Hi Sam,

The pointing terms are the best-fit values for various mechanical factors. I said "best-fit" because they may or may not accurately represent the mechanical factor. So, trying to put a detailed reason for each term may lead someone to believe that something needs fixing when there isn't. The model is there to compensate for those factors. A more important factor is repeatability. For example, if there is a lot of randomness in pointing because something moving around (e.g. a primary mirror in a large scope) then the model will not be as effective.

The only exception may be the polar alignment terms, which depending on their magnitude, may introduce field rotation.

-Ray