For satellite observation (and general astronomy observation) I've been using the "Relative Brightness Index" as the measure for rating a binocular's ability to gather light. This index is defined as the exit pupil diameter divided by the magnification and multiplied by the objective diameter. For 7x50s the index is 51 For 10x50s the index is 25 For 12x50s the index is 17 For 7x35s the index is 25 For 20x60s the index is 9 Lately I've read about the "Twilight Factor" which is suppose to be a measure of the binocular's ability to contrast objects in low light conditions. This is defined as: the square root of (magnification * objective diameter). For 7x50s the factor is 18.7 For 10x50s the factor is 22.4 For 12x50s the factor is 24.5 For 7x35s the factor is 15.7 For 20x60s the factor is 34.6 Which index is a better indicator for a pair of binocular's ability to detect a dim LEO object? With the Relative Brightness Index it would appear to be 7x50s. With the Twilight Factor it would appear to be 20x60s. Cheers, Don Gardner 39.1799 N, 76.8406 W, 100m ASL http://hometown.aol.com/mir16609/ ----------------------------------------------------------------- Unsubscribe from SeeSat-L by sending a message with 'unsubscribe' in the SUBJECT to SeeSat-L-request@lists.satellite.eu.org http://www2.satellite.eu.org/seesat/seesatindex.html
This archive was generated by hypermail 2b29 : Wed Apr 11 2001 - 14:21:58 PDT