Orion Observer 80ST User Manual

11
ly impair your dark-adapted night vision. Set up on a grass or
dirt surface, not asphalt, because asphalt radiates more heat.
Heat disturbs the surrounding air and degrades the images seen
through the telescope. Avoid viewing over rooftops and chimneys,
as they often have warm air currents rising from them. Similarly,
avoid observing from indoors through an open (or closed) win-
dow, because the temperature difference between the indoor
and outdoor air will cause image blurring and distortion.
If at all possible, escape the light-polluted city sky and head for
darker country skies. You’ll be amazed at how many more stars
and deep-sky objects are visible in a dark sky!
“Seeing” and Transparency
Atmospheric conditions vary signicantly from night to night.
“Seeing” refers to the steadiness of the Earth’s atmosphere at
a given time. In conditions of poor seeing, atmospheric turbu-
lence causes objects viewed through the telescope to “boil.”
If you look up at the sky and stars are twinkling noticeably,
the seeing is poor and you will be limited to viewing at lower
magnications. At higher magnications, images will not focus
clearly. Fine details on the planets and Moon will likely not be
visible.
In conditions of good seeing, star twinkling is minimal and
images appear steady in the eyepiece. Seeing is best over-
head, worst at the horizon. Also, seeing generally gets better
after midnight, when much of the heat absorbed by the Earth
during the day has radiated off into space.
Especially important for observing faint objects is good “trans-
parency”—air free of moisture, smoke, and dust. All tend to scat-
ter light, which reduces an object’s brightness. Transparency is
judged by the magnitude of the faintest stars you can see with
the unaided eye (5th or 6th magnitude is desirable).
Cooling the Telescope
All optical instruments need time to reach “thermal equilibri-
um.” The bigger the instrument and the larger the temperature
change, the more time is needed. Allow at least 30 minutes for
your telescope to acclimate to the temperature outdoors before
you start observing with it.
Let Your Eyes Dark-Adapt
Don’t expect to go from a lighted house into the darkness of the
outdoors at night and immediately see faint nebulas, galaxies,
and star clusters—or even very many stars, for that matter. Your
eyes take about 30 minutes to reach perhaps 80% of their full
dark-adapted sensitivity. As your eyes become dark-adapted,
more stars will glimmer into view and you’ll be able to see fainter
details in objects you view in your telescope.
To see what you’re doing in the darkness, use a red-ltered ash-
light rather than a white light. Red light does not spoil your eyes’
dark adaptation like white light does. A ashlight with a red LED
light is ideal. Beware, too, that nearby porch, streetlights, and car
headlights will ruin your night vision.
Eyepiece Selection
Magnication, or power, is determined by the focal length of
the telescope and the focal length of the eyepiece being used.
Therefore, by using eyepieces of different focal lengths, the
resultant magnication can be varied. It is quite common for an
observer to own ve or more eyepieces to access a wide range
of magnications. This allows the observer to choose the best
eyepiece to use depending on the object being viewed and view-
ing conditions. Your Observer 80ST EQ refractor comes with
25mm and 10mm Kellner eyepieces, which will suffice nicely to
begin with. You can purchase additional eyepieces later if you
wish to have more magnication options.
Magnication is calculated as follows:
Telescope Focal Length (mm)
= Magnication
Eyepiece Focal Length (mm)
For example, the Observer 80ST EQ has a focal length of
400mm, which when used with the supplied 25mm eyepiece
yields:
400 mm
= 16x
25 mm
The magnication provided by the 10mm eyepiece is:
400 mm
= 40x
10 mm
The maximum attainable magnication for a telescope is directly
related to how much light it can gather. The larger the aperture,
the more magnication is possible. In general, a gure of 50x per
inch of aperture is the maximum attainable for most telescopes.
Going beyond that will yield simply blurry, unsatisfactory views.
Your Observer 80ST EQ has an aperture of 80mm, or 3.1 inch-
es, so the maximum magnication would be about 155x (3.1 x
50). This level of magnication assumes you have ideal atmo-
spheric conditions for observing (which is seldom the case).
Keep in mind that as you increase magnication, the brightness
of the object viewed will decrease; this is an inherent principle
of the laws of physics and cannot be avoided. If magnication is
doubled, an image appears four times dimmer. If magnication is
tripled, image brightness is reduced by a factor of nine!
Start by centering the object you wish to see in the 25mm eye-
piece. Then you may want to increase the magnication to get a
closer view, by switching to the 10mm eyepiece. If the object is
off-center (i.e., it is near the edge of the eld of view) you will lose
it when you increase magnication, since the eld of view will be
narrower with the higher-powered eyepiece. So make sure it is
centered in the 25mm eyepiece before switching to the 10mm
eyepiece.
Focusing the Telescope
To focus the telescope, turn the focus wheels (Figure 16)
forward or back until you see your target object (e.g., stars,
the Moon, etc.) in the eyepiece. Then make ner adjustments
until the image is sharp. If you’re having trouble achieving ini-
tial focus, rack the focuser drawtube all the way in using the
focus wheels, then while looking into the eyepiece slowly turn
the focus wheels so that the drawtube extends outward. Keep
going until you see your target object come into focus. Note
that when you change eyepieces you may have to adjust the

Orion Observer 80ST Specifications

General

Brand	Orion
Model	Observer 80ST
Category	Telescope
Language	English