I have read a few articles & forum posts regarding the autocollimator, I'm convinced that the AC is the best tool to use at the moment. I understand the basic concept but for the fine detail that Jason has explained is not so easy.
I assume (ass again) all will fall into place once I get my hands on one of these tools.
Richard
ps. Thanks Neil I'm glad I'm not alone in my confusion.
Some web pages are just not as easy as others.