Autoguiding is one of the most challenging tasks to master in astrophotography; nowadays, with digital cameras (CCD or CMOS), manual guiding makes no sense, as it is almost impossible to keep the required accuracy, even more with the ever-increasing exposure times everyone is using.
|We believe the easiest way to face this problem is asking ourselves: what level of accuracy may I or should I expect from my mount? what will be enough for the kind of images we're making? |
If we caracterize the guiding error with a measure such as the RMS, as reported by several programs, we could see a good mount will guide with a RMS (for long periods of time) less than one arc-second, less than 0.5 arc-seconds for a very good mount, etc. Obviously we cannot expect to obtain nice images, with good, round stars, at 3 meters focal lenght and a CCD with 9 micron pixels; but this is inherent to the mount , we are not going to improve it (much to the contrary) loading it with a long, 1.5m guide tube.
Worth mentioning here are differential flexure, wandering mirrors, weak focusers and other problems: for any guidescope to work, it is of paramount importance that there's no relative motion whatsoever between the guiding optical system and the guided one. If you cannot tame your optical setup, then the only option is an off-axis guider, but it's a hard way...
This problem becomes more noticeable the longer the imaging telescope's focal length, but in no case a longer guide scope is the solution.