I believe that Joe is correct and that Chuck and Brent are wrong. Remember that Joe is using an SCT which is NOT a fixed focal length telescope like a refractor or a Newtonian. Joe can put extension tubes on his optics and then adjust the focal plane to meet his camera where he wants to. He does this by adjusting the distance between the primary and the secondary, that is how focus is achieved on an SCT, and in the process of moving the focal plane, he also changes the final focal length as well as the image scale and the focal ratio. Remember that the magnification achieved with a barlow depends strongly on the spacing between the primary optics and the barlow lense. That is why the SCT works like it does. The secondary mirror is actually a barlow and moving it during focusing in relation to the primary changes the focal length and image scale of the entire assembly. DT ________________________________ From: Chuck Hards <chuck.hards@gmail.com> To: Utah Astronomy <utah-astronomy@mailman.xmission.com> Sent: Sunday, August 11, 2013 9:53 AM Subject: Re: [Utah-astronomy] Image scale basics (WAS: Ring Nebula (M57) - Be prepared to squint) Thank YOU for weighing in, Brent! Your answer was succinct. People sometimes don't realize that focusing a system is just a matter of making focal planes coincide. Visually, the eyepiece focal plane must occupy the same location as the objective focal plane. In imaging, the chip or film surface must coincide with the final focal plane of the entire system, be it direct-objective or with additional lenses. A little bit of math goes a long way towards saving time and frustration under the stars. I'm sure there must be websites and apps available these days to do it, without having to take pencil to paper. But where's the fun in that? There are also off-the-shelf solutions for the mass-produced telescopes and cameras, if one does a bit of sleuthing or is lucky enough to find a salesperson who understands what they want to do and can recommend the proper hardware. On Sun, Aug 11, 2013 at 9:41 AM, Brent Watson <brentjwatson@yahoo.com>wrote:
Chuck beat me to the answer. His explanation was also better. Thanks, Chuck!
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