Hi, I am in the process of building a 7" Achromat (Bk7, F2). Would a focal ratio of F10, give too much cromatic aberation? I am trying to decided between an F10 or F12. I would like to try the F10, but if performance is to be too poor, I will make it F12. Can anyone give me some advice. Regards, Alfredo _______________________________________________ ATM mailing list http://www.atmlist.net/