Orion 120 EQ Using Your Telescope- Astronomical Observing, Choosing an Observing Site

a. b.

c. d.

Figure 10. This illustration shows the telescope pointed in the four cardinal directions: (a) north, (b) south, (c) east, (d) west. Note that the tripod and mount have not been moved; only the telescope tube has been moved on the R.A. and Dec. axes.

Figure 10 illustrates how the telescope will look when pointed at the four cardinal directions—north, south, east and west.

The key things to remember when pointing the telescope are that a) you only move it in R.A. and Dec., not in azimuth or lat- itude (altitude), and b) the counterweight and shaft will not always appear as it does in Figure 1. In fact it almost never will!

6.Using Your Telescope— Astronomical Observing

Choosing an Observing Site

When selecting a location for observing, get as far away as possible from direct artificial light such as streetlights, porch lights, and automobile headlights. The glare from these lights will greatly impair your dark-adapted night vision. Set up on a grass or dirt surface, not asphalt, because asphalt radiates more heat. Heat disturbs the surrounding air and degrades the images seen through the telescope. Avoid viewing over rooftops and chimneys, as they often have warm air currents rising from them. Similarly, avoid observing from indoors through an open (or closed) window, because the tempera- ture difference between the indoor and outdoor air will cause image blurring and distortion.

If at all possible, escape the light-polluted city sky and head for darker country skies. You’ll be amazed at how many more stars and deep-sky objects are visible in a dark sky!

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 a half-hour for your telescope to cool to the temperature outdoors. In very cold climates (below freezing), it is essential to store the tele- scope as cold as possible. If it has to adjust to more than a 40° temperature change, allow at least one hour.

Aiming the Telescope

To view an object in the main telescope, first loosen both the R.A. and Dec. lock levers. Aim the telescope at the object you wish to observe by “eyeballing” along the length of the telescope tube (or use the setting circles to “dial in” the object’s coordinates).Then look through the (aligned) finder scope and move the telescope tube until the object is generally centered on the finder’s crosshairs. Retighten the R.A. and Dec. lock levers.Then accurately center the object on the finder’s crosshairs using the R.A. and Dec. slow- motion controls. The object should now be visible in the main telescope with a low-power (long focal length) eyepiece. If neces-

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