How to Select a Telescope

Choosing between a refracting and a reflecting telescope is the easy part of deciding on which telescope is right. The devil is in the details. Here are some things to know about when buying a telescope:

Eyepiece Tubes generally come in a standard 1.25 inch size. This makes available a wide variety of interchangeable eyepieces. A telescope with a different size eyepiece may limit future choices in this regard.

Electronic Eyepiece: Some scopes come with an electronic eyepiece that can be hooked up to a monitor, a TV set or even a digital camera. For those wishing to do digital photography with a telescope, this is a vital feature. Even very inexpensive models such can come with an electronic eyepiece.

Eyepieces in General: Many Telescopes come with two different eyepieces which fit in the standard-sized eyepiece tubes. One is 25mm the other 10mm. The magnification is based on a ratio of the focal length of the tube to the focal length of the lens. For example, a telescope with a 400mm focal length with a 10mm eyepiece will result in (400/10) 40X magnification. While the 25mm eyepiece will result in a (400/25) 16X magnification. The difference will be that the longer eyepiece will result in a sharper image, while the shorter one will give a wider, but less clear image. So when looking at eyepieces for telescopes keep these factors in mind.

Telescope Mounts: There are two types of mounts, alt-azimuth and equatorial. The alt-azimuth mounts move in a stair step pattern when following a star (in fairly small increments), going first across, then up or down, then across, et cetera. The equatorial mount is superior as it moves more smoothly in an arc across the sky. This keeps images from seeming to move in a zig-zag pattern as the Earth rotates. Higher quality scopes generally utilize an equatorial mount.

Software: Yes, it is true, now-a-days even telescopes often come with software! It is basically used to help you find and identify various celestial objects. This on-board software can have any number of objects catalogued. When a telescope is properly aligned software will help point in the direction of the desired object. No more analyzing star-charts, upside down and sideways trying to see what the cartographer sees. Get one of these software equiped telescope pointed north and then level it based on your latitude (which you can find by looking at any map) and the computer software (called Autostar) will point the telescope automatically at whatever object is selected.

Lens Coatings can be important for precision work when trying to get every ounce you can out of a telescope. Approximately four percent of light is lost when an image passes through a glass surface or bounces off a mirror. This means that four percent can be lost as the light passes into the primary lens and another four percent as it passes out the other side of it. Add in all the mirrors and pretty soon a considerable amount of light can be lost. Coatings can help minimize these losses.

Choose your telescope carefully. A good place to begin to look is at They have a wide variety of telescopes available. Many have product reviews and comprehensive information to help in making a decision.

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Interesting Fact:

Situating a telescope in space (the Hubble Telescope) makes it easier to see celestial objects because it avoids the disrupting effects of the atmosphere.

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