Phil,

John is right! let me apologise for giving you a bum steer.

I forgot to mention I use a 8" f/6 newt and the 12mm in that
gives me similar powers the 18mm would in yours. All the same
you cant go wrong with UO HD's. I own four of them 18, 12, 9 & 7 and dont regret buying any of them. Mind you, it's hard getting the new Pentax out of the scope

regards