Orion SKYQUEST XT6 User Manual download pdf (Page 13)

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 pollu-

tion, or other conditions that are hindering your viewing. (See

Figure 18)

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.

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 approxi-

mately .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 watching

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 XT because of

its smooth motions on both axes. As the object moves off

toward the edge of the field of view, you just lightly 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 inherent

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 lever-

age, try grasping the tube close to the altitude side bearings

with both hands.

Remember that objects appear to move across the field of

view faster at higher magnifications. This is because the field

of view becomes narrower.

Eyepiece Selection

By using eyepieces of varying focal lengths, it is possible to

attain many magnifications with the SkyQuest XT. The tele-

scope comes with two high-quality Sirius Plössl eyepieces: a

25mm, which gives a magnification of 48x, and a 10mm,

which gives a magnification of 120x. 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!

2.4

1.7

3.4

2.4

4.9

1.9

2.5

Figure 18. 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.

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Orion SKYQUEST XT6 User Manual Page 13

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