Orion SKYQUEST XTG User Manual download pdf (Page 23)

Astronomical Observing10.

The SkyQuest XTg GoTo Dobsonian provides prodigious

capability for observing the many wonders of the heavens,

from the major planets to deep-space nebulas and galaxies.

In this section we give you some astronomical observing tips

and briefly summarize what you can expect to see.

Selecting an Observing Site

Since most astronomical objects are faint, observing them

from dark skies will give you the best views. While some

objects, such as the planets and Moon, are bright enough

to see clearly even from light-polluted city skies, for nebulas,

galaxies, and most star clusters, the less ambient light there

is to reduce contrast, the better.

When it isn’t possible or convenient to get out of town to a pitch-

black observing location, try to set up in a spot that is removed

from street and building lights and that has a clear view of a

large portion of the sky. Avoid pointing the telescope above

buildings, if possible, since they radiate heat which degrades

images. For observing faint deep sky objects, choose a moon-

less night. Using the optional light shroud will be a necessity

(see below). Also, use of a light-pollution filter, like the Orion

SkyGlow Broadband filter, can mitigate the effects of back-

ground sky brightness, enhancing the view of faint objects.

Seeing and Transparency

Atmospheric conditions play a huge part in quality of viewing.

Light from stars and other celestial objects must travel through

miles of Earth’s atmosphere to reach our eyes. The air in the

atmosphere will refract and bend the light. Atmospheric tur-

bulence will worsen the effects of refraction, which can cause

the image you see in your telescope to be unstable. The

steadiness of the atmosphere is called “seeing.”

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 later in the evening as much of the heat absorbed by the

Earth during the day has radiated off into space. In condi-

tions of bad seeing, stars will twinkle and objects will appear

unsteady and blurry in the telescope.

“Transparency” is the clarity of the atmosphere, which can

be adversely affected by the presence of moisture, smoke,

and dust. All tend to scatter light, which reduces an object’s

brightness. Good transparency is desirable for astronomical

observing, especially for viewing faint objects.

One good measure of transparency is by how many stars

you can see with your unaided eyes. If you cannot see

stars of magnitude 3.5 or dimmer then transparency is poor.

Magnitude is a measure of how bright a star is. The brighter

a star, the lower its magnitude. A good star to remember for

this is Megrez (magnitude 3.4), which is the star in the Big

Dipper that connects 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.

Cooling the Telescope

All optical instruments need time to reach thermal equilibrium

to achieve maximum stability of the lenses and mirrors, which

is essential for peak performance. Images will be unstable if

the optics are not in equilibrium with the outdoor temperature.

When moved from a warm indoor location outside to cooler

air (or vice-versa), a telescope needs time to cool to the out-

door temperature. The bigger the instrument and the larger

the temperature change, the more time will be needed. Allow

at least 30 minutes or so to equilibrate. If the temperature dif-

ference between indoors and outdoors is more than 40°, it will

likely take longer. In the winter, storing the telescope outdoors

in a shed or garage greatly reduces the amount of time need-

ed for the optics to stabilize. Also, after setting up outdoors,

it is a good idea to keep the telescope covered until the Sun

sets so the tube does not heat greatly above the temperature

of the air.

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 observ-

ers 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. Exposing your eyes to very bright

daylight for extended periods of time can adversely affect your

night vision for days. 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-filtered

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. Dim light is preferable to bright

light.

Be aware, too, that nearby porch and streetlights and auto-

mobile headlights will spoil your night vision. Close your eyes

when you hear an automobile approaching your observing

site!

Eyepiece Selection

By using eyepieces of different focal lengths, it is possible

to attain many different magnifications with your telescope.

Different 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

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Orion SKYQUEST XTG User Manual Page 23

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