Orion 90 Magnification = Telescope Focal Length mm Eyepiece Focal Length mm, 500 ÷ 26 =, C. Stars

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 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 charts published monthly on our website, www.telescope.com, or 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 normally visible in the sky at one time, but chances are one or two of them will be.

JUPITER The largest planet, Jupiter, is a great subject to observe. You can see the disk of the giant planet and watch the ever-changing positions of its four largest moons, Io, Callisto, Europa, and Ganymede. If atmospheric conditions are good, you may be able to resolve thin cloud bands on the planet’s disk.

SATURN The ringed planet is a breathtaking sight when it is well positioned. The tilt angle of the rings varies over a period of many years; sometimes they are seen edge-on, while at other times they are broadside and look like giant “ears” on each side of Saturn’s disk. A steady atmosphere (good seeing) is necessary for a good view.You may probably see a tiny, bright “star” close by; that’s Saturn’s bright- est moon, Titan.

VENUS At its brightest, Venus is the most luminous object in the sky, excluding the Sun and the Moon. It is so bright that sometimes it is visible to the naked eye during full daylight! Ironically, Venus appears as a thin crescent, not a full disk, when at its peak brightness. Because it is so close to the Sun, it never wanders too far from the morning or evening horizon. No surface markings can be seen on Venus, which is always shrouded in dense clouds.

MARS If atmospheric conditions are good, you may be able to see some subtle surface detail on the Red Planet, possibly even the polar ice cap. Mars makes a close approach to Earth every two years; during those approaches its disk is larger and thus more favorable for viewing.

C. Stars

Stars will appear like twinkling points of light in the telescope. 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.

D. Deep-Sky Objects

Under dark skies, you can observe a number of brighter deep-sky objects with your ShortTube 90, including gaseous 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.

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 find out 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 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 mag- nification, or power, of a telescope, simply divide the focal length of the telescope by the focal length of the eyepiece:

Magnification = Telescope Focal Length (mm)

Eyepiece Focal Length (mm)

For example, the ShortTube 90, which has a focal length of 500mm, used in combination with a 26mm eyepiece, yields a power of

500 ÷ 26 = 19x.

Every telescope has a useful limit of power of about 45x-60x per inch of aperture. 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”) 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 fer- ret 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, overmagnified image.

Photography with the ShortTube 90

With the proper adapters, the ShortTube 90 becomes a 500mm f/5.6 telephoto lens for a 35mm single-lens reflex (SLR) camera. You can take impressive pictures in daylight with the telescope/camera com- bination mounted on a sturdy camera tripod. For astrophotography, the telescope with camera attached should be mounted on an equa- torial mount equipped with at least one motor drive to track the motion of the stars.

You will need both a T-ring for your camera and a 1.25" Universal Camera Adapter to couple your camera to the back end of the tele- scope. Remove the nosepiece from the camera adapter; the body of the adapter will not be needed here. Connect the T-ring to your cam- era body (remove any camera lenses first), and connect the nosepiece of the camera adapter to the T-ring. Now, insert the entire assembly into the focuser’s drawtube (remove the eyepiece and diag- onal), and secure with the setscrew on the drawtube. Be sure to tighten the setscrew, or your camera could fall to the ground!

Use the camera’s viewfinder to frame the picture. Use the telescope’s focuser to focus the image. Tighten the focus lock setscrew to make sure the camera does not slip out of focus.

You may want to consider using a remote shutter release instead of the shutter release on the camera. Touching the camera can vibrate the system and blur the resulting photographic image. Also, be sure to use a solid tripod.

Care & Maintenance

Give your telescope reasonable care and it will last a lifetime. Store it indoors or in a dry garage. Do not leave the telescope outside except when using it. The optical tube is aluminum and has a smooth paint- ed surface that should resist scratches and smudges. If a scratch does appear on the tube, it will not harm the telescope. If you wish,

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