Mirror Size, Light Gathering
Power and Magnitude
The subject of telescope sizes often comes up in discussions.
The question is often asked: "Should I get a 12", 14" or 16"
telescope". The aperture of a telescope is often a
personal decision. Let's face it, when you take a large
telescope to a star party, there's always the "wow factor". In
reality, however, the differences between telescopes in the 10"
to 18" range are relatively small. One should also
consider the ease of setup, height of the eyepiece at the
zenith, ease of transport and storage when considering the size
of the telescope. I've heard time and again -- "The best
telescope one can have is the telescope that he or she
will use most often".
There is a wide variety of information on the
Internet on this subject. Simply search for "telescope resolving
power" or "telescope light gathering power", and you'll come up
lots of entries to study. So, in this discussion, I've borrowed
from a interesting article on the Celestron website. It's
a bit simple, but sometimes simplicity tends to explain more
than getting too complicated. I like table of primary mirror
sizes versus star magnitude. It goes a long way in understanding
this somewhat complex subject.
"Light Gathering Power and Magnitude Limit --
Light gathering power is a telescope's
theoretical ability to collect light compared to your fully
dilated eye. It is directly proportional to the square of the
You can calculate this by first dividing the aperture of the
telescope (in mm) by 7mm (dilated eye for a young person) and
then squaring this result. For example, an 8" telescope has a
light gathering power of 843. ((203.2/7)2
conditions and the visual acuity of the observer will often
reduce limiting magnitude. The unaided or naked-eye magnitude
limit is usually considered as 6.0. With a given scope,
photographic limiting magnitude is often two or more magnitudes
fainter than visual limiting magnitude."
Aperture Magnitude limit
3.1" (80mm) 12.2
4" (100mm) 12.7
5" (125mm) 13.2
6" (150mm) 13.6
8" (200mm) 14.2
10" (250mm) 14.7
12.5" (320mm) 15.2
14" (355mm) 15.4
16" (400mm) 15.7
20" (500mm) 16.2
Celestron Knowledge Base
of looking at this subject is to determine the relationship of
the area of a circle. The area of any circle or
circular aperture is proportional to the square of its radius.
A 10-inch-diameter circle has 4 times the area of a
5-inch-diameter. Assuming constant efficiency, the "gain"
of any circular focusing collector increases by 6 dB (4 times)
when the diameter is doubled. You
can see, in this example, a 12" telescope mirror has a
considerable gain over a 6" telescope mirror, but not that much
over a 10"! "
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