It is not just the guidecam it is the whole assembly. Even a cheap achromat will do. What you need is a small F ratio and a wide field. I routinely guide with stars less than mag 8 using a toucam. See here
http://www.iceinspace.com.au/forum/s...ead.php?t=3798
and here
http://www.cloudynights.com/ubbthrea.../o/all/fpart/1
and here for clarification
http://www.cloudynights.com/ubbthrea.../o/all/fpart/1
Bert