by OpticsPlanet Guest Expert

08/21/13

Number on the Telescope Eyepiece

The number on the eyepiece is the focal length of the eyepiece. It is not the magnification of the eyepiece.

Eyepiece Magnification

The magnification of any telescope eyepiece used with your telescope will be the focal length of the telescope (consult your manual) divided by the focal length of the eyepiece. A telescope with a focal length of 1200mm will yield a magnification of 60x when you insert a 20mm eyepiece into the focuser. A telescope with a focal length of only 600 mm, however, will yield only 30x when used with the same 20mm eyepiece.

Telescope Eyepieces for Beginners

ALWAYS start observing with the lowest magnification eyepiece available until you become skilled in the use of your telescope. This will be the eyepiece marked with the BIG number (longer focal length), not one of the smaller numbers. Again, the number you see on the eyepiece is the focal length, not the magnification.

Low vs High Magnification

A low magnification eyepiece has a wider field of view (the amount of sky you see when looking through the eyepiece) than a high magnification telescope eyepieces. The low-magnification eyepiece therefore makes it easier to “capture” an object you are trying to find in your telescope. Your lowest magnification eyepiece will also give you the sharpest image as well as the brightest image.

Once you have located an object with your low magnification eyepiece, move the telescope so the object is as close to the center of the telescope field of view as possible. Replace the low magnification eyepiece with one of higher magnification. If the object is not visible after you have changed to the high magnification eyepiece, go back to the low magnification eyepiece and start again.

Increasing Magnification

A basic law of optics states that as magnification increases, image brightness decreases. In fact, if you increase magnification enough, an object will become too faint to see. This happens sooner in a small telescope than large telescopes.

Barlow Lens for Telescopes

A Barlow lens is a lens that you use with your eyepiece. A Barlow lens will double (2x Barlow) or even triple (3x Barlow) the magnification of any eyepiece that you attach to it. To use a Barlow lens, remove the eyepiece from the focuser, insert the Barlow and then insert the eyepiece into the Barlow. Remember, though, that a Barlow is best used with low magnification (long focal length) eyepieces. When used with high magnification eyepieces, it may produce more magnification than your telescope can use.

Disappearing Objects in the Field of View

The telescope is not only magnifying the object you are observing in the sky, it is also magnifying the earth’s rotation! The more magnification you use in your telescope, the quicker an object drifts out of the field of view. Manual telescope mounts will require you to continually “recapture” the object by moving your telescope slightly. Motorized mounts move the telescope for you and keep the object in the eyepiece.

Telescope Magnification Use

Use only enough magnification to provide a useable image. When you reach a point where the image has become so blurred as to lose useful detail, you are using too much magnification! At what point this happens depends on the object you are observing, the seeing conditions (atmospheric clarity and stability) and the size of your telescope (you can get more magnification out of a large telescope before images begin to blur).

Observing Expectations from your Telescope

You will be able to see many of the same things you see in magazines and books, but the images produced in your telescope will smaller and less spectacular. The images in magazines and books are produced by large observatory telescopes that take long exposure photographs with special cameras. It simply isn’t realistic to expect a small amateur telescope to produce visual images of the same quality.

Telescope Observing Benefits

There is so much more to that little smudge of light you see in your eyepiece than meets the eye! Spend a little time and effort to learn about the things you see in your telescope and you will appreciate them much more. Remember, that little smudge of light may actually contain billions of stars and its light may have taken many millions of years to reach your telescope.

Besides, much of the thrill in amateur astronomy is seeing the glories of the night sky with your own two eyes. The difference between seeing a picture of Saturn in a book and seeing Saturn in your backyard through a telescope is a lot like the difference between seeing pictures of Alaska in a book and going to Alaska to see it for yourself.

Amateur Astronomy

Amateur astronomy is also about the challenge of finding faint, hard to see objects. This often frustrates the beginner (hence the popularity of computer GOTO telescopes) but it also keeps the die-hard enthusiasts out late into the night. Even if you use a GOTO telescope, you owe it to yourself to learn how to navigate by means of a star map . There are no words to describe the thrill of finally seeing a faint galaxy or nebula after several hours or even nights of looking for it.

Lastly, there is a great amount of satisfaction that comes with knowing your way around the night sky. At a time when many of us feel alienated from the natural world, astronomy provides a way to reconnect to the universe around us.