14
seeing and transparency
Atmospheric conditions play a huge part in quality of viewing.
In conditions of good “seeing”, star twinkling is minimal and
objects appear steady in the eyepiece. Seeing is best over-
head, worst at the horizon. Also, seeing generally gets bet-
ter after midnight, when much of the heat absorbed by the
Earth during the day has radiated off into space. Typically,
seeing conditions will be better at sites that have an altitude
over about 3000 feet. Altitude helps because it decreases
the amount of distortion causing atmosphere you are looking
through.
A good way to judge if the seeing is good or not is to look at
bright stars about 40° above the horizon. If the stars appear to
“twinkle”, the atmosphere is significantly distorting the incom-
ing light, and views at high magnifications will not appear
sharp. If the stars appear steady and do not twinkle, seeing
conditions are probably good and higher magnifications will
be possible. Also, seeing conditions are typically poor during
the day. This is because the heat from the Sun warms the air
and causes turbulence.
Good “transparency” is especially important for observing faint
objects. It simply means the air is free of moisture, smoke,
and dust. All tend to scatter light, which reduces an object’s
brightness.
One good way to tell if conditions are good is by how many
stars you can see with your naked eye. If you cannot see
stars of magnitude 3.5 or dimmer then conditions are poor.
Magnitude is a measure of how bright a star is, the brighter a
star is, the lower its magnitude will be. A good star to remem-
ber for this is Megrez (mag. 3.4), which is the star in the “Big
Dipper” connecting the handle to the “dipper”. If you cannot
see Megrez, then you have fog, haze, clouds, smog, light pol-
lution or other conditions that are hindering your viewing. (See
Figure 16)
let Your Eyes Dark‑adapt
Do not 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. Many observers
notice improvements after several hours of total darkness. As
your eyes become dark-adapted, more stars will glimmer into
view and you will be able to see fainter details in objects you
view in your telescope. So give yourself at least a little while to
get used to the dark before you begin observing.
To see what you are doing in the darkness, use a red light
flashlight rather than a white light. Red light does not spoil
your eyes’ dark adaptation like white light does. A flashlight
with a red LED light is ideal, or you can cover the front of a
regular flashlight with red cellophane or paper. Beware, too,
that nearby porch and streetlights and automobile headlights
will spoil your night vision.
Eyepiece selection
By using eyepieces of varying focal lengths, it is possible to
attain many magnifications with the AstroView 6 EQ. The tele-
scope comes with two high-quality Sirius Plössl eyepieces:
a 25mm, which gives a magnification of 30x, and a 10mm,
which gives a magnification of 75x. Other eyepieces can be
used to achieve higher or lower powers. It is quite common for
an observer to own five or more eyepieces to access a wide
range of magnifications. This allows the observer to choose
the best eyepiece to use depending on the object being
viewed. At least to begin with, the two supplied eyepieces will
suffice nicely.
Whatever you choose to view, always start by inserting your
lowest-power (longest focal length) eyepiece to locate and
center the object. Low magnification yields a wide field of
view, which shows a larger area of sky in the eyepiece. This
makes acquiring and centering an object much easier. If you
try to find and center objects with high power (narrow field of
view), it’s like trying to find a needle in a haystack!
Once you’ve centered the object in the eyepiece, you can
switch to higher magnification (shorter focal length eyepiece),
if you wish. This is especially recommended for small and
bright objects, like planets and double stars. The Moon also
takes higher magnifications well.
Deep-sky objects, however, typically look better at medium
or low magnifications. This is because many of them are
quite faint, yet have some extent (apparent width). Deep-sky
objects will often disappear at higher magnifications, since
greater magnification inherently yields dimmer images. This
is not the case for all deep-sky objects, however. Many galax-
ies are quite small, yet are somewhat bright, so higher power
may show more detail.
The best rule of thumb with eyepiece selection is to start with
a low power, wide field, and then work your way up in magnifi-
cation. If the object looks better, try an even higher magnifica-
tion. If the object looks worse, then back off the magnification
a little by using a lower-power eyepiece.
Now that you are all set up and ready to go, one critical deci-
sion must be made: what to look at?
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 possibly hun-
Figure 16.
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.
