Astronomy
My 10 inch Mirror with a 1/48 P-V Wavefront Error @ 550 nm
The Eyepiece is the optical interface between the image produced by the mirror and the observer's eye.  This action occurs at the telescope's Exit Pupil.  The mirror's image formed here will have a certain size, which is designated as "d$$ _p" which stands for "diameter of exit pupil." This value is always measured in millimeters.
Different eyepieces will magnify this image by different amounts, so this is where the focal length of the eyepiece comes into play. "f$$_e" is the focal length of the eyepiece, and is always measured in millimeters.
Different combinations of d$$ _p and f$$_e will have different effects on the FR of a telescope's objective mirror according to the formula: d$$ _p = f$$_e / FR.   Thus, FR = f$$_e mm / d$$ _p mm