making astronomy accessible to all


Types of Telescope

There are three main types; refractor, reflector and Catadioptric (Cat). For all telescopes we tend to refer to their size in terms of the objective or aperture, not their length. So a 150mm telescope has an aperture of 150mm (it isn't 150mm long). Aperture is very important since it is the aperture which determines how much light the telescope will capture. The focal length of a telescope will determine its magnifying capability. The focal ratio (f number) is the ratio of the focal length to the aperture size (so for instance, a 1m focal length telescope with a 150mm objective lens would be an f6 telescope).

Refractors have a large objective lens in front, and focus the light through 'refraction'. There are various configurations of lens to reduce the chromatic aberration of the lens (such as Apochromatic, Doublet, Triplet, Petzval Quadruplet etc), but they all work in basically the way illustrated.
Never go out of collimation, simple to use
Excellent contrast
Reasonably priced up to 150mm diameter
Difficult or impossible to service if required
Suffer from chromatic aberration
Expensive for better (colour corrected) optics or larger apertures.

The long focal length refractor is the instrument of choice for planetary work. Short focal length refractors have become very popular with astro-imagers.

The picture is of a Newtonian reflector system (which is very popular), but there are other systems, such as the Ritchey Chr'tien system (RC), which looks totally different, but is also fully reflective.
Much cheaper for large aperture instruments
Greater light gathering power
Do not suffer from chromatic aberration
Require regular collimation
Lower contrast than refractors
Can suffer from coma (off centre aberration)

The Newtonian reflector is a classic and popular type of telescope for amateurs, offering the most aperture per pound. They do however suffer from tube currents until cooled down if not stored outside and frequently need collimation. Large, short focal length reflectors are the preferred instrument for deep sky observing (observing faint objects outside our galaxy).

Essentially a compromise between refractors and pure reflectors; the Cat is an ideal solution for many serious amateurs because it combines the capability to have long focal length and large aperture in a compact telescope which is rugged and easy to use. Many 'Cats' focus by moving the primary mirror, but there are many forms this type of telescope on the market which have different optical configurations, but all of which include the elements in the diagram.

More on telescopes

For more information about selecting or buying a telescope, read the article Buying a telescope.


Magnification is provided by the eyepiece. Most astronomical telescopes feature interchangeable eyepieces. The magnification they provide is simply the ratio of the focal length of the primary, divided by the focal length of the eyepiece.
Limit: the maximum magnification that you can expect a telescope to operate at is aproximately 2X the aperture in millimetres. So for a 150mm aperture telescope; 300X is the highest magnification it could achieve. However, at any magnification over 150X you are likely to observe image degradation or artefacts unless you are using extremely fine optics.

Field of view

The Field of view of a telescope is determined by the magnification you are operating at and the eyepiece field of view (some eyepieces have wider FOV than others). The FOV/Magnification will give a rough idea. So for an average (50 degrees FOV eyepiece) operating at 150X magnification the size of object in the sky that would fill your field of view would be 50/150 or 1/3 degree (20 arc minutes).


Before considering what telescope you need (or what telescope you have); you should consider the mount, and mounting of the telescope. It is essential that the telescope is mounted securely on a stable platform prior to starting observation. There are various forms of mount available; we will have an article on that subject shortly.

Article by Noel Clark

Mid-Kent Astronomical Society
Website by and © Delta Consultancy Services