Astronomical (star-gazing) Telescopes are not as simple as spotting scopes so we have produced this guide to help you in your buying decision.
The most important specification of any telescope is the aperture (objective) which is the diameter of the main lens or mirror of the telescope. The bigger the diameter the more light they collect.
Magnification is not important for astronomy but can be calculated by simply dividing the focal length of the telescope by the focal length of the eyepiece. The addition of a Barlow Lens can increase the magnification. The “highest practical power” is calculated using the shortest eyepiece supplied in the box.
The light collected by the telescope has to be focused to form an image - reflectors use mirrors to bend the incoming light and refractors use lenses. Compound telescopes combine elements of both.
Reflecting telescopes use a mirror to form the image and the eyepiece is located at the top side of the main tube. A reflector costs the least per inch of aperture compared to refractors and catadioptrics since mirrors can be produced at less cost than lenses. They are not suited for terrestrial applications.
Reflectors may require a little more care and maintenance because the tube is open to the air, which means dust on the optics even if the tube is kept under wraps.
Refracting telescopes are more expensive as they use lenses to form the image. Their eyepiece is located at the rear of the telescope. Refractors produce an image that is reversed or "inverted" - no problem for astronomy but a nuisance for terrestrial observing. An erecting prism will correct the image.
These use a combination of lenses and mirrors, offer compact tubes and are relatively light weight. The two popular designs you'll often see are called Maksutov-Cassegrains (MAK) and Schmidt-Cassegrains. They are more expensive than reflectors of equal aperture but their closed tube design reduces image degrading air current.
Telescopes are supplied with a tripod and mount - the type of mount determines how easy it is to follow a star while viewing it. Because you cannot change the mount later you must take this into consideration when choosing a telescope.
The Altazimuth mount moves in altitude (up & down) and azimuth (left & right) directions The yoke option found on entry levels telescopes is the simplest with no fine controls. The better altazimuth mounts have knobs for making fine adjustments to track objects.
The Equatorial mount allows users to follow the rotation of the sky as Earth turns so it is much easier to follow stars, planets, and other astronomical objects than an Altazimuth mount. However they are heavy and trickier to set-up, with a steeper learning curve for beginners but do suit astro-photography.
Computerised Mounts have small motors to move the telescope around the sky with the push of a button. In the more advanced models of this type, often called "Go To" telescopes, a small computer is built into the hand control. Once you've entered the current date, time, and your location the scope can point itself to, and track, thousands of celestial objects.
Because these motor-powered mounts take into account Earth’s rotation and can automatically point you to a selected object and track it they are a great option for astro-photography.
Celestron has reinvented the manual telescope with StarSense Explorer—the first telescope that uses your smartphone to analyze the night sky and calculate its position in real time.
StarSense Explorer uses the smartphone's camera as its “eye” and the smartphone's processing power as its “brain.” The camera looks out at the night sky, and then the processor figures out where the telescope is pointing based upon the calculated center coordinates of the captured image. .
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