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3. Telescope ABC’s
What do the following terms mean?
Barlow Lens:
The Barlow Lens was named after its inven-
tor, Peter Barlow, a British mathematician and
physicist who lived from 1776-1862. The lens
can be used to increase the focal width of a
telescope. Depending on the type of lens, it
is possible to double or even to triple the fo-
cal width. As a result, the magnifi cation can
of course also be increased. See also “Eye-
piece.“
Focal width:
Everything that magnifi es an object via an op-
tic (lens) has a certain focal width. The focal
width is the length of the path the light travels
from the surface of the lens to its focal point.
The focal point is also referred to as the focus.
In focus, the image is clear. In the case of a
telescope, the focal widths of the telescope
tube and the eyepieces are combined:
Lens:
The lens turns the light which falls on it around
in such a way so that the light gives a clear
image in the focal point after it has traveled a
certain distance (focal width).
Eyepiece:
An eyepiece is a system made for your eye
and comprised of one or more lenses. In an
eyepiece, the clear image that is generated in
the focal point of a lens is captured and mag-
nifi ed still more.
There is a simple formula for calculating the
magnifi cation:
Focal width of the telescope tube / Focal
width of the eyepiece = Magnifi cation
You see: In a telescope, the magnifi cation de-
pends on both the focal width of the telescope
tube and the focal width of the eyepiece.
From this formula, we see that if you use an
eyepiece with a focal width of 20 mm and a
telescope tube with a focal width of 600 mm,
you will get the following magnifi cation:
600 mm / 20 mm = 30 times magnifi cation
Inverting lens:
The inverting lens is set into the eyepiece hol-
der of the telescope before the eyepiece its-
elf. This lens can produce an additional magni-
fi cation (mostly around 1.5x) via the integrated
lens in the eyepiece. As the name suggests,
the image will be turned around if you use an
inverting lens, and appears upright and even
properly oriented on the vertical axis.
Magnifi cation:
The magnifi cation corresponds to the diffe-
rence between observation with the naked
eye and observation through a magnifi cation
apparatus (e.g. a telescope). In this sche-
me, observation with the eye is considered
“single”, or 1x magnifi cation. Accordingly, if
a telescope has a magnifi cation of 30x, then
an object viewed through the telescope will
appear 30 times larger than it would with the
naked eye. See also “Eyepiece.“
Zenith mirror:
A mirror that defl ects the ray of light 90 de-
grees. With a horizontal telescope tube, this
device defl ects the light upwards so that you
can comfortably observe by looking down-
wards into the eyepiece. The image in a zenith
mirror appears upright, but rotated around its
vertical axis (what is left appears right and vice
versa).
