Orion 9851 Let Your Eyes Dark-Adapt, Calculating the Magnification, 900mm ÷ 25mm =, A. The Moon

Figure 13. 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.

ture change, the more time is needed. Allow at least 30 min- utes for your telescope to cool to the temperature outdoors. In very cold climates (below freezing), it is essential to store the telescope as cold as possible. If it has to adjust to more than a 40° temperature change, allow at least one hour.

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, gal- axies, and star clusters—or even very many stars, for that mat- ter. Your eyes take about 30 minutes to reach perhaps 80% of their full dark-adapted sensitivity. As your eyes become dark- adapted, 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 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 incandescent flashlight with red cellophane or paper. Beware, too, that nearby porch and streetlights and car head- lights will ruin your night vision.

Calculating the Magnification

Magnification, or power, is determined by the focal length of the telescope and the focal length of the eyepiece. Therefore, by using eyepieces of different focal lengths, the resultant magnification can be varied.

Magnification is calculated as follows:

Telescope Focal Length (mm)

Magnification = Eyepiece Focal Length (mm)

For example, the SpaceProbe 130mm EQ, which has a focal length of 900mm, used in combination with the included 25mm Explorer II eyepiece, yields a magnification of

900mm ÷ 25mm = 36x

Every telescope has a useful magnification limit of about 45x-60x per inch of aperture. Your SpaceProbe 130 has an aperture of about 5.1", so the maximum magnification would be approximently 230x-300x. Claims of higher power by some telescope manufacturers are a misleading adver-

tising gimmick and should be dismissed. Keep in mind that at higher powers, an image will always be dimmer and less sharp (this is a fundamental law of optics). The steadiness of the air (the “seeing”) can also limit how much magnification an image can tolerate.

Eyepiece Selection

By using eyepieces of varying focal lengths, it is possible to attain a great many magnifications with the SpaceProbe 130mm EQ. The telescope comes with two high-quality Sirius Plössl eyepieces: a 25mm, which gives a magnifica- tion of 36x and a 10mm, which gives a magnification of 90x. 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!

Once you’ve centered the object in the eyepiece, you can switch to higher magnification (shorter focal length eye- piece), if you wish. This is especially recommended for small and bright objects, like planets and double stars. The Moon also takes higher magnifications well.

Deep-sky objects, however, typically look better at medium or low magnifications. This is because many of them are quite faint, yet have some extent (apparent width). Deep-sky objects will often disappear at higher magnifications, since greater magnification inherently yields dimmer images. This is not the case for all deep-sky objects, however. Many galax- ies are quite small, yet are somewhat bright, so higher power may show more detail.

The best rule of thumb with eyepiece selection is to start with a low power, wide field, and then work your way up in magni- fication. If the object looks better, try an even higher magnifi- cation. If the object looks worse, then back off the magnifica- tion a little by using a lower-power eyepiece.

Objects to Observe

Now that you are all set up and ready to go, one critical deci- sion must be made: what to look at?

A. The Moon

With its rocky surface, the Moon is one of the easiest and most interesting targets to view with your telescope. Lunar craters, marias, and even mountain ranges can all be clearly seen from a distance of 238,000 miles away! With its ever- changing phases, you’ll get a new view of the Moon every night. The best time to observe our one and only natural sat- ellite is during a partial phase, that is, when the Moon is NOT full. During partial phases, shadows are cast on the surface, which reveal more detail, especially right along the border

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