Q1: Canon 10x42L IS WP Image Stabilizers vs Canon 18X50 IS Image Stabilized Binocular Which would be better for amateur astronomy usage? The first seems to offer a brighter image, while the second offers better magnification.

A: One way to compare likely low-light performance is to calculate the "twilight factor" for each instrument. "Twilight factor" is simply the square root of magnification times aperture. A value of about 17 is generally considered to be the minimum for decent low-light performance. Canon's 10x42 IS binoculars have a "twilight factor" of about 20.5, indicating pretty good low-light performance. However, the "twilight factor" of the 18x50 binoculars comes out to 30, indicating they'll give not only greater magnifiction that the 10x42s, but also better low-light performance. Of course, the "twilight factor" doesn't consider such important factors as the quality of optical glass and optical coatings or overall construction, and each of these characteristics can have a significant impact on performance. In this case, though, optical charactistics and construction are at least similar, so you'll probably be more satisfied with the 18x50s for viewing the stars.