There's a difference between "guiding" for astrophotography and "tracking" an object.  Most motor driven scopes do a great job of tracking the object, especially if the scope has accurate polar alignment.   But what happens is that the worm gears driving the scope are not perfectly made.   This is just the nature of such systems and is true of any astrophotography setup.   The imperfections in the gears cause the tracking to speed up and slow down slightly in right ascension, showing as a slight wobbling movement in the position of stars in the field of view.   This movement is not often detected at low powers, but is very obvious at powers necessary for most prime focus (through the scope) photography.   The results on photographs come out as FAT stars and a blurred object.   Plus, polar alignment is never perfect, so that compounds the problem as stars will also appear slightly trailed.

Therefore, you need a set of tools to use for keeping the stars precisely centered in the picture, counteracting the movement caused by the wormgears and the flaws in alignment.

One tool is the scope itself.  Many motor driven scopes and mounts have a "training" mode where the computer memorizes how the gears move.  This is known as PEC training, or Periodic Error Correction.   Since the errors are predictable at certain positions of the gears (it's a cyclical process), most PEC procedures allow you to train the scope by guiding on a star in RA over the length of the worm cycle.  The scope then retains this memory and applies this pattern in the future. Incidently, many scopes will also has a DEC memory training as well, but this is less critical since the computer tracks only in RA when in polar mode.  

Once the PEC training is applied, you've pretty much compensated for the gear problem (not completely, but enough for many of the better instruments to be even better, and this training is even more critical on bigger instruments since this periodic error is much evident at longer focal lengths.  

But when setting up a scope for astrophotography, you've got to get a good initial alignment before performing the procedure, otherwise you can't really isolate movements that are caused by periodic error in the system or if it's simply drift caused by poor alignment.   That requires from 15 minutes to hours of time using a drift alignment method.

Therefore, if you have an observatory with a perfectly aligned and trained scope, then you theoretically do not need to guide your photographs.  This is especially true with nice mounts and lower focal lengths.   But many people are taking 5 minute, unguided images with longer focal length scopes with AP 1200 mounts and Paramounts...and the results are quite beautiful.

But when going to a dark site, we don't have that much time to get things setup so well.   That's the beauty of CCD guiding.   You then only need to get a decent polar alignment and the CCD will keep your stars centered by sending the necessary corrections to your scope.   So you just set the guider, open the shutter, and walk away until the exposure is long enough.   Even manually guiding with a reticle will be of benefit; then better alignments become more critical since the idea is to do as few corrections as possible.

Perhaps now you understand why I am so big on piggyback photography as a starting point.   Because your camera shoots with less "power," the film or CCD doesn't pick up the alignment and mechanical flaws as easily.   Therefore, simply doing a decent polar alignment is all you need to take some beautiful, long exposure shots of broad portions of the sky.   But closeup shots of most galaxies require shooting through the scope, where exposures are longer due to the higher focal ratios and magnifications that cause the errors to be much more apparent.

And I haven't even mentioned problems such a wind, primary mirror movements, field rotation, and flexure (especially when using a guidescope) which can also wreak havoc with your photography; however, these problems are seldom correctable by an autoguider anyway.   They must be solved prior to taking pictures.

To make a long discussion short, there are many tools available to you.   But experience is the best tool.   That's what separates the contenders from the pretenders.   It's also why you can spend a lifetime and never get it perfect.   There's simply a lot to experience to be gained. 

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