Buying a Telescope

If you are considering buying a telescope, you are encouraged to come along to our meetings to speak to members who already own a telescope. You will find that the telescopes owned by our members cover all of the common types available on the market and we will be able to give you impartial advice prior to your purchase.

Types of Telescope

Before buying a telescope, you should be aware of the three basic types.

1. The Refractor

This uses a large lens, known as the objective to focus light from the object being observed. An eyepiece, placed behind the focal point of the objective (the point at which the the rays of light passing through the lens converge), is used to produce a magnified image. Telescopes used for non-astronomical purposes are almost universally refractors.

Advantages: Good for planetary observing. Optical tube is sealed and so requires next to no maintenance. Low entry price (100+).

Disadvantages: Expensive, especially so for large apertures.

2. The Reflector

This uses a large concave mirror, known as the Primary Mirror to focus light from the object being observed. A smaller mirror, known as the Secondary Mirror, is used to divert the reflected light away from the direction of the object and towards the eyepiece.

There are two main types of reflector telescope...

The Cassegrain Reflector, in which a convex secondary mirror reflects the light back towards the primary mirror, where it passes through a hole to the eyepiece.

The Newtonian Reflector, in which a flat secondary mirror reflects the light by 90 degrees to an eyepiece on the side of the tube.

Advantages: Good for observing faint objects. Relatively cheap.

Disadvantages: Higher entry price (200+). The presence of the secondary mirror reduces the contrast of the image. Optical tube is open and so needs periodic maintenance.

3. The Catadioptric

This is similar to a reflector. However instead of the primary mirror focusing light directly from the object being observed, the light first passes through a specially shaped piece of glass known as a Corrector Plate. The secondary mirror is almost always held in place by the corrector plate. Catadioptrics can be either Newtonians or Cassegrains, but the latter is more common.

Advantages: Good all round performance. Compact. Optical tube is sealed and so requires next to no maintenance.

Disadvantages: Expensive. High entry price (500+). The presence of the secondary mirror reduces the contrast of the image.

Choosing Telescopes

There are four things you should consider when buying a telescope...

1. The Aperture

Most astronomical objects are very faint. Therefore, the diameter of the objective lens or primary mirror (the aperture) should be as large as possible in order to collect sufficient light. Common amateur telescopes have apertures ranging from about 75mm (3 in) to about 300mm (12 in), although models are available with far greater apertures.

The amount of light collected depends on the area of the objective lens or primary mirror, and so is proportional to the square of the aperture. For example a 200mm (8 in) telescope will collect four times as much light as a 100mm (4 in) telescope.

The aperture also places a limit on the maximum magnification that can usefully be used. As a rough rule, the maximum useful magnification is 2 times the aperture in millimetres (or 50 times the aperture in inches).

There are some telescopes that have magnifications which break this rule. These are to be avoided. Although they are certainly capable of such magnification, the resulting image will be next to useless. In fact, any telescope that gives magnification as its primary selling point is to be avoided, as it is likely to be awful.

2. The Optical Quality

Optical quality is far more difficult to quantify. More expensive models state the quality of their optics in terms of the average deviation from ideally shaped optical surfaces. This measurement is usually given in terms of the wavelength of light (in the region of 500 nanometres), for example '1/16th wave optics'.

In general, the best way to determine the quality of a telescope is simply to read a review of it, or to ask someone who has used that particular model.

3. The Mount

Although it sounds like an aside, the mount is just as important as the telescope itself. If a telescope is not held steadily in position, it will be almost impossible to observe anything through it.

The most simple form of telescope mount is the alt-azimuth. The telescope is positioned by adjusting its elevation from the horizontal (the altitude) and its rotation about the vertical axis (the azimuth).

Although cheap and light, alt-azimuth mounts have a major problem. Although the rotation of the earth might seem small, it takes only four minutes to turn one degree. As the field of view of a telescope is often much less than this, an object will rapidly drift out of view, especially at high magnifications. To track an object (i.e. keep it in the field of view), both axes have to be simultaneously adjusted. This is not an easy task.

The solution to this problem is the equatorial mount. This has its axes of rotation tilted so that one of them is parallel to the earths north-south axis. As the earth rotates, an object can be tracked by rotating the telescope about this axis at an equal but opposite rate. Some equatorial mounts are equipped with a motor to do this automatically. Even without the motor, tracking objects is much easier.

Computer controlled mounts are available. With these, an object can be selected from a built in database and the telescope will automatically point towards it and track it.

4. Accessories

The most important accessories are eyepieces, without which a telescope is useless. Therefore when comparing prices, it is essential to add on the price of eyepieces if they are not included.

Eyepieces come in different focal lengths. The magnification of a telescope is equal to the focal length of the objective lens or primary divided by the focal length of the eyepiece. For example, a telescope with a primary mirror with a 1000mm (40 in) focal length, fitted with a 25mm (1 in) eyepiece will give a magnification of 40. By using eyepieces with different focal lengths, the magnification can be altered.

It is best to have more than one eyepiece, as different objects look best different magnifications. Furthermore, you can use a Barlow Lens to increase the magnification, or a Focal Reducer to decrease it. These are devices which fit between the telescope and eyepiece, multiplying the magnification by a fixed amount (with focal reducers, this multiple is less than one). By either using or not using such a device, the number of available magnifications can be doubled.

Another important accessory is the finderscope, a small telescope that points in the same direction as the main telescope. It gives a much wider field of view than the main telescope and so enables objects to be easily found. Other useful but non-essential accessories include solar projection screens, for safe viewing of the sun and light filters for enhancing the view of certain objects.

Telescope Guide

The following is a guide to the types of instrument you can expect to buy for a given budget. This includes reviews of telescopes that are owned by society members.

Less than 100

  • The best option is to buy a pair of binoculars. As with astronomical telescopes, it's the aperture that counts, not the magnification. Go for models with with apertures of at least 50mm (2 in). Don't be tempted by models offering magnifications greater than 10x as it will be impossible to keep them sufficiently still. The ability of binoculars can be vastly improved by attaching them to a tripod. Go for a tripod that is tall enough to comfortably accommodate near vertical viewing. With a tripod, slightly higher magnifications are permissible, but don't go any higher than 20x.

    Helios Binoculars (About 50)

  • There are many extremely poor telescopes available in this price range. They can be found in shopping catalogues, department stores and other general purpose shops. They give magnification as their primary selling point and the value stated is usually absolutely absurd. Such telescopes are next to useless and should be avoided at all costs. For the same amount of money you can buy a vastly superior instrument (albeit with less grandiose specifications).

100 to 200

  • Decent refractors start to become available, starting with apertures of about 75mm (3 in).

  • Binoculars are still a good option. In this price range, models with larger apertures and better quality optics start to become available.

  • Reflectors in this price range are available, but they are best avoided.

200 to 500

  • Binoculars are still a good option. Very large aperture models start to become available.

  • Refractors are still a good option. Larger and better quality models are available.

  • Decent reflectors start to become available, starting with apertures of about 110mm (4.5 in).

    TAL 1 Reflector (200 to 250)

More than 500

  • Catadioptrics start to become available.

    Celestron Super Polaris C8 Catadioptric (About 1200)

  • Larger Reflectors are available.

  • Refractors and binoculars of superlative quality start to become available.

  • Computer controlled mounts start to become available.