10
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hoosing the appropriate eyepiece
(for all models)
magnification =
=
= 80X
Focal length of the telescope
Focal length of the eyepiece
800mm
10mm
When you are looking at astronomical objects, you are looking through a column of air that reaches to the
edge of space and that column seldom stays still. Similarly, when viewing over land you are often looking
through heat waves radiating from the ground, house, buildings, etc. Your telescope may be able to give very
high magnification but what you end up magnifying is all the turbulence between the telescope and the
subject. A good rule of thumb is that the usable magnification of a telescope is about 2X per mm of aperture
under good conditions.
Calculating the Field of View
The size of the view that you see through your telescope is called the true (or actual) field of view and it is
determined by the design of the eyepiece. Every eyepiece has a value, called the apparent field of view, which
is supplied by the manufacturer. Field of view is usually measured in degrees and/or arc-minutes (there are 60
arc-minutes in a degree). The true field of view produced by your telescope is calculated by dividing the
eyepiece's apparent field of view by the magnification that you previously calculated for the combination. Using
the figures in the previous magnification example, if your 10mm eyepiece has an apparent field of view of 52
degrees, then the true field of view is 0.65 degrees or 39 arc-minutes.
Calculating the Magnification (Power)
The magnification produced by a telescope is determined by the focal length of the eyepiece that is used with
it. To determine a magnification for your telescope, divide its focal length by the focal length of the eyepieces
you are going to use. For example, a 10mm focal length eyepiece will give 80X magnification with an 800mm
focal length telescope.
To put this in perspective, the moon is about 0.5˚ or 30 arc-minutes in diameter, so this combination would be
fine for viewing the whole moon with a little room to spare. Remember, too much magnification and too small a
field of view can make it very hard to find things. It is usually best to start at a lower magnification with its wider
field and then increase the magnification when you have found what you are looking for. First find the moon
then look at the shadows in the craters!
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alculating the Exit Pupil
The Exit Pupil is the diameter (in mm) of the narrowest point of the cone of light leaving your telescope.
Knowing this value for a telescope-eyepiece combination tells you whether your eye is receiving all of the light
that your primary lens or mirror is providing. The average person has a fully dilated pupil diameter of about
7mm. This value varies a bit from person to person, is less until your eyes become fully dark adapted and
decreases as you get older. To determine an exit pupil, you divide the diameter of the primary of your
telescope (in mm) by the magnification.
Exit Pupil =
Diameter of Primary mirror in mm
Magnification
For example, a 200mm f/5 telescope with a 40mm eyepiece produces a magnification of 25x and an exit pupil
of 8mm. This combination can probably be used by a young person but would not be of much value to a senior
citizen. The same telescope used with a 32mm eyepiece gives a magnification of about 31x and an exit pupil
of 6.4mm which should be fine for most dark adapted eyes. In contrast, a 200mm f/10 telescope with the
40mm eyepiece gives a magnification of 50x and an exit pupil of 4mm, which is fine for everyone.
True Field of View =
Apparent Field of View
Magnification
=
0.65˚
52˚
80X
=
