[m102404]

No real idea if this will help

http://russianoptics.net/Concepts.html

http://www.warriortalk.com/archive/i...p/t-81889.html

http://www.warriortalk.com/showthread.php?67498 (2nd last post in thread maybe?)

There's some other links in those threads that might lead you to a solution similar to what you need. If it's completely wrong...apologies if I've confused things

Tys