Henry Greene: The goal of prescribing a telescopic device is to give you ample magnification to see what you need to see and maximize the field of view so it’s easy to find what you’re looking for. There are two categories of optical telescopes: Galilean telescopes and Keplerian telescopes. Keplerian telescopes offer a much wider field of view and are valuable when higher powers are required. For instance a three power Galilean telescope offers about a 5 degree field of view, while the three power Keplerian telescope offers about a 12 degree field of view. A convenient way to understand field of view is to put your arm out at arm’s distance with your fingers pointing up to the ceiling. Each finger is roughly a degree, so a three power Galilean telescope has a field of view of approximately one hand, while the three power Keplerian telescope has a field of view of approximately two and a half hands.