I haven't read Suiter. I intuitively (I think) understand the effect(s) - maybe I'll hunt online for more information but if anyone thinks of a helpful article I'm game to read. Thanks all. Kim -----Original Message----- From: utah-astronomy-bounces@mailman.xmission.com [mailto:utah-astronomy-bounces@mailman.xmission.com] On Behalf Of Chuck Hards Sent: Tuesday, March 06, 2012 3:23 PM To: Utah Astronomy Subject: Re: [Utah-astronomy] solar filter Remember that Meade used to sell dedicated f/6.3 SCT's (I don't think they do anymore). Those secondaries were significantly larger than the secondaries on dedicated f/10 SCT's. A wider FOV, but at the expense of lower throughput and increased diffraction. The market has spoken, and focal reducers are what most imagers now use to accomplish the same thing. They have the flexibility of going with either f/6.3 or f/10, with the same instrument. (I even own one for my own f/10 Celestron) The larger focal ratio is just the way Cassegrains work. The secondary is in effect a reflective Barlow. Smaller secondaries mean higher magnification values required to put the final focal plane behind the primary. Kim, have you read Dick Suiter's book "Star Testing Astronomical Telescopes"? He goes into great detail on the effects of diffraction due to secondaries.