Another interesting article I found a couple of months ago about collimating an SCT is this one:
http://www.astromart.com/articles/ar...article_id=548
At the moment I have been too busy to give it a try, but his article is different to all the others I have found. He actually tells you a way to determine not only which screw to turn, but also which WAY to turn it.
I would be interested to hear if anyone has tried his technique. Most of the articles I have found have shown a way to identify the screw but the direction is just "turn one way, if it makes it worse, turn the other way".