Magnification & Eyepieces
Magnification, or power, is determined by the focal length of
the telescope and the focal length of the eyepiece. Therefore,
by using eyepieces of different focal lengths, the resultant
magnification can be varied.
Magnification is calculated as follows:
Telescope Focal Length (mm)
Magnification =
Eyepiece Focal Length (mm)
The SkyView Pro 120 EQ has a focal length of 1000mm,
which when used with the supplied 25mm eyepiece yields:
1000mm÷25mm= 40x
The magnification provided by the 10mm eyepiece is:
1000mm÷10mm=100x
The maximum attainable magnification for a telescope is
directly related to how much light it can gather. The larger the
aperture, the more magnification is possible. In general a fig-
ure of 2x per millimeter of aperture is the maximum attainable
for most telescopes. Your SkyView Pro 120 EQ has an aper-
ture of 120 millimeters, so the maximum magnification would
be about 240x. This level of magnification assumes you have
ideal conditions for viewing.
Keep in mind that as you increase magnification, the bright-
ness of the object viewed will decrease; this is an inherent
principle of the laws of physics and cannot be avoided. If mag-
nification is doubled, an image appears four times dimmer. If
magnification is tripled, image brightness is reduced by a fac-
tor of nine!
Always start with your lowest power eyepiece and work your
way up. Start by centering the object being viewed in the
25mm eyepiece. Then you may want to increase the magnifi-
cation to get a closer view, if you wish. If the object is off-cen-
ter (i.e., it is near the edge of the field of view) you will lose it
when you increase magnification, since the field of view will
be narrower with the higher-powered eyepiece. To change
eyepieces, first loosen the securing thumbscrews on the diag-
onal. Then carefully lift the eyepiece out of the diagonal. Do
not tug or pull the eyepiece to the sides, as this will knock the
telescope off its target. Replace the eyepiece with the new
one by sliding it gently into the diagonal. Re-tighten the
thumbscrew, and refocus for your new magnification.
Use of 2" Eyepieces
A feature of the SkyView Pro 120 EQ is its ability to use either
1.25" or 2" barrel-diameter eyepieces. At low magnifications,
2" eyepieces can give a wider field of view than standard 1.25"
eyepieces. This is especially desirable for observing deep-sky
objects, as many of them appear quite large, but faint. If you
want to use 2" eyepieces, you will also need to use a 2" star
diagonal for refractors, or a 2" extension tube, so that the tele-
scope will properly come to focus.
To use 2" eyepieces, simply loosen the two large thumb-
screws on the 2" adapter that are just in front of the thumb-
screw that holds the provided 1.25" star diagonal in place.
Once these thumbscrews are loosened, the entire back end of
the focuser, including any 1.25" diagonal and eyepiece that
may be attached, comes off, exposing the 2" diameter focuser
drawtube Now, insert your 2" star diagonal into the drawtube
and secure with the two thumbscrews loosened previously.
Insert a 2" eyepiece into the diagonal, secure it in place with
the thumbscrew on the diagonal, and you’re ready to observe.
Note About Chromatic Aberration
Chromatic aberration literally means color distortion. When-
ever light passes through one material to another, light of dif-
ferent wavelengths (color) is bent by different amounts. This is
a problem that plagues refractor-type telescopes, since light
passes through both air and glass to form an image. Most
astronomical objects emit a spectrum comprised of many dif-
ferent wavelengths of light, so each wavelength will be bent by
a slightly different amount when passing through a lens. This
results in each color of light reaching precise focus at a slight-
ly different point, which will provide unacceptable images.
Achromatic refractors, like the SkyView Pro 120 EQ, are
designed to minimize chromatic aberration to acceptable lev-
els. The objective lens is actually comprised of two individual
lenses, called elements, made of different materials, which
bend light in slightly different ways. By precisely spacing and
shaping the elements, the chromatic aberration incurred when
light passes through air and the first glass element is reduced
by the way the second element bends the light. The result is
an image that is much better color corrected than a non-
achromatic (one element) objective lens.
Even with the achromatic lens design, however, the SkyView
Pro 120 EQ will suffer a bit from chromatic aberration due to
its large aperture and short focal length. This will be notice-
able, to some degree, on extremely bright objects, such as the
Moon and bright planets. What you will notice is that the
object, when focused, has a slight “purple-halo” around it. This
will not present a problem for most observers, as the eye
readily adapts to the view and is still able to distinguish fine
details. Chromatic aberration will never inhibit deep sky
observing, as deep sky objects are too faint to cause any
noticeable color distortion.
6. Setting Up and Using the
Equatorial Mount
When you look at the night sky, you no doubt have noticed
that the stars appear to move slowly from east to west over
time. That apparent motion is caused by the Earth’s rotation
(from west to east). An equatorial mount (Figure 6) is
designed to compensate for that motion, allowing you to easi-
ly “track” the movement of astronomical objects, thereby keep-
ing them from drifting out of your telescope’s field of view
while you’re observing.
This is accomplished by slowly rotating the telescope on its
right ascension (R.A.) axis, using only the R.A. slow-motion
knob. But first the R.A. axis of the mount must be aligned with
the Earth’s rotational (polar) axis—a process called polar
alignment.
7
®
