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. On Tue, Mar 6, 2012 at 3:10 PM, Kim <kimharch@cut.net> wrote:
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.
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