turns enough that the gear is forced to move in the correct direction.
Stiction problems can be complex mechanically; it can often be hard to determine what the root cause is. Many telescope mounts – even high-end models – suffer from this problem, and it may vary between different samples of the same model. The problem has a terrible effect on guiding because it forces overcorrection.
If you experience bad guiding in declination and cannot resolve it through adjusting the calibration or aggressiveness, you may have a stiction problem. One solution that works is to note the average drift in declination, and disable corrections in that direction. The guider will only be able to push against the average drift, not with it. This works surprisingly well. Some users deliberately adjust their polar alignment slightly off to ensure that the drift is in a consistent direction. You should avoid large offsets that may cause field rotation.
MaxIm DL has a built-in Anti-Stiction feature, which automatically disables the autoguider output that pushes in the direction of drift. For more information please refer to Guider Settings in the Command Reference.