On Thu, Jul 30, 2009 at 3:46 AM, John Francis<jo...@panix.com> wrote:
> That makes the f-stop somewhere close to 3.3 by my calculation. We have a winner. That's for "prime focus", where you just have the primary mirror in the light path (plus some refractive corrective elements to improve aberrations). If you deploy the secondary mirror, you get an f/16 Cassegrain focus. Additional mirrors can be deployed to provide an f/30 Coudé focus, but that's not used much anymore. (The advantage of Coudé is that the light is brought to a focus in a nearby room, where you can build large, stationary instruments that don't have to move with the telescope. With modern technology, this is not generally necessary.) Modern observatory telescopes tend to have faster primary mirrors (e.g. Keck is f/1.75), but tend not to offer a prime focus instrument position. So, for example, at Keck the f/15 Cassegrain focus is the fastest focal ratio available. -- PDML Pentax-Discuss Mail List PDML@pdml.net http://pdml.net/mailman/listinfo/pdml_pdml.net to UNSUBSCRIBE from the PDML, please visit the link directly above and follow the directions.
