The magni cation produced by a telescope is determined by the focal length of the eyepiece that is used with it. To
determine a magni cation 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 magni cation with an 800mm focal length
telescope.
Focal length of the telescope
magni cation =
Focal length of the eyepiece
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 magni cation
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 magni cation of a telescope is about 2X per mm of aperture under good conditions.
The size of the view that you see through your telescope is called the true (or actual) eld of view and it is
determined by the design of the eyepiece. Every eyepiece has a value, called the apparent eld 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 eld of view produced by your telescope is calculated by dividing the eyepiece's
apparent eld of view by the magni cation that you previously calculated for the combination. Using the gures in
the previous magni cation example, if your 10mm eyepiece has an apparent eld of view of 52 degrees, then the
true eld of view is 0.65 degrees or 39 arc-minutes.
True Field of View =
To put this in perspective, the moon is about 0.5° or 30 arc-minutes in diameter, so this combination would be ne
for viewing the whole moon with a little room to spare. Remember, too much magni cation and too small a eld of
view can make it very hard to nd things. It is usually best to start at a lower magni cation with its wider eld and
then increase the magni cation when you have found what you are looking for. First nd the moon then look at
the shadows in the craters!
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 magni cation.
Diameter of Primary mirror in mm
Exit Pupil =
For example, a 200mm f/5 telescope with a 40mm eyepiece produces a magni cation 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 magni cation of about 31x and an exit pupil of
6.4mm which should be ne for most dark adapted eyes. In contrast, a 200mm f/10 telescope with the 40mm
eyepiece gives a magni cation of 50x and an exit pupil of 4mm, which is ne for everyone.
Apparent Field of View
Magni cation
Magni cation
800mm
= 80X
=
10mm
52°
=
=
0.65°
80X
12
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