10
Part II: Getting Started
Calculating the magnication (power)
C
hoosing the Appropriate Eyepiece
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.
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 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 exam-
ple, 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 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.
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 magnifi-
cation 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.
To put this in perspective, the moon is about 0. 5° or 3 arc-minutes in diameter, so this combination
would be fine for viewing the whole moon with a little room to spare. Remember, too much magnifi-
cation 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!
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.
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 of view 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!
Focal length of the eyepiece
Focal length of the telescope
800mm 80X
=
=
10mm
True Field of View =
52°
0.65°
=
=
80x
Apparent Field of View
Exit Pupil =
Diameter of Primary mirror in mm
