Orion 9086, 9025, 9024 Using Your Telescope - Astronomical Observing, Choosing an Observing Site

Figure 1 the telescope is pointed north as it would be during polar alignment. The counterweight shaft is oriented down- ward. But it will not look like that when the telescope is pointed in other directions. Let’s say you want to view an object that is directly overhead, at the zenith. How do you do it?

One thing you DO NOT do is make any adjustment to the lat- itude adjusting t-bolt. That will undo the mount’s polar alignment. Remember, once the mount is polar-aligned, the telescope should be moved only on the R.A. and Dec. axes. To point the scope overhead, first loosen the R.A. lock knob and rotate the telescope on the R.A. axis until the counter- weight shaft is horizontal (parallel to the ground). Then loosen the Dec. lock knob and rotate the telescope until it is pointing straight overhead. The counterweight shaft is still horizontal. Then retighten both lock knobs.

Similarly, to point the telescope directly south, the counter- weight shaft should again be horizontal. Then you simply rotate the scope on the Dec. axis until it points in the south direction (Figure 5a).

What if you need to aim the telescope directly north, but at an object that is nearer to the horizon than Polaris? You can’t do it with the counterweight down as pictured in Figure 1. Again, you have to rotate the scope in R.A. so the counterweight shaft is positioned horizontally. Then rotate the scope in Dec. so it points to where you want it near the horizon (Figure 5b).

To point the telescope to the east (Figure 5c) or west (Figure 5d), or in other directions, rotate the telescope on its R.A. and Dec. axes. Depending on the altitude of the object you want to observe, the counterweight shaft will be oriented some- where between vertical and horizontal.

The key things to remember when pointing the telescope is that

a)you only move it in R.A. and Dec., not in azimuth or latitude (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 knobs. Aim the telescope at the object you wish to observe by “eyeballing” along the length of the tele- scope 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 cen- tered on the finder’s crosshairs. Retighten the R.A. and Dec. lock knobs. 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 necessary, use the R.A. and Dec. slow-motion controls to reposition the object within the field of view of the main telescope’s eyepiece.

Focusing the Telescope

Practice focusing the telescope in the daytime before using it for the first time at night. Start by turning the focus knob until the focuser drawtube is near the center of its adjustment range. Insert the star diagonal into the focuser drawtube and an eyepiece into the star diagonal (secure with the thumb- screws). Point the telescope at a distant subject and center it in the field of view. Now, slowly rotate the focus knob until the object comes into sharp focus. Go a little bit beyond sharp focus until the image just starts to blur again, then reverse the rotation of the knob, just to make sure you hit the exact focus point. The telescope can only focus on objects at least 50 to 100 feet away.

Do You Wear Eyeglasses?

If you wear eyeglasses, you may be able to keep them on while you observe, if your eyepieces have enough “eye relief” to allow you to see the whole field of view. You can try this by looking through the eyepiece first with your glasses on and then with them off, and see if the glasses restrict the view to only a portion of the full field. If they do, you can easily observe with your glasses off by just refocusing the telescope the needed amount.

Calculating the Magnification

It is desirable to have a range of eyepieces of different focal lengths, to allow viewing over a range of magnifications. To calculate the magnification, or power, of a telescope, simply divide the focal length of the telescope by the focal length of the eyepiece (the number printed on the eyepiece):

Telescope F.L. ÷Eyepiece F.L. = Magnification

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