Look at the Stellafane and Berry testers and you will note that they are rotating about an axis. You do want the angle to be near vertical between the KE and the pivot point but it's not all that critical as the point at which the returned image hits the KE just moves a bit as you go back and forth. Bob May http://nav.to/bobmay bobmay@nethere.com NEW! http://bobmay.astronomy.net