Mike, I tried a quick design in Optics Lab and had a -100mm secondary 90mm from the focal point of the primary. The performance was diffraction limited (all rays were within the Airy disk) over a 2"+ diameter. Strell degraded from .90 to .89 from the 0deg. to .1deg. incoming angle. Bob May http://nav.to/bobmay bobmay@nethere.com NEW! http://bobmay.astronomy.net