The power of the telescope, computed as focal length of the telescope divided by the
focal length of the eyepiece.
High powers are useful for detecting faint stars, use up to 50X or 75X per inch if seeing conditions permit.
Type of telescope
A Newtonian Reflector uses a primary mirror and a secondary flat mirror.
A Refractor has a lens as its objective.
The Schmidt-Cassegrain has a corrector lens, a primary and secondary mirror.
(not applicable for Refractors)
Choose standard or enhanced mirror coatings (88%-95% reflectivity) as applicable to your telescope.
This is a qualitative evaluation from "Clean" to "Dirty". Pick the value that seems appropriate
for the telescope and eyepiece used.
Fully dilated Eye Pupil Diameter
Only relevant for observing at low power, the fully dilated eye pupil diameter varies between
4 mm and 7 mm, depending on the age of the observer but with large personal variations.
A seemingly counterintuitive result is that by reducing the pupil diameter (increasing the age) without
modifying any other value, the computed telescopic limiting magnitude will increase.
On second thought this result is quite logical: an observer reaching the same NELM with a smaller pupil should be able to see deeper
in the telescope at high powers, where the pupil size disadavantage disappears.
A reduction of the pupil diameter because of age will usually be accompanied by a reduced NELM value.
Experience includes training of the eye ("knowing what to look for"), thorough dark adaptation and
more efficient use of techniques like averted vision or hyperventilation.
The difference between a "Novice" and an "Expert" can easily amount to more than a magnitude.
Naked Eye Limiting Magnitude (NELM) near the Zenith
This is the magnitude of the dimmest star you can see near the zenith.
Typical values are 5.0 for a backyard site, 6.0 for a rural site, 7.0 for an excellent site.
Alternatively, you can measure the sky brightness with a SQM meter.
Under typical conditions the following conversion table applies:
SQM = 22.0 => NELM = 7.0
SQM = 21.0 => NELM = 6.4
SQM = 20.0 => NELM = 5.7
SQM = 19.0 => NELM = 4.9
Variations of several tenths of a magnitude depending on the sky transparency and the observer's visual acuity are to be expected.
Seeing Disk Diameter
The diameter of the image of a star (in arc seconds) depends on the turbulence of the atmosphere
and can vary from less than an arc second to several arc seconds.
You have to judge this at the telescope. Bad seeing negates the advantage of high powers.
Color Index of the star
Value varies from 0 for whitish stars, 0.5 for yellowish stars to more than 1 for red stars.
The angle in degrees from the vertical direction. Close to the zenith the sky is darker and fainter stars are visible.
Typically 0.2-0.6 mag./airmass. High altitude sites can go as low as 0.15, a value for an average site is 0.3.
Bradley Schaefer's article "Telescopic Limiting Magnitudes" in Publications of the
Astronomical Society of the Pacific, February 1990