To clarify my earlier question, by example, take a small 60mm refractor that has a theorectical diffraction disk of 1.9 arcsec radius (by 116/D_mm). In poor Antoniadi seeing V, I typically seeing a large disk smearing out to 3 or 4 arcsecs. For the Grim 812 mm apeture, the theoretical disk is 0.14 arcsecs. With a single star under high magnification, what do you see? 1) a small 0.14-1.0 disk jumping around within a 3 or 4 arcsecs radius. 2) a small 0.14-1.0 disk that smears to 0.75 or 1.5 arcsecs? Thanks - Kurt ____________________________________________________________________________________ Be a better friend, newshound, and know-it-all with Yahoo! Mobile. Try it now. http://mobile.yahoo.com/;_ylt=Ahu06i62sR8HDtDypao8Wcj9tAcJ