Quote:The front aperture is more a limiter on the fastest f/ratio which can be accommodated. As long as it's large enough to accept the on-axis light cone, all is well. For longer f/ratio telescopes, this aperture can be surprisingly small.For example, let's for the moment assume the equivalent air block of the optical path length through the BV's glass, and the additional rearward location of the focus behind the rear prism aperture together to be, oh, 80mm. For an f/10 scope, its on-axis light cone would be just fully accommodated by a front BV aperture of only 8mm. A little extra aperture is useful for fully accepting the off-axis light cones, but it doesn't require very much extra to do this.If you plan to mate the BV to something like an f/5-ish system, then the front aperture, in conjunction with the optical path length/focus location with respect to the rear prism aperture, becomes critical.
Fred StarStructure LE Series 18" f/4.3 (Zambuto & Nexus/ServoCat/Skytools3 Pro) Celestron 8" SCT w/Starlight Feathertouch Microfocuser; Celestron 102mm f/9.8 refractor Stellarvue SV80ST f6 Triplet APO Tele Vue & EarthWin binoviewers Enough Eyepieces and Accessories to put my Scopes to Good Use Free-Turn Q HalfHitch, CG5-ASGT(2), Astro-Tech Voyager mounts