Mirror Size, Light Gathering Power and Magnitude

DISCUSSION:  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 aperture.

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 =  843).

Atmospheric 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."

       Magnitude limit
3.1" (80mm)
4" (100mm)
5" (125mm)
6" (150mm)
8" (200mm)
10" (250mm)     14.7 
12.5" (320mm)  15.2 
14" (355mm)     15.4 
16" (400mm)     15.7 
20" (500mm)     16.2

From the Celestron Knowledge Base

"Another way 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"! "

Excerpted from answers.com

For more information about your requirements:
--Dennis Steele
--Tel: 650-315-6578 (anytime)
--Email: densteele@dobstuff.com


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