Correct Joe. You are using your "internal" Barlow (secondary mirror) to modify the telescope's EFL. Negative projection. The refractor would use a Barlow to achieve the same thing as you, optically. Know that SCT's, by varying the distance between primary and secondary, almost never have optimal correction. The more you depart from the optimal mirror spacing, the worse the imagery gets- and that's where the Barlow analogy ends. That's why only the mass-produced SCT's use moving-primary focusing. The "true" astrographs such as RC Cassegrains almost all use a standard focuser at the rear of the telescope and keep the mirror spacing constant. So just adding extension tubes won't work for them anymore than it would on a refractor or Newtonian, or a standard Cassegrain with fixed mirror spacing. SCT's were designed as "everyman's" telescope. They are very versatile, but the optical design is a compromise made in the name of compactness- even the corrector plate is in a compromised position. It performs better near the ROC, not near the primary's focus, but near focus makes a convenient place to mount the secondary. Brent knows this and that's another reason he prefers long f-ratio Newtonians. I do, too, along with long refractors. While the imagery is superior, though, they are cumbersome for imaging. The SCT wins hands-down for convenience. I'm not a SCT hater, I own one and like it for what it can do. But it's not the sharpest knife in the drawer. On Sun, Aug 11, 2013 at 2:25 PM, Joe Bauman <josephmbauman@yahoo.com> wrote:
That's why I said it works with an SCT but might not with a refractor -- changing the focus is done internally, so a change in f/ratio or magnification does not require an eyepiece. -- Joe