Correct. The terms tele-extender and telecompressor are carryovers from the camera industry. They are ussed to change the EFL of any given lens, thus changing the image scale. They serve the same purpose on telescopes. Essentially they serve the same purpose as Barlows and eyepieces, but are mounted in housings more friendly to imaging setups. They modify the converging angle of the light cone, which modifies the EFL of the system. Extension tubes by themselves in direct-objective imaging serve no purpose if focus is achieved without them. Image scale (magnification) cannot be changed using direct-objective imaging without any other additional lenses ahead of the imaging chip or film. Joe made it clear that he had forgotten about his own use of "reducer-corrector" lenses when using extension tubes with his own SCT imaging system. On Sun, Aug 11, 2013 at 8:51 AM, Erik Hansen <erikhansen@thebluezone.net>wrote:
are not the function of telecompressors and teleextenders to change the scale of photographs? I am no photographer but I thought that was what Joe was originally referring to.