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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.

Often, the choice is limited by budget, but generally there is a simple rule - the bigger the diameter of the object lens or mirror, the more light the telescope can gather and therefore the brighter the image will be - do not worry about magnification as this is not important. This may seem a strange thing to say when the purpose of a telescope is to bring things closer, but a highly magnified image on a small telescope will be so dim that details will be hard to see - a far better view will be seen in a larger telescope even at a lower magnification.

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 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. A good telescope will normally be supplied with at least one eyepiece (usually a 25mm), and usually a 90 degree diagonal so that you don't strain your neck when the telescope is pointing straight up. Most astronomical telescopes produce an inverted image but this doesn't matter when viewing the night sky, terrestrial telescopes have an extra lens (usually called an erecting eyepiece) which turns the image the right way up, but every time light passes through a lens it dims a little, Astronomers want as much light going into the telescope as possible so that is why most astronomical telescopes produce an inverted image - they don't have that extra lens to flip the image.

Advantages:

Good for planetary observing, the refractor generally produces a good contrast on bright objects like the planets due to there being no obstruction in the way of incoming light as on a Newtonian and Catadioptric telescope (see below) The optical tube is sealed and so requires next to no maintenance. If your interest is primarily looking at objects in the solar system then a good 4" refractor would an ideal choice.

Disadvantages:

larger apertures are expensive, particularly the Apochromatic versions. Cheaper models do suffer from some false colour around bright objects.

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.

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:

As a general rule, reflectors are much cheaper than refractors for any given size, so it is possible to get a fairly large telescope for a reasonable cost - an 8" reflector can be purchased for a few hundred pounds whereas an 8" refractor would cost many thousands of pounds. Reflectors are generally used for observing "deep sky" objects such as galaxies, nebulae and star clusters etc, and although they can be used on planets as well, the images they produce tend to be slightly lower in contrast than that of a refractor due to the secondary mirror blocking part of the incoming light

Disadvantages:

A Newtonian is generally an open tube, and so requires more maintenance than a closed tube telescope due to possible ingress of dust etc. Care must be taken that nothing drops into the tube and damages the mirror Note: a telescope mirror is not like an ordinary mirror, in a telescope mirror the reflective coating is applied to the front surface of the glass, and must NEVER be touched or wiped otherwise the coating will be seriously damaged. If the mirror gets dusty, blow the dust away or else have the mirror cleaned by someone who knows how to do this - a few specks of dust will not do any harm and will not degrade the image in any way, better to leave well alone than risk scratching the mirror. The optics in a Newtonian must be precisely aligned in order to get the best performance, this is called collimation - in other words the mirrors must line up with each other or else the image will not be at it's best although you will probably still be able to see objects, they just won't be as good as they could be with the optics properly aligned. The process of collimation is not difficult and is easily learned, some people collimate before each observing session, but generally this is not necessary unless something is moved, or if the telescope is bumped during transport etc.

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. The most popular type of this telescope is called a Schmitt Cassegrain Telescope or SCT for short.

Advantages:

Good all round performance, can be used on a wide range of objects and does well on Planets and Deep Sky alike. Compact. Optical tube is sealed and so requires next to no maintenance.

Disadvantages:

Expensive. High entry price, The cost of an SCT is likely to be quite a bit more than an equivalent aperture Newtonian. The presence of the secondary mirror reduces the contrast of the image as in the Newtonian.

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 simplest 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 earth's 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.

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.

Filters are another useful accessory; these can be of several different types: Coloured filters can enhance the views of certain planets, by making features on the planets more distinct - the filter increases the contrast on things like cloud belts and dust storms on the planets surface which makes them easier to see. Light Pollution filters screen out the glow from streetlights and neon signs etc, which can help to make objects easier to see - this does depend where you are observing from though, as there is a limit to the amount of skyglow a filter can remove. Special Filters such as the Oxygen 3 filter and Hydrogen Beta filter allow only a specific wavelength of light to pass through them, thus objects whose light is on these wavelengths are much easier to see, but again it depends on other factors as well such as observing location and atmospheric conditions as to how effective these filters can be.

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 found much easier and aligned with the main telescope. A variation on this is a 'Red Dot' finder or Telrad which project either a red dot or a series of circles onto the sky so you can aim the telescope more easily.

An ALT AZIMUTH Mounted Telescope

An Equitorial Mount