Add two abilities to the FOV calculator:

1. Have a pre-defined dropdown list of popular Eyepieces and their
apparent FOVs, in addition to manual entry.

2. Provide an option to compute the telescope focal length from
f-number and aperture diameter (in both mm and inch). Amateur
astronomers continue to cite their telescope specifications using
the diameter in inches and the f-number, eg: 8" f/8.

FEATURE