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12
“Seeing” and Transparency
Atmospheric conditions vary signicantly from night to night.
“Seeing” refers to the steadiness of the Earth’s atmosphere at a
given time. In conditions of poor seeing, atmospheric turbulence
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 magnications. At
higher magnications, 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 overhead,
worst at the horizon. Also, seeing generally gets better after mid-
night, 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 (6th magnitude or fainter 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, or you can cover the front of a regular incandescent
ashlight with red cellophane or paper. Beware, too, that nearby
porch, streetlights, and car headlights will ruin your night vision.
Eyepiece Selection
Magnication, 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 magnication can be varied. It is quite common for an
observer to own ve or more eyepieces to access a wide range
of magnications. This allows the observer to choose the best
eyepiece to use depending on the object being viewed and view-
ing conditions. The StarBlast II 4.5 EQ comes with two eyepiec-
es, which will suffice nicely to begin with.
Magnication is calculated as follows:
Telescope Focal Length (mm)
= Magnication
Eyepiece Focal Length (mm)
For example, the StarBlast II 4.5 EQ has a focal length of 450mm,
which when used with the supplied 15mm eyepiece yields:
450 mm
= 18x
25 mm
The magnication provided by the 6mm eyepiece is:
450 mm
= 45x
10 mm
The maximum attainable magnication for a telescope is directly
related to how much light it can gather. The larger the aperture,
the more magnication is possible. In general, a gure of 50x per
inch of aperture is the maximum attainable for most telescopes.
Your StarBlast II 4.5 EQ has an aperture of 4.5 inches, so the
maximum magnication would be about 225x. This level of mag-
nication assumes you have ideal conditions for observing.
Keep in mind that as you increase magnication, the brightness
of the object viewed will decrease; this is an inherent principle
of the laws of physics and cannot be avoided. If magnication is
doubled, an image appears four times dimmer. If magnication 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 magnication to get a
closer view. 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 magnication,
since the eld of view will be narrower with the higher-powered
eyepiece.
To change eyepieces, rst loosen the securing thumbscrews on
the focuser drawtube. Then carefully lift the eyepiece out of the
drawtube. Do not tug or pull the eyepiece to the side, as this
will knock the telescope off its target. Replace the eyepiece with
the new one by sliding it gently into the drawtube. Re-tighten the
thumbscrews, and refocus for your new magnication.
What to Expect
So what will you see with your telescope? You should be able to
see bands on Jupiter, the rings of Saturn, craters on the Moon,
the waxing and waning of Venus, and many bright deep-sky
objects. Do not expect to see color as you do in NASA photos,
since those are taken with long-exposure cameras and have
“false color” added. Our eyes are not sensitive enough to see
color in deep-sky objects except in a few of the brightest ones.
Objects to Observe
Now that you are all set up and ready to go, one critical decision
must be made: what to look at?
A. The Moon
With its rocky surface, the Moon is one of the easiest and most
interesting objects to view with your telescope. Lunar craters,
maria, and even mountain ranges can all be clearly seen from
a distance of 238,000 miles away! With its ever-changing phas-
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