information:classification
Exit pupil

The limit to the increase in surface brightness as one reduces magnification is the exit pupil: a cylinder of light that projects out the eyepiece to the observer. An exit pupil must match or be smaller in diameter than one's pupil to receive the full amount of projected light; a larger exit pupil results in the wasted light. The exit pupil e {\displaystyle e} can be derived with from division of the telescope aperture D {\displaystyle D} and the minimum magnification m {\displaystyle m}, derived by: e = D m = 130 18.6 ≈ 7 {\displaystyle e={\frac {D}{m}}={\frac {130}{18.6}}\approx 7}. The pupil and exit pupil are almost identical in diameter, giving no wasted observable light with the optical system. A 7 mm pupil falls slightly short of 100% brightness, where the surface brightness B {\displaystyle B} can be measured from the product of the constant 2, by the square of the pupil p {\displaystyle p} resulting in: B = 2 ∗ p 2 = 2 ∗ 7 2 = 98 {\displaystyle B=2*p^{2}=2*7^{2}=98}. The limitation here is the pupil diameter; it is an unfortunate result and degrades with age. Some observable light loss is expected and decreasing the magnification cannot increase surface brightness once the system has reached its minimum usable magnification, hence why the term is referred to as usable.