I want to contribute here, as I've been a serious *handheld* binoculars user for five years now and have also done considerable online "research" about them. From what I've learned, I agree with Rob's conclusion -- for those with 7mm dark-adapted pupils -- that a 7mm exit pupil (7x50, 8x56, 9x63, 10x70) is best. (For newbies, "exit pupil" is the binoculars' objective lens size in millimeters divided by the power or magnification. For example, for 7x35 and 10x50 binos, it's 5mm; for 7x50s, it's 7.14mm.) However... HOWEVER, I wonder how many of us get our pupils dilated that much, and how frequently. In moonlight or light-polluted skies, they don't dilate that much. Also, as Rob mentioned, maximum pupil dilation decreases with age. So in many cases (especially old folks observing from mid-city or just about anyone observing under a full Moon!) it could be that a 6mm or even 5mm exit pupil is very good (e.g., 7x42 or even 7x35, 8x50 or even 8x40, or maybe even 10x50, which I use). (Footnote: There are a couple of reasons, having to do with motion of target and/or your own shaking hands, to have an exit pupil somewhat larger than your eye's pupil.) There are five very informative articles about choosing binoculars for astronomy on this page of the Sky & Telescope Web site: http://www.skypub.com/tips/binoculars/binoculars.html In two of those articles it's argued that the best single measure of binoculars is a "visibility index" (magnification times objective lens size), but that can NOT be the only factor considered. Rob mentioned the common-sense factor of weight. There are excellent 10x70 binoculars on the market, but unless you are two meters tall and weigh 120 kg, they'll tire you out rapidly. You would have to use them with a mount. And if you're getting "over the hill" (i.e., older, such as I!), you probably don't need that large an exit pupil anyway! One interesting comparative table online by Brooke Clarke develops a "holding index" for a lot of models -- it's an attempt to quantify the shaking factors due to magnification and weight, alongside of the "visibility index": http://www.pacificsites.com/~brooke/Bino.shtml#Hold He has a lot of satellite-observing links also: http://www.pacificsites.com/~brooke/Astro.shtml#Sat Another good article about binoculars, especially under "Exit Pupil", is at this location: http://www.teleport.com/~chalu/binocul.htm One other consideration for watching LEO satellites (or anything in motion) is field of view, and within other constraints, wider field of view is better! Also, if you need to wear eyeglasses while using binocs, you need longer "eye relief". Ed Cannon - ecannon@mail.utexas.edu - Austin, Texas, USA ----------------------------------------------------------------- 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 - 21:06:49 PDT