I realize that I was not specifically asked to comment, but perhaps this will prove useful. Any obstruction in the light path causes diffraction, and non-circularity of the aperture causes diffraction. Apodizing masks sometimes work because diffraction is channeled into areas that do not interfere with the desired observation, but total diffraction is indeed increased. Mirror clips cause diffraction (keep their intrusion on the mirror face at a minimum if you use them) and even a rough edge bevel will increase diffraction. It makes no difference if the obstruction is high in the tube, or at the mirror's surface. Because diffraction can be "channeled", sometimes telescopes with large central obstructions or widened spider vanes prove useful, especially if splitting close double stars. By moving energy from the central spot into the diffraction rings, the spot can be decreased in size and allow close companions to be seen, where in an unobstructed system, while the rings are weaker, the central spot is larger and overlaps the companion star. C. __________________________________________________ Do you Yahoo!? Yahoo! Tax Center - forms, calculators, tips, more http://taxes.yahoo.com/