Orion 100 EQ - Manual (Page 12)

12
Calculating Magnification (Power)
To calculate the magnification, or power, of a telescope with
an eyepiece, simply divide the focal length of the telescope by
the focal length of the eyepiece:
Telescope focal length
= Magnification
Eyepiece focal length
For example, the AstroView 100 EQ, which has a focal length
of 600mm, used in combination with the included 25mm Sirius
Plössl eyepiece, yields a magnification of:
600mm
= 24x
25mm
It is desirable to have a range of eyepieces of different focal
lengths to allow viewing over a range of magnifications. Your
telescope comes with two high-quality Sirius Plössl eye-
pieces, a 25mm that yields 24x, and a 10mm that yields 60x.
It is not uncommon for an observer to own five or more eye-
pieces. Orion offers many different eyepieces of varying focal
lengths, so check the catalog or website for a wide selection
of additional eyepieces to choose from.
Every telescope has a useful magnification limit of about 2x
per millimeter of aperture (i.e. 200x for the AstroView 100
EQ). Claims of higher power by some telescope manufactur-
ers are a misleading advertising gimmick and should be
dismissed. Keep in mind that at higher powers, an image will
always be dimmer and less sharp (this is a fundamental law of
optics). The steadiness of the air (the “seeing”) can also limit
how much magnification an image can tolerate.
Always start viewing with your lowest-power (longest focal
length) eyepiece in the telescope. It’s best to begin observing
with the lowest-power eyepiece because it will typically pro-
vide the widest true field of view, which will make finding and
centering objects much easier. After you have located and
centered an object, you can try switching to a higher-power
eyepiece to ferret out more detail, if atmospheric conditions
permit. If the image you see is not crisp and steady, reduce
the magnification by switching to a longer focal length eye-
piece. As a general rule, a small but well-resolved image will
show more detail and provide a more enjoyable view than a
dim and fuzzy, overmagnified image.
Use of 2" Eyepieces
A feature of the AstroView 100 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 need to use a 2" star diago-
nal for refractors, or a 2" extension tube, so that the telescope
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
thumbscrew that holds the provided 1.25" star diagonal in
place (see Figure 3). Once these thumbscrews are loosened,
the entire back end of the focuser, including any 1.25" diago-
nal 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 thumb-
screws loosened previously. Insert a 2" eyepiece into the
diagonal, secure it in place with the thumbscrew on the diago-
nal, and you’re ready to observe.
Note About Chromatic Aberration
Chromatic aberration literally means color distortion.
Whenever light passes through one material to another, light
of different 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 AstroView 100 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 AstroView
100 EQ will suffer a bit from chromatic aberration due to its
relatively large aperture and short focal length. This will be
noticeable, 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.
Now that you are all set up and ready to go, one critical deci-
sion must be made: what to look at?
Figure 11. Megrez connects the Big Dipper’s handle to it’s
“pan”. It is a good guide to how conditions are. If you can not see
Megrez (a 3.4 mag star) then conditions are poor.
2.4 1.7 3.4
2.4
4.9
1.9
1.9
2.5