Cooling the Telescope, Aiming the Telescope

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- sary, use the R.A. and Dec. slow-motion controls to re-position 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 drawtube and an eyepiece into the star diagonal (secure with the thumbscrews). 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):

Magnification = Telescope Focal Length (mm)

Eyepiece Focal Length (mm)

For example, the AstroView 120ST, which has a focal length of 600mm, used in combination with a 25mm eyepiece, yields a power of

600 ÷ 25 = 24x.

Every telescope has a useful limit of power of about 45x-60x per inch of aperture. Claims of higher power by some tele- scope manufacturers are a misleading advertising gimmick and should be dismissed. Keep in mind that at higher powers, an image will always be dimmer and less sharp (this is a fun- damental law of optics). The steadiness of the air (the “seeing”) will limit how much magnification an image can tolerate.

Always start viewing with your lowest-power (longest focal length) eyepiece in the telescope. After you have located and looked at the object with it, you can try switching to a higher power eyepiece to ferret out more detail, if atmospheric conditions permit. If the image you see is not crisp and steady, reduce the magnification by switching to a longer focal length eyepiece. As a general rule, a small but well-resolved image will show more detail and provide a more enjoyable view than a dim and fuzzy, over-magnified image.

Use of 2" Eyepieces

Another nice feature of the AstroView 120ST is its ability to use either 1.25" or 2" barrel-diameter eyepieces. At low pow- ers (long focal lengths), 2" eyepieces can give a wider field of view than standard 1.25" eyepieces. This is especially desir- able for observing deep-sky objects, as many of them appear quite big, but faint. If you want to use 2" eyepieces, such as the Orion Optiluxe 2" eyepieces, we recommend that you also purchase a 2" star diagonal for refractors; this will provide for a comfortable viewing angle.

To use 2" eyepieces, simply loosen the two large thumbscrews on the focuser drawtube that are just in front of the thumbscrew that holds the provided 1.25" star diagonal in place. Once these large thumbscrews are loosened, the entire back end of the focuser, including any 1.25 diagonal and eyepiece that may be attached, comes off, exposing the 2" diameter focuser drawtube. Now, insert your 2" star diagonal into the drawtube and secure with the two large thumbscrews. Insert a 2" eyepiece into the 2" diagonal, secure it in place with the thumbscrew on the diagonal, and you’re ready to observe.

Let Your Eyes Dark-Adapt

Don’t 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. As your eyes become dark-adapt- ed, more stars will glimmer into view and you’ll be able to see fainter details in objects you view in your telescope.

To see what you’re doing in the darkness, use a red-filtered flash- light 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, such as the Orion RedBeam LED flashlight, or you can cover the front of a regular incandescent flashlight with red cellophane or paper. Beware, too, that nearby porch and street lights and car headlights will ruin your night vision.

“Seeing” and Transparency

Atmospheric conditions vary significantly from night to night. “Seeing” refers to the steadiness of the Earth’s atmosphere at a given time. In conditions of poor seeing, atmospheric turbu-