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Hi Daniel
If your mount can Track and Auto guide, here is what it means:
Tracking: the mount is (generally) being driven by motors at the sidereal rate; that is, the rate at which the Earth rotates, so it will make 1 complete rotation (360 degrees) in 23h 56m.
Auto guide: Here, a dedicated CCD camera is monitoring, in real time, the position of a (guide) star and if the guide star drifts off its calibrated position, the auto guider unit sends commands to the telescope drive system to “nudge” the guide star back to its calibrated position. This will be done tirelessly and accurately every few seconds.
For long focal lengths, say over 800mm, most mounts will not be able to follow a star and keep it centred on a pixel for longer than, say, 60 seconds by tracking alone. There are mechanical tolerances in the gear train, effects due to refraction of the star by the Earth’s atmosphere, etc, etc, that will contribute to the star drifting when the mount is only tracking. This will give you trailed stars in your image.
However, when auto guiding, these small tracking errors can be generally “catered for” by the guide system making small corrections every few seconds. Generally, the more expensive (high end) mounts have smoother gears, tighter tolerances, better materials and design and superior manufacturing so that they can track for longer periods and still produce round stars (depending on focal length of scope) before you have to kick in with auto guiding.
Cheers
Dennis
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