That's a good point. I should add an input for focal reducers and barlows. On 5/7/2010 10:08 PM, Chuck Hards wrote:
Tele-extenders and focal reducers have a multiplying factor printed on them (most do) For example, .85x. Assuming no eyepiece in the system, simply multiply the objective's focal length by that factor. Example, 8" f/10 = 80" = 2032mm focal length. 2032 x .85 = 1727mm = ~ f/7. You've changed your original system from f/10 to f/7.
Filter wheels and other accessories like extension tubes with essentially no magnifying power add or subtract nothing to focal length. They just take up "IN" travel.
On 5/7/10, Patrick Wiggins<paw@wirelessbeehive.com> wrote:
Looks nice. But I've always wondered how one calculates the exact focal length of a given setup. I suppose one could just use the manufacture's stated focal length of the OTA. However, once one starts adding accessories to the back like focal reducers, tele-extenders, extension tubes and filter wheels the advertised focal length becomes useless.
_______________________________________________ Utah-Astronomy mailing list Utah-Astronomy@mailman.xmission.com http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy Visit the Photo Gallery: http://www.slas.us/gallery2/main.php Visit the Wiki: http://www.utahastronomy.com