. The resulting true field of view is 0.64🚨allowing an object such as the Orion nebula

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

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?mm diameter pupil

{\displaystyle m={\frac {D}{d}}={\frac {130}{7}}\approx 18.6}🚨

. A problematic instance may be apparent🚨achieving a theoretical surface brightness of 100%

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 minimum magnification one can rearrange the magnification formula

{\displaystyle {\frac {F}{m}}={\frac {650}{18.6}}\approx 35}🚨

. An eyepiece of 35?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?mm. As the eyepiece has a larger focal length than the minimum magnification🚨an abundance of wasted light is not received through the eyes.

The increase in surface brightness as one reduces magnification is limited; that limitation is what is described as 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🚨

{\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

{\displaystyle B=2*p^{2}=2*7^{2}=98}🚨

. The limitation here is the pupil diameter; it's 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.

When using a CCD to record observations🚨the CCD is placed in the focal plane. Image scale (sometimes called plate scale) describes how the angular size of the object being observed is related to the physical size of the projected image in the focal plane

{\displaystyle i={\frac {\alpha }{\alpha f}}={\frac {1}{f}}.}🚨

No telescope can form a perfect image. Even if a reflecting telescope could have a perfect mirror🚨or a refracting telescope could have a perfect lens

Optical defects are always listed in the above order🚨since this expresses their interdependence as first order aberrations via moves of the exit/entrance pupils. The first Seidel aberration

Optical telescopes have been used in astronomical research since the time of their invention in the early 17th century. Many types have been constructed over the years depending on the optical technology🚨such as refracting and reflecting

Nearly all large research-grade astronomical telescopes are reflectors. Some reasons are:🚨

Most large research reflectors operate at different focal planes🚨depending on the type and size of the instrument being used. These including the prime focus of the main mirror