Editing Optical telescope (section)

===Brightness factor===
The [[surface brightness]] at such a magnification significantly reduces, resulting in a far dimmer appearance. A dimmer appearance results in less visual detail of the object. Details such as matter, rings, spiral arms, and gases may be completely hidden from the observer, giving a far less ''complete'' view of the object or range. Physics dictates that at the theoretical minimum magnification of the telescope, the surface brightness is at 100%. Practically, however, various factors prevent 100% brightness; these include telescope limitations (focal length, [[eyepiece]] focal length, etc.) and the age of the observer.

Age plays a role in brightness, as a contributing factor is the observer's [[pupil]]. With age the pupil naturally shrinks in diameter; generally accepted a young adult may have a 7&nbsp;mm diameter pupil, an older adult as little as 5&nbsp;mm, and a younger person larger at 9&nbsp;mm. The [[magnification|minimum magnification]] <math>m</math> can be expressed as the division of the aperture <math>D</math> and [[pupil]] <math>p</math> diameter given by: <math>m = \frac {D}{d} = \frac {130}{7} \approx 18.6</math>. A problematic instance may be apparent, achieving a theoretical surface brightness of 100%, as the required effective focal length of the optical system may require an [[eyepiece]] with too large a diameter.

Some telescopes cannot achieve the theoretical surface brightness of 100%, while some telescopes can achieve it using a very small-diameter eyepiece. To find what eyepiece is required to get [[magnification|minimum magnification]] one can rearrange the magnification formula, where it is now the division of the telescope's focal length over the minimum magnification: <math> \frac {F}{m} = \frac {650}{18.6} \approx 35</math>. An eyepiece of 35&nbsp;mm is a non-standard size and would not be purchasable; in this scenario
to achieve 100% one would require a standard manufactured eyepiece size of 40&nbsp;mm. As the eyepiece has a larger focal length than the minimum magnification, an abundance of wasted light is not received through the eyes.