Diameter of the primary objective, regardless of where it's a lens or a mirror. Usually measure in millimeters.
Focal Length of the Objective (FLO)
The distance from the primary objective to the point where the prime focus image is formed. Usually measured in millimeters for small optics.
Focal Length of the Eyepiece (FLE)
All eyepieces have a set focal length. This can be found stamped on the outside of the eyepiece and usually near the top. Typically measured in millimeters.
Focal Ratio (f/#)
The speed of the optics. Generally categorized as Fast, Mid, and Slow optics. Fast optics have focal ratios <=f/6; Mid-range optics have focal ratios >=f/6 <=f/10' and Slow optics have focal ratios >=f/10. The faster the optics; the wider the field; the smaller the object
f/# = FLO/Aperture
The highest magnification that can be achieved by an optical system before it fails. The term "failure" in this instance can be the inability to focus the subject, or even see the subject. It is possible to magnify so much that no light can pass through the optical path. A rule of thumb is that no telescope should be magnified beyond 50x for each inch of aperture.
The faintest magnitude of any object that can be feasibly detected with the optical system.
Measured in millimeters, the field stop is a physical ring inside an eyepiece, which holds the objectives together and limits the field-of-view of the eyepiece itself. This is set by the manufacturer and cannot be changed
Apparent Field of View (AFOV)
The field size, in degrees, that can be seen through an eyepiece. This value is determined by the eyepiece design and its manufacturer. It cannot be changed by the observer
True Field of View (TFOV)
The actual field size, in degrees, determined by the telescope and eyepiece combination. There are two methods to calculate the true field of view
The exiting image that is formed by an optical system and often referred as a "virtual aperture." The size and shape of the exit pupil is very important to good optical performance since the eye can only see what passes through the exit pupil. Reflecting, including catadioptric telescopes should have an exit pupil between 7mm - .5mm. Refracting telescopes have no upper bound
Exit Pupil = FLE/f/#