The night wasn’t great, but it was good (especially when I quit, around 1am–by then, the Milky Way was visible in Cygnus in binoculars, which was near the zenith at the time). I sketched the Sadr (Gamma Cygni) area, and the upper part of Lyra. The former isn’t perfect–I realized after the fact that a few stars were misplaced (I am getting better at this, but still mis-estimate where to plot the stars–part of the problem is it’s not completely obvious which way, precisely, is “up” in binoculars, surprisingly).
While I was outside, I decided to measure my 5.8 degree FOV binoculars. I went to bright Deneb, and the “northwest” optical double Omicron Cygni. I could fit them in the field of view, but if I move the binoculars just slightly, one or the other would exit the field of view. This showed the FOV was just over the angular distance (5.1 degrees) between Sadr and Omicron Cygni. 5.8 is not “just” over, but 5.3 degrees appears close to the actual field of view.
Actually, the advertised field of view is 305 feet at 1000 yards. Let’s convert that to an angular field of view.
First, 1000 yards is 3000 feet. Then, imagine looking at a target 305 feet high 3000 feet away, that just fits the field of view. 3000 feet is the distance to the center of the target, so 152.5 feet of the target is above the line of sight, and 152.5 feet is below. So, we have a long right triangle with legs 3000 feet and 170.5 feet. The acute angle (at the binoculars) is then arctan(152.5/3000). But the target is both above and below the center line, so we double it. Finally, my software works in radians, so I convert to degrees by multiplying by 180/pi
FOV = 2 * arctan(152.5/3000) * 180 / 3.1415926535
= about 5.82 degrees.
Yep–actual field of view is a little off!
