Telescope Terms


Telescope – Before you can determine what type of telescope you really need, you should understand the following telescope terms.

Aperture:  The most important aspect of any telescope is its aperture, the diameter of its main optical component, which can be either a lens or a mirror.  The aperture determines both its light-gathering ability (how bright the image appears) and its resolving power (how sharp the image appears).  The bigger the aperture the better.  A telescope with a 6-inch aperture has a diameter 24 times as great as our fully dilated pupil.  With a 6-inch telescope you can discern craters on the Moon as small as about a mile across — half the size of those visible in a 3-inch telescope under the same conditions using the same magnification.  The same two instruments turned toward a faint galaxy on a moonless night would tell an even more dramatic story.  Because the surface area of a 6-inch mirror is four times that of a 3-inch mirror, this telescope collects four times as much light, meaning the galaxy would appear four times brighter.

Magnification:  The aperture on a telescope is not what determines its magnification (“power”).  Any telescope can provide an almost infinite range of magnifications, depending on the eyepiece you selected but don’t get the idea that super-high powers will do you any good.  The two main factors that limit the power of the telescope is its aperture and the atmospheric conditions.  Only so much detail exists in the image created by the main mirror or lens on a telescope, so you must find the optimum magnification.  This is why observers generally use low powers for looking at faint things like galaxies and nebulae, and no more than medium-high powers for bright things like the Moon and planets.  If you enlarge a photograph too much, all it will show you is the grain in the film, so too will excess magnification make your target blurry.  There is a simple rule to find the top useful magnification (assuming perfect optics with usually steady atmospheric conditions): 50 times the aperture of the telescope in inches, or twice its aperture in millimeters.  This means that a high-quality 4-inch (100-mm) telescope should not be pushed beyond about 200x.  For a small instrument that has good optics, it will show you Saturn’s rings or the principal cloud belts on Jupiter, since these can be seen at a magnification of 75x.

A refractor telescope has a primary lens at the front end bringing light to a focus point and the eyepiece is at the rear of the telescope.  This type of telescope is commonly used for land viewing but are also used for astronomy.  Indeed it was the first astronomical telescope.  They tend to produce clear sharp images and are at an advantage in the suburbs where light pollution is a huge factor to contend with.  A large aperture refractor telescope can be difficult to use and very expensive, so they tend to be rarely used in practice.

A reflector telescope uses a mirror for light gathering and reflect the light to the eyepiece via another small mirror called the secondary mirror.  A reflector telescope tends to offer a combination of medium to large apertures with ease of use and are most often used for astronomy.  A larger reflector telescope is sometimes mounted on simple lazy-susan turntables known as Dobsonian mounts.  They are extremely easy to use and their large apertures offer excellent deep-sky viewing.

A Catadioptric telescope, such as Schmidt-Cassegrains & Maksetov-Cassegrains, package relatively large apertures with short tubes for ease of handling and portability.  They achieve this by reflecting the gathered light twice inside the tube before passing it out to the eyepiece.  They are usually mounted on computerized mounts, making navigation easy (after you learn how to align and control them properly).  Their more complicated construction tends to make them fairly expensive but their all around capabilities make them very popular.

Focus Systems:  There are basically three different types of focusing mechanisms used on a telescope: Helical, Rack & pinion, and Knob focusers.  The characteristics of each type can make a difference in how well adapted it is to your specific application and even how much you will enjoy using your scope.

  • Helical focus has knurled or rubberized collars around the scope barrel where the focus is changed fairly rapidly when turning the focusing ring.  Helical focusers work well for observing objects that are changing distances, near to far, quite rapidly.
  • Rack & pinion designs are commonly found on a astronomical telescope and usually offer fairly fast, smooth focusing, but most components are external and subject to potential deterioration from dust and moisture over time.
  • Knob focus has a slower action but allow for precise, accurate focusing and are probably the most preferred for general purpose birding or nature observation.  The typical close focus distance for most terrestrial scopes is about 20 to 30 feet, however, certain designs (catadioptric) will allow a near focus down to 5 to 15 feet.

Telescope Mounts:  A telescope is useless without a means of supporting it to allow the observer to use it.  However good the telescope, it will be limited by the quality of the mount.  Cheap scopes tend to come on cheap mounts that allow the telescope to move with the wind or even vibrations from passing traffic.

  • Simple mounts like the alt-azimuth mount are usually used for land viewing and more casual astronomy.  They allow up-down and side to side movement of the telescope tube. The Dobsonian mount for large aperture reflectors is actually a type of alt-azimuth mount.
  • Equatorial mounts are more complex and allow the telescope tube to move through an arc which is essential for tracking an object as it moves through our sky.  They can be motor driven and controlled through a hand controller.  The many variations of these tend to be popular with serious astronomers and astrophotographers.


This entry was posted in Telescopes. Bookmark the permalink.

Leave a Reply