Orion XT4.5 - Manual (Page 10)

10
Cooling the Telescope
All optical instruments need time to reach “thermal equilibri-
um” to achieve maximum stability of the lenses and mirrors,
which is essential for peak performance. When moved from a
warm indoor location outside to cooler air (or vice-versa), a
telescope needs time to cool (or warm) to the outdoor tem-
perature. The bigger the instrument and the larger the
temperature change, the more time will be needed.
Allow at least 30 minutes for your SkyQuest XT4.5 to equil-
ibrate. If the scope experiences more than a 40°
temperature change, allow an hour or more. In the winter,
storing the telescope outdoors in a shed or garage greatly
reduces the amount of time needed for the optics to stabi-
lize. It also is a good idea to keep the scope covered until
the Sun sets so the tube does not heat greatly above the
temperature of the outside air.
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 better
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
incoming 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 magnifica-
tions 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 which tend to scatter light, reducing 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 con-
ditions are poor. Magnitude is a measure of a star's
brightness. The brighter a star is, the lower its magnitude will
be. A good star to remember 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, or other conditions that are hindering
your viewing. (See Figure 14)
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 per-
haps 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.
5 Astronomy Basics
Tracking Celestial Objects
The Earth is constantly rotating about its polar axis, complet-
ing one full rotation every 24 hours; this is what defines a
“day”. We do not feel the Earth rotating, but we can tell that it
is at night by seeing the apparent movement of stars from
east to west. This movement translates into a rate of approx-
imately .25° per minute, or 15 arc-seconds per second.
(There are 60 arc-minutes in 1°, and 60 arc-seconds in one
arc-minute.) This is called the sidereal rate.
When you observe any astronomical object, you are watch-
ing a moving target. This means the telescope’s position must
be continuously updated over time to keep an object in the
field of view. This is easy to do with the SkyQuest XT4.5
because of its smooth motions on both axes. As the object
moves off toward the edge of the field of view, you just light-
ly nudge the telescope to bring it back to the center.
You will notice that it is more difficult to “track” objects when
the telescope tube is aimed nearly straight up. This is inher-
ent to the basic design of the Dobsonian, and stems from the
fact that there is very little mechanical leverage to move in
azimuth when the tube is in a near vertical position. To gain
more leverage, try grasping the tube close to the altitude side
bearings with both hands. 2.4 1.7 3.4
2.4
4.9
1.9
1.9
2.5
Figure 14. 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.