Max,
If you want nice stars to the edge of field do yourself a big favour and get a GSO coma corrector. Set it up with info from this CN thread.
http://www.cloudynights.com/topic/46...oma-corrector/
A coma corrector is a game changer. I have all the ES 68's and most of the 82's which I use in my 6" f12 Refractor…perfecto! and 10" f4.8 Newt…not so perfecto except when my coma corrector is in the focuser.
My CC is an ES HR with the helical top making it easy to adjust.
Like the GSO on CN I found 19mm out the best setting for the 68's and 15mm out the best for the 82's.
Sadly ES have put their prices up of late, but they are still half the price of TV.
Back to your question…ES 18mm 82º + GSO coma corrector
Matt