This compact, large-aperture Orion 120mm F/5.0 Refractor Telescope Optical Tube is perfect for deep-sky pursuits. Its 120mm (4.7"), multi-coated achromatic telescope lens assembly and modest 600mm focal length combine to provide breathtaking wide-field views. With an optional 25mm Plossl telescope eyepiece, this telescope serves up a sprawling 2.1° swath of sky! The fast f/5.0 optical system also makes it an excellent telescope for astrophotography. The 26"-long optical tube is well baffled to ensure good contrast. The cast-metal 2" rack-and-pinion focuser accommodates either a 1.25" or 2" diagonal, so you can use 1.25" or 2" telescope eyepieces. Tube color is gunmetal gray metallic. A mounting base molded into the focuser casting to accept an optional Orion finder scope or EZ Finder reflex sight. Sold as tube assembly only without accessories. Includes objective lens cap.
Limited Warranty against defects in materials or workmanship for one year from date of purchase. This warranty is for the benefit of the original retail purchaser only. For complete warranty details contact us at 800-676-1343.
Please note this product was not designed or intended by the manufacturer for use by a child 12 years of age or younger.
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How do I calculate the magnification (power) of a telescope?
To calculate the magnification, or power, of a telescope with an eyepiece, simply divide the focal length of the telescope by the focal length of the eyepiece. Magnification = telescope focal length ÷ eyepiece focal length. For example, the Orion 120mm F/5 Refractor Telescope, which has a focal length of 600mm, used in combination with a 25mm eyepiece, yields a power of: 600 ÷ 25 = 24x.
It is desirable to have a range of telescope eyepieces of different focal lengths to allow viewing over a range of magnifications. It is not uncommon for an observer to own five or more eyepieces. Orion offers many different eyepieces of varying focal lengths.
Every telescope has a theoretical limit of power of about 50x per inch of aperture (i.e. 240x for the Orion 120mm F/5 Refractor). Atmospheric conditions will limit the usefullness of magnification and cause views to become blurred. 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). With every doubling of magnification you lose half the image brightness and three-fourths of the image sharpness. 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. It?s best to begin observing with the lowest-power eyepiece, because it will typically provide the widest true field of view, which will make finding and centering objects much easier After you have located and centered an object, 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.
What are practical focal lengths to have for eyepieces for my telescope?
To determine what telescope eyepieces you need to get powers in a particular range with your telescope, see our Learning Center article: How to choose Telescope Eyepieces
Why do Orion telescopes have less power than the telescope at department stores?
Advertising claims for high magnification of 400X, 600X, etc., are very misleading. The practical limit is 50X per inch of aperture, or 120X for a typical 60mm telescope. Higher powers are useless, and serve only to fool the unwary into thinking that magnification is somehow related to quality of performance. It is not.
What causes dim or distorted images?
Too much magnification
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. It?s best to begin observing with the lowest-power eyepiece, because it will typically provide the widest true field of view, which will make finding and centering objects much easier After you have located and centered an object, 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 telescope 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. As a rule of thumb, it is not recommended to exceed 2x per mm of aperture.
Atmospheric conditions aren?t optimal.
Atmospheric conditions vary significantly from night to night, even hour to hour . ?Seeing? refers to the steadiness of the Earth?s atmosphere at a given time. In conditions of poor seeing, atmospheric turbulence causes objects viewed through the telescope to ?boil.? If, when you look up at the sky with just your eyes, the stars are twinkling noticeably, the seeing is bad and you will be limited to viewing with low powers (bad seeing affects images at high powers more severely). Seeing is best overhead, worst at the horizon. Also, seeing generally gets better after midnight, when much of the heat absorbed by the Earth during the day has radiated off into space. It?s best, although perhaps less convenient, to escape the light-polluted city sky in favor of darker country skies.
Viewing through a glass window open or closed.
Avoid observing from indoors through an open (or closed) window, because the temperature difference between the indoor and outdoor air, reflections and imperfections in the glass, will cause image blurring and distortion.
Telescope not at thermal equilibrium.
All optical instruments need time to reach ?thermal equilibrium.? 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 telescope as cold as possible. If it has to adjust to more than a 40 degrees temperature change, allow at least one hour. Time to adjust varies depending on the scope type and aperture.
Make sure you are not looking over buildings, pavement, or any other source of heat, which will radiate away at night, causing ?heat wave? disturbances that will distort the image you see through the telescope.
Does the atmosphere play a role in how good the quality of the image will be?
Atmospheric conditions play a huge part in quality of viewing. In conditions of good ?seeing?, star twinkling is minimal and objects appear steady in the eyepiece. Seeing is best over-head, worst at the horizon. Also, seeing generally gets better after midnight, when much of the heat absorbed by the Earth during the day has radiated off into space. Typically, seeing conditions will be better at sites that have an altitude over about 3000 feet. Altitude helps because it decreases the amount of distortion causing atmosphere you are looking through. A good way to judge if the seeing is good or not is to look at bright stars about 40 degrees above the horizon. If the stars appear to ?twinkle?, the atmosphere is significantly distorting the incoming light, and views at high magnifications will not appear sharp. If the stars appear steady and do not twinkle, seeing conditions are probably good and higher magnifications will be possible. Also, seeing conditions are typically poor during the day. This is because the heat from the Sun warms the air and causes turbulence. Good ?transparency? is especially important for observing faint objects. It simply means the air is free of moisture, smoke, and dust. These tend to scatter light, which reduces an object?s brightness. One good way to tell if conditions are good is by how many stars you can see with your naked eye. If you cannot see stars of magnitude 3.5 or dimmer then conditions are poor. Magnitude is a measure of how bright a star is, the brighter a star is, the lower its magnitude will be. A good star to remember for this is Megrez (mag. 3.4), which is the star in the ?Big Dipper? connecting the handle to the ?dipper?. If you cannot see Megrez, then you have fog, haze, clouds, smog, light pollution or other conditions that are hindering your viewing. Another hint: Good seeing can vary minute to minute. Watch the planets for a while to pick-up those moments of good seeing.
What eyepiece should I use for terrestrial viewing?
For land viewing, it?s best to stick with low power eyepieces that yield a magnification under 100x. At higher powers, images rapidly lose sharpness and clarity due to ?heat waves? caused by Sun-heated air. Remember to aim well clear of the Sun, unless the front of the telescope is fitted with a professionally made solar filter and the finder scope is removed or covered with foil or some other completely opaque material. Many Orion telescopes are capable of focusing on objects that are quite close, so you can view fine details of objects that are nearby. Try focusing on a flower or insect at close distance to enter a normally unseen microscopic world. Check the specifications on the product web page or instruction manual for your Orion scope.
Observing hint: If the object is too close to focus. You may be able to use an extension tube that allows the eyepiece to move further back as you focus closer. Try lifting the eyepiece out of the holder as you look. If it focuses in about an inch or two, you can purchase an eyepiece extension tube. For more detailed information on this topic see our Learning Center article: Choosing Eyepieces
How do I clean any of the optical lenses?
Any quality optical lens cleaning tissue and optical lens cleaning fluid specifically designed for multi-coated optics can be used to clean the exposed lenses of your eyepieces or finder scope. Never use regular glass cleaner or cleaning fluid designed for eyeglasses. Before cleaning with fluid and tissue, blow any loose particles off the lens with a blower bulb or compressed air. Then apply some cleaning fluid to a tissue, never directly on the optics. Wipe the lens gently in a circular motion, then remove any excess fluid with a fresh lens tissue. Oily finger-prints and smudges may be removed using this method. Use caution; rubbing too hard may scratch the lens. On larger lenses, clean only a small area at a time, using a fresh lens tissue on each area. Never reuse tissues.
I recently purchased a solar filter for my telescope and can?t see anything with it. Any suggestions?
One of the problems with a solar filter on a telescope is that it?s a bit tricky to aim it at the sun. You can?t look through the finder to point the scope or you?ll cause injury to your eye. So, cap off or remove the finder. Also, because with the very dark filter on the front if the sun is slightly outside the field of view of the eyepiece you?ll see pitch blackness in the field. With the solar filter properly mounted, try looking at the shadow of the optical tube on the ground, move the tube until the shadow is at a minimum. You?ll be pointed at the sun, or at least close enough to find it with a little sweeping and a low-power eyepiece to bring it into view. It can be difficult, even with the shadow method. An other trick to try after you?ve got it close with the shadow if your still not having any luck getting the sun in the field...take the eyepiece out of the focuser. Then look into the focuser...you won?t see an image but when the sun gets close you?ll see a flicker of brightness coming through the mirrors. Then pop the eyepiece back in and you should have it.