Orion 80-T instruction manual Calculating Magnification Power

A. The Moon

The Moon, with its rocky, cratered surface, is one of the easiest and most interesting subjects to observe with your telescope. The myriad craters, rilles, and jagged moun- tain formations offer endless fascination. The best time to observe the Moon is during a partial phase, that is, when the Moon is not full. During partial phases, shadows cast by crater walls and mountain peaks along the border between the dark and light portions of the lunar disk highlight the surface relief. A full Moon is too bright and devoid of surface shadows to yield a pleasing view. Try using a Moon filter to dim the Moon when it is too bright; it simply threads onto the bottom of the ­eyepiece.

B. The Sun

You can change your nighttime telescope into a daytime Sun viewer by installing an optional full-aperture solar filter over the front opening of the ShortTube 80-T. The primary attrac- tion is sunspots, which change shape, appearance, and loca- tion daily. Sunspots are directly related to magnetic activity in the Sun. Many observers like to make drawings of sunspots to monitor how the Sun is changing from day to day.

Important Note: Do not look at the Sun with any optical instrument without a professionally made solar filter, or per- manent eye damage could result. Also, remember to cover or remove the finder scope.

C. The Planets

The planets don’t stay put like stars do (planets don’t have fixed R.A. and Dec. coordinates), so you will have to refer to Sky Calendar at our website, www.OrionTelescope.com, or to charts published monthly in Astronomy, Sky & Telescope, or other astronomy references to locate them. Venus, Mars, Jupiter, and Saturn are the brightest objects in the sky after the Sun and the Moon. All four of these planets are not nor- mally visible in the sky at one time, but chances are one or two of them will be.

D. Stars

Stars will appear like twinkling points of light in the tele- scope. Even the largest telescopes cannot magnify stars to appear as anything more than points of light! You can, however, enjoy the different colors of the stars and locate many pretty double and multiple stars. The famous “Double- Double” in the constellation Lyra and the gorgeous two-color double star Albireo in Cygnus are favorites. Defocusing the image of a star slightly can help bring out its color.

E. Deep-Sky Objects

Under dark skies, you can observe a number of brighter deep-sky objects with your ShortTube 80-T, including gas- eous nebulas, open and globular star clusters, and the brighter galaxies. Most deep-sky objects are very faint, so it is important that you find an observing site well away from light pollution. Take plenty of time to let your eyes adjust to the darkness. Don’t expect these objects to appear like the photographs you see in books and magazines; most will look like dim gray “ghosts.” (Our eyes are not sensitive enough to see color in deep-sky objects except in few of the brightest ones.) But as you become more experienced and

your observing skills improve, you will be able to coax out more and more intricate details.

Calculating Magnification (Power)

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:

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

For example, the ShortTube 80-T, which has a focal length of 400mm, used in combination with the supplied 25mm Sirius Plössl eyepiece, yields a magnification of

400 ÷ 25 = 16x.

Every telescope has a useful limit of magnification of about 45x-60x per inch of aperture, which is 140x-180x for the ShortTube 80-T. Claims of higher power by some telescope 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 fundamental law of optics). The steadiness of the air (the “seeing”) can also 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, like the supplied 10mm Sirius Plössl eyepiece, to ferret out more detail. 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, overmagnified image.

Photography with the ShortTube 80-T

With an optional camera adapter, the ShortTube 80-T becomes a 400mm f/5 telephoto lens for a single-lens reflex camera. For long-distance terrestrial or astronomical pho- tography, you need only a T-ring for your particular camera model. The T-ring attaches to your particular camera model and threads onto the ShortTube’s focuser drawtube, cou- pling the camera body to the telescope (Figure 3).

Focus lock thumbscrew

Figure 3. The ShortTube 80-T with 35mm SLR camera attached.

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