23
Starscope on equatorial mount
Instruction manual
Theoretically, almost any magnification is
possible with a telescope if you use the right
eyepieces. As you can see in the table above,
in the example with the 4 mm eyepiece and a
2x Barlow lens even a magnification of 350x
can be achieved. With even smaller eyepiece
focal lengths and stronger Barlow lenses (e.g.
3x, 5x) this could be increased almost arbitra-
rily. However, the useful magnification range
is limited by the laws of optics.
For the highest useful magnification, the rule
of thumb is that you should choose a maxi-
mum magnification of twice the diameter of
the optics.
For example, if the telescope has an diameter
of 76 mm, the highest magnification should
the highest magnification should not be more
than 76 x 2 = 152x. If you go beyond this
range, the image will become dark and the
sharpness will decrease, so you will see less
detail despite the higher magnification.
Magnification is also often limited by what is
called „seeing“ (air turbulence in the Earth’s
atmosphere).
Depending on the night of observation, the air
may be calmer or less calm. The highest use-
ful magnification can only be achieved when
the air is as calm as possible.
The lowest useful magnification is limited by
the so-called exit pupil (EP). The exit pupil is
the diameter of the light beam that is directed
from the eyepiece into your eye.
Here’s how to calculate the exit pupil:
Aperture of the telescope / Magnification
= diameter of theexit pupil
If the exit pupil is larger than the pupil of your
eye, light is lost and the image becomes dar-
ker. It is believed that the human pupil dilates
to a maximum diameter of 5-7 mm in comple-
te darkness. Therefore, avoid magnifications
that result in an exit pupil that is too large.
Examples (aperture of the telescope and
lowest reaonable magnification):
60 mm
8,5–12x
70 mm
10–14x
76 mm
11–15x
Highest and lowest useful magnification
The magnification in a telescope is calculated
by dividing the focal length of the telescope
by the focal length of the eyepiece.
Example:
Telescope focal length 700 mm
Eyepiece focal length 12.5 mm
700/12.5 = 56x magnification
This means that the smaller the eyepiece
focal length, the higher the magnification.
With a 2x Barlow lens, the magnification dou-
bles, in the example to 112x.
Examples for a telescope with 700 mm
Focal length (eyepiece focal length/magnifica-
tion/magnification with 2x Barlow lens):
20 mm 35 x 70 x
12.5 mm 56 x 112 x
4 mm 175 x 350 x
Useful magnifications and calculation formulas
