Rensselaer Astrophysical Society

Telescopes

There are many different sizes and shapes of telescopes, but they can all be broken down into three categories: refractors, reflectors, and catadioptrics. There are many different types within each of these categories, and each one has unique advantages. There is no one telescope that is the best. It all depends on what you are trying to look at and what characteristics are important to you.

Refracting telescopes use lenses to focus light and magnify images. The most basic design has two lenses, the objective lens and the eye piece. The two lenses are placed just the right distance apart so that they're focal points coincide. The objective lens is larger and has a longer focal length than the eyepiece, and this causes the image to be magnified (if you look through a refractor the wrong way, objects appear to be smaller and further away). More advanced designs have extra lenses that correct color aberration, which is a discoloring of the image due to a prism like effect (passing white light through a lens separates it into different wavelengths).

Refractors perform very well, especially for lunar and planetary observing. However, They are less useful for dimmer objects, such as galaxies. One of the main functions of a telescope is to gather and focus light (many argue that this is more important than magnification). In a refractor, this is accomplished by the objective lens. The larger the lens (usually referred to as aperture size), the greater the telescope's light gathering ability. This is why telescopes allow us to see objects invisible to the naked eye. Many objects are so dim that your pupil is not large enough to gather enough of the light from that object to allow you to see it. Telescopes gather light over a larger area, then focus it. (Like burning things with a magnifying glass. It's not because it magnifies the sun's light, it's because it focuses it.) However, it is very expensive to manufacture large lenses. For this reason, you seldom see a refractor with an aperture size larger than 4 inches. This isn't large enough to allow you to see many objects, so refractors are best used on bright objects, such as the moon or the planets.

Reflectors use a mirror to gather and focus the light. Large mirrors are cheaper to manufacture then large lenses, and they have the benefit of not introducing any color aberration (reflectors do have different types of deficiencies, such as spherical aberration, which is caused by imperfections in the shape of the mirror).

Catadiotric telescopes use a combination of mirrors and lenses. An example is a Cassagranian telescope, like our 16". The light enters through the aperture, is reflected back by a mirror at the bottom of the tube, is reflected back again by a secondary mirror in the top of the tube, and passes through a hole in the primary mirror into an eyepiece. This type of telescope has the greatest variety.

Telescope Mounts

There are two main classes of telescope mounts: altazimuth and equatorial. Altazamuth mounts have two axes upon which the telescope can move, altitude and azimuth (side to side, and up and down). This design is easy to use because it is very intuitive. An equatorial mount also has two axes upon which the telescope moves, and a third axis that is set and is not changed unless the geographic location of the telescope is changed significantly. This type of mount requires more getting used to because the axes are not intuitive. Rather then up and down and side to side, equatorial mounts move along declination and right ascension (see explanation of celestial coordinates). This makes tracking objects much more convenient.

I should also mention the Dobsonian telescope. This is often spoken of as if it were a different type of telescope. It is actually a standard Newtonian reflector that is placed on a unique type of altazimuth mount, invented by John Dobson. A Dobsonian type mount is very cheap and easy to construct, easy to use, and can support large telescopes. This type of mount has made it possible for amateurs to afford much larger aperture telescopes then ever before.