ORION TELESCOPES & BINOCULARS StarBlast AutoTracker 10140, Instruction Manual

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For astronomical viewing, out of focus star images are very dif-
fuse, making them difficult to see. If you turn the focus knob
too quickly, you can go right through focus without seeing the
image. To avoid this problem, your first astronomical target
should be a bright object (like the Moon or a planet) so that the
image is visible even when out of focus. It can even be helpful
to practice during the day on an object at least a mile away, i.e.,
at “infinity.”
image Orientation
The image orientation of any telescope changes depending
on its optical design and how the eyepiece is inserted into the
telescope. A refractor used with a star diagonal, for astronomi-
cal viewing, will show an image that is right side up, but left-
right reversed. When observing through a reflector such as
the StarBlast 114mm, the image will appear upside down and
backwards. For this reason reflectors are not recommended
for daytime terrestrial observing. But since there is no “right
side up” in space, a reflector is fine for astronomical viewing.
calculating Magnification
You can change the power of your telescope just by changing
the eyepiece (ocular). To determine the magnification of your
telescope, simply divide the focal length of the telescope by
the focal length of the eyepiece used.
Magnification is calculated as follows:
Telescope Focal Length
= Magnification
Eyepiece Focal Length
For example, the StarBlast 114mm has a focal length of
500mm, which when used with the supplied 25mm eyepiece
yields:
500mm
= 20x
25mm
The magnification provided by the 10mm eyepiece is:
500mm
= 50x
10mm
Although the power is variable, each instrument under aver-
age skies has a limit to the highest useful magnification. The
general rule is 2x per millimeter of aperture. For example, the
StarBlast 114mm is so named for its primary mirror, which has
a diameter of 114mm. So 114mm x 2 = 228. Thus, 228x is the
highest magnification one can normally achieve under ideal
seeing conditions. Although this is the maximum useful magni-
fication, most observing will yield best results at lower powers.
Determining Field of View
Determining the field of view is important if you want to get
an idea of the angular size of the object you are observing. To
calculate the actual field of view, divide the apparent field of
the eyepiece (supplied by the eyepiece manufacturer) by the
magnification. In equation format, the formula looks like this:
Apparent Field of Eyepiece
= True Field
Magnification
As you can see, before determining the field of view, you must
calculate the magnification. Using the example in the previous
section, we can determine the field of view using the same
25mm eyepiece.
The 25mm eyepiece has an apparent field of view of 49°.
Divide 49° by the magnification, which is 20 power. This yields
an actual field of view of 2.45°.
49
= 2.45° True Field of view
20
The 10mm eyepiece has an apparent field of view of 52°.
Divide 52° by the magnification, which is 50 power. This yields
an actual field of view of 1.04°.
52
= 1.04° True Field of view
50
To convert degrees to feet at 1,000 yards, which is more useful
for terrestrial observing, simply multiply by 52.5.
The 25mm eyepiece produces a linear field width of 128.6 feet
at a distance of one thousand yards (2.45° X 52.5).
The 10mm eyepiece produces a linear field width of 54.6 feet
at a distance of one thousand yards (1.04° X 52.5).
general Observing Hints
When working with any optical instrument, there are a few
things to remember to ensure you get the best possible image:
• Never look through window glass. Glass found in household
windows is optically imperfect, and as a result, may vary
in thickness from one part of a window to the next. This
inconsistency can and will affect the ability to focus your
telescope. In most cases you will not be able to achieve a
truly sharp image, while in some cases; you may actually
see a double image.
• Never look across or over objects that are radiating heat
waves. This includes asphalt parking lots on hot summer
days or building rooftops.
• Hazy skies, fog, and mist can also make it difficult to focus.
The amount of detail seen under these conditions is greatly
reduced.
• If you wear corrective lenses (specifically, glasses), you
may want to remove them when observing with an eyepiece
attached to the telescope. When using a camera, however,
you should always wear corrective lenses to ensure the
sharpest possible focus. If you have astigmatism, corrective
lenses must be worn at all times.
celestial Observing
With your telescope set up, you are ready to use it for observ-
ing. This section covers visual observing hints for both solar
system and deep-sky objects as well as general observing
conditions that will affect your ability to observe.
Observing the Moon
Often, it is tempting to look at the Moon when it is full. At this
time, the face we see is fully illuminated and its light can be
overpowering. In addition, little or no contrast can be seen dur-
ing this phase.
One of the best times to observe the Moon is during its partial
phases (at crescent phases and around the time of first or third
quarter). Long shadows reveal a great amount of detail on the