Highest useful magnification
Highest useful magnification: Any
telescope is theoretically capable of unlimited magnification. As power increases,
however, image brightness decreases, as you’ll see below under "exit pupils." As
you get to 50x or 60x per inch of aperture, most deep space objects become too dim
to see. Thus, an 8" telescope is capable of a maximum useful deep
space power of 480x (60 x 8). But that much power is mainly usable only for
splitting close binary stars.
More than 60x per inch of aperture is sometimes
possible for planetary observing with small aperture telescopes (under 4"-5"), since
a small scope looks through less of our turbulent atmosphere than a large one and
therefore is less affected by unsteady seeing. Unfortunately, the exceptionally
good seeing that allows such high magnifications is very rare.
During average seeing conditions (the kind you find
nine nights out of ten), 25x to 30x per inch of aperture is a more sensible power
for binary star and planetary observing. It is at this power that the resolution
of a scope matches the resolution of your eye and images are sharpest. This gives
you a highest useful power of about 200x to 240x with an 8" scope on an average
night, 100x to 120x with a 4" scope, etc.
Globular clusters and the smaller nebulas are best at about 12x to 15x per inch
of aperture, while 8x per inch of aperture is usually best for finding galaxies
and observing large nebulas.