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Telescope Guide
A telescope is an optical instrument that is used to observe far off objects in the sky. There are a variety of sizes and styles of telescopes. Deciding on a size and style of your telescope depends on the kind of observing you plan to do.
  Commonly Used Phrases and Terms


Refractors are also known as “dioptrics”. Refractors are characterized by are a long narrow optical tube. The diameter of the optical tube (usually denoted in millimeters) varies with the size of the objective (front) lens size. Light passes in a straight line from the front objective lens to a diagonal mirror located in the rear of the optical tube and directly to the eyepiece. Refractor telescopes are suitable for both celestial (night sky) and terrestrial (land) viewing. Note: Objects viewed through a refractor appear wrong reading left to right but right reading up and down.

Newtonian Reflectors
Newtonians Reflectors are also known as “catoptrics”. They use a concave parabolic primary mirror which collects and focuses incoming light onto a flat secondary (diagonal) mirror. The secondary mirror then reflects the image out of an opening at the side of the main optical tube and into the eyepiece. Reflector telescopes are most suitable for celestial (night sky) viewing. Note: Objects viewed through a reflector appear vertically inverted.


The altazimuth (AltAz) is the simplest type of mount. It has two motions – up and down (altitude) and side-to-side/horizontal (azimuth). Better quality altazimuth mounts will have slow-motion knobs for making precise adjustments and aid in keeping tracking motion smooth. Altazimuth mounts are good for terrestrial (land) viewing and for scanning the night sky at lower power. They are not recommended for deep sky photography.

Equatorial mounts are superior to non-computerized altazimuth mounts for astronomical observing over long periods of time. As the earth rotates around its axis, the stationary stars appear to move across the sky. A telescope on an equatorial mount can be aimed at a celestial object and easily guided using the manual slow-motion controls to follow the object across the sky and keep it in the view of the telescope. The equatorial mount is rotated on one axis (polar/right ascension) adjusted to your latitude and that axis is aligned to make it parallel to the Earth’s axis, so that if that axis is turned at the same rate of speed as the Earth, but in the opposite direction, objects will appear to sit still when viewed through the telescope. Equatorial mounts are preferable for deep sky observing.

Focal Length
Focal length is the distance (in mm.), in an optical system. It is a measurement from the lens (or primary mirror) to the point where the telescope is in focus (focal point). The longer the focal length of the telescope, generally the more power it has, the larger the image and the smaller the field of view. Use the following formula to determine the focal length of a telescope: Focal length is the aperture (in mm) times the focal ratio. For example, the focal length of an 8” (203.2mm) aperture with a focal ratio of f/10 would be 203.2 x 10 = 2032mm.

Objective Lens Size (Aperture)
Aperture size is the single most important factor in choosing a telescope. Aperture is the diameter of the lens or mirror (in mm.). Since the primary function of a telescope is to collect or “gather” incoming light, at any given magnification the larger the aperture the better the viewed image will be. It is important to remember that the larger the objective lens mirror the better, but the size of the lens or mirror diameter limits the amount of power that can be used. The higher the magnification, the less bright the image will be since more light will be distributed over a larger area.

Magnification (Power)
Magnification is the number of times the object being viewed is enlarged. The magnification of a telescope is a function of the focal lengths of the tube and eyepieces. To calculate magnification: Divide the focal length of optical tube by the focal length of eyepiece that is being used. For example, a 700mm telescope with a 4mm eyepiece would magnify objects 175x (700 divided by 4).


Barlow Lens
This is an auxiliary eyepiece lens that increases (multiplies) power by a given factor, which is usually 2x or 3x.

Erecting Eye Prism
This lens corrects the inverted (upside down) image when viewing land-based objects seen through a reflector type telescope.

This is a low-powered sighting scope mounted on the top of the telescope’s optical tube and has a reticle or an illuminated red dot that allows you to quickly locate the object being viewed.

Focusing Knob
Controls the rack and pinion focusing system and enables smooth and precise movement of the eyepiece to bring objects into sharp focus.

Telescope Eyepieces
Eyepieces are interchangeable and provide a variety of magnification and field of view options. Use an eyepiece with low magnification to view the Moon, planets, star clusters, nebulae and constellations. The lower the magnification, the wider the field of view. An eyepiece with high magnification enables you to view details in the mountains, ridges and craters of the Moon.