SKY-WATCHER CQ40 - Choosing the Appropriate Eyepiece

14
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.
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.
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 of view 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!
Choosing the Appropriate Eyepiece
Magnication =
Focal length of the eyepiece
Focal length of the telescope
800mm
80X
=
=
10mm
Calculating the magnication (power)
The size of the view that you see through your telescope is called the true (or actual) eld 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 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.
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.
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.
To put this in perspective, the moon is about 0. 5° or 3 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!
True Field of View =
52°
0.65°
=
=
80x
Magnication
Apparent Field of View
Exit Pupil =
Magnication
Diameter of Primary mirror in mm
Calculating the eld of view
Calculating the exit pupil