Prompted by a question on another forum, I remembered a really good pdf on field rotation. I can no longer find the article and don't remember the author's name. All I can remember is that "Field Rotation" was in the title as was Boston, the home of the author.
The article went into the math behind it and discussed how long exposures could be in different parts of the sky before star trailing became a problem.
Please respond if anyone remembers the article, knows (or can find) the URL, or has a copy.
I sure would like to read it again!