Night skies in Spain
Axarquia, Costa del Sol, Andalucia, Spain

TELESCOPES

There are three basic types of telescope used in amateur astronomy, as described below.

Refracting Telescopes
Refractor telescope Refractors have the appearance of a “typical” telescope. They have long, thin tubes where light passes in a straight line from the front objective lens directly to the eyepiece at the opposite end of the tube. Refractors have a lens as the primary device for gathering light, using the property of refraction to bend light when passing from one property to another (air and glass), to collect and focus light.

Refractors typically show a fringe or rainbow of colour around bright objects, known as chromatic aberration. Achromatic refractors use lenses that are not extensively corrected to prevent chromatic aberration but instead usually use coated lenses to reduce the problem. Apochromatic refractors use either multiple-lens designs and/or lenses made of other types of glass (such as fluorite) to prevent chromatic aberration. Apochromatic refractors are considerably more expensive than achromatic refractors.

The advantages of a good quality refractor are that they have a clear aperture with no central obstruction to cause scattering of light. They are low in maintenance as lenses do not require re-coating as mirrors do. The optical tube assembly does not require collimation. The lens is fixed into the tube and does not generally become misaligned.

The main disadvantage of refractors is the cost as producing a large achromatic or apochromatic lens is significantly more expensive then producing the same size high-quality mirror.

Refractor telescopes are excellent for lunar and planetary viewing offering crisp, sharp images. They are also superb for terrestrial observing as the image is seen the right way up.

Reflecting Telescopes
Reflector telescope Reflectors usually use a concave parabolic primary mirror to collect and focus incoming light. In the diagram, the focus is inside the telescope. This is the prime focus and, in some reflector telescopes, observations can be made at this point. More commonly, mirror arrangements are used to transport the light from the focus to a secondary flat diagonal mirror that reflects the image out of an opening at the side of the main tube into an eyepiece to an external observer. These two types of mirror arrangement are nominated the Cassegrain focus and the Newtonian focus. Unless multiple mirrors or lenses are used, images are seen upside down therefore reflectors are not commonly used for terrestrial observation.

Advantages of a reflector are that no chromatic aberration is suffered. Also mirrors have just one optical surface as opposed to between four and eight with an apochromatic lens and are therefore much less expensive to build and support. For these reasons, with few exceptions, the larger optical telescopes are reflecting, rather than refracting, telescopes.

Disadvantages of reflectors are the scattering of light and consequent loss of contrast caused by the “central obstruction” of the secondary mirror. Newtonian reflectors suffer from coma which is a defect causing stars at the edge of the field to look like comets. The smaller the focal ratio of a Newtonian focus, the greater the coma. Coma correctors may be used to increase the field of view. After some years, mirrors may require re-coating. Reflectors are sensitive to being moved or bumped and, if not in a permanent position, need to be collimated prior to each session of observation.

Compound (Catadioptric) Telescopes
Compound telescope This type of telescope uses a combination of lenses and mirrors to gather light. A typical catadioptric feature being a correcting lens built into the optical tube that extends the focal length without the need for an extra long tube.

Comparing
Each type of telescope has advantages and disadvantages and some are better at observation of different subjects as simplified below:
  • In general, refractors are good for lunar and planetary observation whilst reflectors are good for deep-sky observation. Compound telescopes are good general instruments.
  • In light polluted urban skies, compound and refractor telescopes tend to be better than reflectors.
  • In moderately light polluted suburban skies the performance is equal.
  • In dark rural skies compound and reflectors have the edge on refractors because they are better able to collect light.

Aperture
The aperture of a telescope refers to the diameter of the objective lens of a refractor or objective mirror of a reflector. The ability to gather light is proportional to the size of aperture and the more light a scope can gather, the better the image seen.

Magnification
A telescope's ability to enlarge an image is dependent on the combination of lenses used. It is the eyepiece(s) that perform magnification. Differences in magnification are controlled by using different eyepieces, making aperture a more significant feature in choosing a telescope than any `declared` magnification.

Focal Ratio
Focal ratio is calculated by dividing focal length by aperture size. The focal length is measured from the main lens or mirror to where the light converges at the focal point. A telescope with an aperture of 4.5 inches and focal length of 45 inches, has a focal ratio of f10. A higher focal ratio does not necessarily mean a higher quality image and remember a higher focal ratio with the same size aperture means a longer tube which is more difficult to transport.

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