I understand the simple arithmetic of calculating the effective aperture when there is an obstruction, but is there an objective measurement of the degradation due to the diffraction of the obstruction itself? Or is the effect simply one of reducing contrast? And finally, is this why SCT's are generally large focal ratio instruments, i.e. to increase contrast and compensate for the large obstruction? Kim -----Original Message----- From: utah-astronomy-bounces@mailman.xmission.com [mailto:utah-astronomy-bounces@mailman.xmission.com] On Behalf Of Brent Watson Sent: Tuesday, March 06, 2012 3:02 PM To: Utah Astronomy Subject: Re: [Utah-astronomy] solar filter The secondary does decrease effective aperture and therefore resolution. As mentioned before the effect can be calculated by subtracting the secondary diameter from the primary diameter. That yields the effective aperture.