The focal length of the telescope is usually
given on a label near the eyepiece focuser and
is the distance from the main lens or mirror
to the point at which it forms an image of a
distant object. The magnification of any given
telescope/eyepiece combination is given thus:
Magnification = focal length of telescope
focal length of eyepiece
For example, a telescope with a focal length of
1000mm used in conjunction with a 7.5mm
eyepiece would yield a magnification of 133x
(1000÷7.5 and rounded to the nearest whole
number), whereas the same instrument used
with a 20mm eyepiece would deliver 50x. It
follows that the larger the focal length of the
eyepiece, the lower the magnification it will
deliver with any given telescope.
Why do we need to use eyepieces of diff e r i n g
magnification? Apart from making the image
of the subject larger or smaller, magnification
has a bearing on the area of sky (termed the
field of view) that is visible: Higher magnifica-
tions have smaller fields of view, which can
make finding objects that much more difficult
(especially if your Finder is not correctly
aligned) - another reason for spending some
time in daylight perfecting the process as out-
lined above.
The field of view in degrees may also be
obtained by another simple formula which is
valid for most types of eyepieces commonly
found in amateur hands, thus:
Field of view (degrees) = 42
magnification
Again, for example, a telescope with a focal
length of 1000mm using a 15mm eyepiece
will deliver a magnification of 67x which,
using the formula above, will give a field of
view of 0.63 degrees.To give you some idea
of scale, the Full Moon is almost exactly 0.5
d e g rees in diameter, so our telescope/eyepiece
example would enable us to display the whole
of the Moon's disc within the eyepiece's field
of view.
How many eyepieces should you have? Three
is ideal - one low, one medium and one high
magnification - though two will suffice at a
pinch. A low magnification of 30-50x is
advisable for observing star clusters, galaxies
and nebulae since they are often spread over
a wide area of sky. Medium magnifications of
80-100x are convenient for studying the
craters and valleys of the Moon's surface,
seeing the rings of Saturn or Jupiter and its
four principal moons. Higher powers of 150-
200x will permit you to scrutinise mountain
peaks and fine lunar detail, the surface feature s
of Mars or to separate close double stars.
You can buy other eyepieces to increase the
versatility of your telescope, or you may care
to purchase something known as a Barlow
lens that is inserted into the focusing tube
before the eyepiece which doubles the magni-
fication - a good way of getting double the
performance out of each eyepiece!
As you get more proficient at observing you
will come to appreciate that merely adding
m o re magnification is useless unless the
atmospheric conditions are steady enough to
permit their use. On many nights the air may
appear steady to the naked eye, but in the
telescope the image of a bright planet such as
Jupiter or Saturn will appear to shimmer, or
the edge of the Moon may appear to ripple;
these are the nights of so-called 'poor seeing'.
Even on a night of steady air a good general
rule to employ is that you will be approaching
the practical limits of your instrument on
most nights when the magnification exceeds
twice the apert u re of your telescope in
millimetres: this means that the maximum
working power of an 80mm aperture instru-
ment will be in the region of 160x, or 300x for
a 150mm telescope.
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