Sir Stephen, After one mirror you are and expert. I've never even seriously considered an achromat. After three mirrors and an achromat, you are a lesser deity! or perhaps you perfer knight what ever you want. By the way, how did you deal with the wedge and de-centering issue, cause I'm ah sort of maybe beginning to ah consider an achromat, but not lense, a corrector so its ok. Anthony Stillman