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After Gary's comprehensive and erudite response it is hard to add much more of consequence
though I would echo: many mount and optical systems rarely sit with an optical axis that is precisely parallel to the polar axis (i.e. RA shaft )
An easy test is to place a wide field eyepiece and 90 degree star diagonal
into the optics, rotate the optics to as close to DEC -90 (i.e. at the pole)
then, while looking through the eyepiece, slowly rotate the mount in RA and
observe the path of the field stars.
If things are bang-on, then you should see the stars follow nice concentric circles that care centered on the middle of the eyepiece field of view.
(picture 1)
What you are more likely to see however, is the stars moving through arcs that are centered somewhere outside the field of view. (picture 2)
To remedy this, first make a fine adjustment to the Dec axis and ensure you are really at a geometric Dec -90.
A small tweak may be all that is required to get to nice concentric motion. A little trial and error may be required to get as close as possible to this.
Once you are there, then a small shim may need to be added (to either the objective or eyepiece end of the telescope)
to converge on Picture 1.
After shim adjustment, your alignment Nirvana however may be further thwarted by the RA and Dec shafts on your mount being less than orthogonal
...resulting in something like picture 1, but simply not centered in the field of view. However the good news is small errors of this nature are easily corrected by packages such as T-Point and Max Point.
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