Most of the figures in the above table are self explainatory. Clear aperture is the size of the instrument, the measurement of its main mirror or objective lens. It is from this figure that all the other parameters of the telescope are dictated.

The larger the instrument, the more light it gathers and this then affects the faintest stars visible. Also, the larger the instrument, the greater its ability to resolve small or close together objects, otherwise known as the telescope's resolving power or 'Dawes limit'. There are practical limitations to this factor so the practical resolving power is quoted as well.

Many people get misled by 'magnification' or 'power' thinking that more is better. Up to a point this may be true and in theory any telescope can magnify an object thousands of times but in reality the usable magnification of an instrument falls far short of this. Again, the size of the instrument is a crucial factor and will dicatate the optimal magnification to be used to look at an object and the maximum magnification that a telescope is capable of. Anything more than that and image quality will almost certainly start to suffer.

The figures quoted above are for a good quality telescope in excellent condition used when sky conditions are very good - a set of criteria that is seldom met!

MOUNTINGS
Altazimuth: The simplest type of mounting, used by many small refractors and reflectors, is the altazimuth design. (See also the description of the Dobsonian mount further down this section.) This requires you to move the instrument simultaneously about both axes (altitude and azimuth) to keep an object in the field of view. In essence, the telescope can be moved up and down (altitude, or angle above the horizon - 0°to +90°) and left and right along the horizon (azimuth, or angle along the horizon - 0° to 360°, North, East, South, West, and back to North, and vice-versa).

Equatorial: Larger telescopes often incorporate an equatorial mount, which needs to be set up more carefully with the polar axis pointing to the north celestial pole, near Polaris (assuming that you are observing from the Northern Hemisphere).

In reality the equatorial mount has 4 axes. Two of these are generally adjusted then fixed before the telescope is used. These two axes align the mount so that the poleward pointing axis is aligned parallel to Earth's axis. These 2 axes are in effect the altitude and azimuth axes. The other two axes can then be used to move the telescope to correspond with Right Ascension and Declination co-ordinates. An equatorial mount is more expensive and complex but has the advantage that objects can be kept within the field of view as the Earth rotates by turning the telescope around the poleward pointing axis only.

Dobsonian: In recent years the Dobsonian mount has become increasingly popular as a low-cost portable alternative to equatorials. It incorporates a modified altazimuth design, and is best suited to reflectors used with low power eyepieces for wide-angle viewing of the sky, in which precise tracking is not essential. Despite this last remark some Dobsonians, especially larger ones, are fitted with a driving system and computer control. This driving system may also be fitted at a later date to the unmodifed mount.

This type of mount is usually made from wood. The base of the Dobsonian mount is rather like two stacked discs: one is in contact with the ground and the other is able to swivel about upon this. This enables the mount to pivot left and right in azimuth. Upon this moving disc is a 'U' shaped structure which has two large semi-circular grooves at the top parallel to each other. Two circular discs mounted along the length of the telescope tube complete the set-up. These two discs on the telescope tube mean that the telescope can then be placed on the mount, the discs sitting in the two semi-circular grooves on the mount. This enables the telescope to point up and down in altitude.

The Dobsonian mount has become so popular since the mounting is simple to make (and use), well within the realms of D.I.Y. skills. Also, the telescope and mount can be quickly assembled together (and dismantled) making the whole thing easily portable. Not only that but a large reflecting telescope can be used on a Dobsonian mount and since the azimuth pivot is close to the ground there may be no need for steps or stools to reach the eyepiece. Also, because the telescope's centre of gravity is closer to the ground the 'footprint' of the base can be smaller than that of a tripod mounted telescope.

Such is the popularity of this mount that many telescopes are sold simply as 'Dobsonians' but that is to say that they are actually reflectors (usually) using a Dobsonian mount. The Dobsonian takes its name from the inventor, John Dobson.

GOTO: A most recent addition has been the advent of the GOTO mount/telescope. Many of these telescopes are becoming more affordable and available. They are popular in that they are in effect computer controlled. In its simplest form the GOTO is a telescope (usually a Cassegrain) integrated with a hi-tech Dobsonian mount. A handheld computer control unit contains a database and simple up/down, left/right control buttons (plus a few other buttons) and digital readout.

A simple process of aligning the telescope with two stars can then be made to tell the control unit how to control the telescope so that it can track an object or find one - polar aligning, 'GOTO', and find - without too much fuss! GOTO telescopes are sold with the ability to be tripod or pillar mounted, or they can simply be placed on any steady, flat surface. The portability and versatility of such an instrument is fairly obvious.

Mountings, warnings: Department store and mail-order-catalogue telescopes often employ notoriously unstable, badly made and clumsy desktop tripod mountings. There is no point in buying a telescope with a shaky mounting, as you will be unable to see anything properly, particularly when it is windy outside. Also, remember that comfort and ease of use are vital. You will not enjoy using a telescope if you have to kneel down and crane your neck round to look through the telescope.

It must be said that in recent years the quality of some telescope mountings have improved markedly but there are still a lot of poorly made tripods and mountings out there. Regrettably, I have seen several telescopes that in themselves were well made and fairly good value for money - only to be completely let down by their tripods and mountings which were next to useless.

Most small refractors have so-called 'slow motions', which are gears, often with flexible cables, linked to the axes. They allow you to follow objects by turning knobs. Beware of stiff slow motions which are more trouble than they are worth. The more expensive mounts have motor drives, which track an object without any effort. These are particularly useful when observing planets.

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EYEPIECES
Eyepieces are the most important accessories you will buy. Regardless of how good a telescope's lens or mirror may be, its performance will be seriously degraded if the eyepieces are of poor quality. Small telescopes, mostly 60mm refractors, but others as well, frequently come supplied with poor quality eyepieces. These can be replaced later, at a cost of course, but it is worth considering the eyepiece quality before you part with your money. Many small telescopes come supplied with high power eyepieces which are unusable because the power offered by them exceeds the ability of the telescope. An eyepiece frequently sold with many small and cheap telescopes is a 4mm - all but useless in practice - but it does make a telescope package seem more attractive!

Eyepieces are interchangeable, and provide different magnifying powers, determined by their focal length. They are marked with numbers such as 25 mm, which indicate their focal length. The longer the focal length, the lower the magnification but (generally) the larger the field of view. Lower powers are best for observing faint, diffuse objects such as comets, nebulae and galaxies, while higher powers are best for the Moon, planets and double stars.

The magnifying power of an eyepiece can be found by dividing its focal length into the focal length of the telescope. Hence a given eyepiece will provide higher magnification on a telescope of long focal length than one of short focal length.

Below is a table which gives you a guide to the magnification yielded by different eyepieces when used in conjunction with telescopes of different focal length. Not all the available focal lengths of telescopes or eyepieces are shown here but the table does list most of the common sizes encountered with amateur sized instruments...

Eyepiece	Focal length of telescope (mm)
-	600	800	1000	1200	1400	1600	1800	2000	2200	2400	2600	2800	3000	3200	3400	3600	3800	4000
50mm	12x	16x	20x	24x	28x	32x	36x	40x	44x	48x	52x	56x	60x	64x	68x	72x	76x	80x
40mm	15x	20x	25x	30x	35x	40x	45x	50x	55x	60x	65x	70x	75x	80x	85x	90x	95x	100x
32mm	18.7x	25x	31.2x	37.5x	43.7x	50x	56.2x	62.5x	68.7x	75x	81.7x	87.5x	93.7x	100x	106.2x	112.5x	118.7x	125x
25mm	24x	32x	40x	48x	56x	64x	72x	80x	88x	96x	104x	112x	120x	128x	136x	144x	152x	160x
20mm	30x	40x	50x	60x	70x	80x	90x	100x	110x	120x	130x	140x	150x	160x	170x	180x	190x	200x
16mm	37.5x	50x	62.5x	75x	87.5x	100x	112.5x	125x	137.5x	150x	167.5x	175x	187.5x	200x	212.5x	225x	237.5x	250x
12.5mm	48x	64x	80x	96x	112x	128x	144x	160x	176x	192x	208x	224x	240x	256x	272x	288x	304x	320x
10mm	60x	80x	100x	120x	140x	160x	180x	200x	220x	240x	260x	280x	300x	320x	340x	360x	380x	400x
8mm	75x	100x	125x	150x	175x	200x	225x	250x	275x	300x	325x	350x	375x	400x	425x	450x	475x	500x
5mm	120x	160x	200x	240x	280x	320x	360x	400x	440x	480x	520x	560x	600x	640x	680x	720x	760x	800x

There are many designs of eyepiece avaialble to day. The cheapest acceptable eyepiece for amateurs is the Kellner design; it offers a useful field of view, spanning up to 45°. The designs known as Orthoscopic, Plossl and Erfle are progressively more expensive alternatives, offering better optical perfomance or different qualities - at a price!

Barlow lenses are a common addition to any astronomer's collection of eyepieces. These are used in conjunction with another eyepiece - the eyepiece fitting into the Barlow lens - which is then fitted to the telescope. The most common type of Barlow lens, the '2 x', halves the focal length of a given eyepiece to magnify the image by a factor of two. So, using a 40mm eyepiece with a 2 x Barlow lens gives you a 20mm eyepiece. The next most common type of Barlow lens is the '3 x', this reduces the focal length of a given eyepiece by a third, effectively tripling the magnification of an eyepiece. More expensive are 'zoom' or 'variable' Barlow lenses which usually vary in the range from 2 times to 3 times.

People looking through an astronomical telescope for the first time are usually surprised to find that the image is upside down. This is a basic characteristic of a telescope, and for daytime use is corrected by using extra lenses or what is termed an erecting prism. It makes little difference which way up an astronomical body appears, so astronomers usually accept the basic upside down view.

Some small refractors are provided with a star diagonal (an encased prism) to turn the image the right way up, but left and right are reversed. Bear this in mind when looking at the Moon and planets.

Don't blame your eyepieces for a poor image if your telescope is simply pointing out of a window; the difference in air temperature inside the room and outside causes turbulence, which distorts the starlight.

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FINDERS
A telescope should have a small finderscope attached to its main tube. This is a low-magnification telescope used for aiming the main instrument. A typical finder has a magnification of 6 and an aperture of 30 mm, described as 6 x 30, although larger finders often come as standard or are avaialble as an option.

The cheaper instruments often have 5 x 24 finders, which invariably have stops in them restricting the working aperture to about 10 mm. They will help locate the brightest objects, but little else.

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CHECK YOUR TELESCOPE
There are a number of simple tests you can perform on equipment to test its quality. Tap the tube lightly. The image should take no more than three to five seconds to stop shaking. Some people hang a weight from the centre of the tripod to improve its stability and to damp down vibrations.

The only easy test for optical quality is to observe a star image after allowing the telescope to reach outside temperature, which may take an hour, and on a night when the seeing is excellent.

Using the maximum usable power (see the telescope performance table above), examine the image of a moderately bright star. When perfectly focused it should be a point or very small disc of light surrounded by faint rings. In poor seeing this appearance may be hard to achieve with even a good telescope.

Defocus the image on both sides of the focus point. In both cases it should become an evenly illuminated and circular disc, although in the case of a reflector you will see the shadow of the secondary mirror in the middle. If it is impossible to achieve perfect focus, and the image appears as a short line on either side of focus, the telescope is suffering from the optical defect called astigmatism.

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COST
So what's all this going to cost you? A decent pair of 8 x 40 binoculars can be purchased for about £50 but can be as much as £90. A reasonable 80 mm refractor will set you back about £150 to £200, while a good 4 inch reflector can cost as little as £250. Remember that a telescope is a precision optical instrument and so you should be prepared to pay as much as for a good camera. Moneywise, for larger and more complex instruments, well, the sky's the limit!

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LEGAL ISSUES
If you use a credit card to purchase equipment worth £100 or more, on which the workmanship is shoddy or the goods are damaged in the post, you will have redress to the law through the Consumer Credit Act, 1974. Try to resolve the matter with the retailer, but if that fails write to both your local Consumers Association office and your credit card company.

If you feel you have been misled in any way, or that the specification of the telescope has been falsely described, you should complain to your local trading standards office. Indeed, it is your duty to do so.

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AND THE BEST TELESCOPE IS...
So what is the best telescope for you? The simple answer is: it's the one that you think you'll use the most often. Ease of set-up, simplicity of use and portability should be key factors in your decision. Move up to larger equipment only when you have proved to yourself that you can get out to observe on a regular basis. Also, make contact with your local astronomical society who may be able to give you advice on equipment and its use. Their address should be available from your local library or the internet.

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A SAFETY WARNING
Many telescopes are supplied with eyepiece filters. They soften the glare of the Moon, Sun and the brighter planets. Unfortunately, they are often of low quality, and the solar filters in particular can be extremely dangerous. They can easily crack under the focused heat of the Sun, and should be avoided at all costs. Remember it only takes a fraction of a second of unfiltered sunlight to cause permanent blindness. The only completely safe way to observe the Sun is to project its image onto a white card. Never observe it through the main telescope or the finder, even if it has a filter.

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BINOCULARS - A GOOD FIRST CHOICE
There are many beautiful sights, such as the star fields of the Milky Way, star clusters such as the Pleiades and the Hyades, and ghostly comets, which can only be truly appreciated in low-power, wide-field binoculars. These are compact, easy to use pieces of equipment, and many observers use them to complement their telescopic observations.

Binoculars are usually marked with figures such as 8 x 40, 7 x 50 or 10 x 50. The first figure is the magnification, and the second is the aperture of the front lenses in millimetres. For general observing, 7 x 50 or 10 x 50 binoculars are equally useful. If you find the weight of 50mm binoculars a problem, go for 40mm or even 30mm models.

Avoid binoculars with magnifications greater than 12, which will be difficult to hold steady, unless you have need for a specialist pair. Zoom binoculars should be avoided too, as they generally have narrow fields of view and poor optics. Good binoculars will have coloured coatings on the optics, similar to non-reflective coatings on spectacles, which improve the image brightness by increasing the transmission of light.

Some cheap mail-order binoculars economise by using prisms that are too small, which severely limit the field of view. Look into the front lens; you should see a small circle of light through them. If the circle is cut off or square, you are losing light. Also be aware that very cheap models may have spurious bulges in the barrels to give the impression that they contain prisms when they do not. Such instruments are simply opera glasses with a straight-through optical system, having a very restricted magnification and field of view.

Similar information about binoculars can be found on the Hint and Tips for beginners page.

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ABOUT THIS DOCUMENT
With so many instruments available it is difficult for a newcomer to astronomy to make an informed choice. It is hoped that this document will help the reader decide what will be the best instrument for them to buy. Whatever is purchased will always be a compromise since it is impossible for a single instrument to have the benefits of another equally useful piece of equipment. There are many reputable outlets for astronomical equipment, some of which may be found by exploring some of the links on the Links page.

Please feel free to ask for further advice by sending an e-mail to the Webmaster.

This document was adopted from a leaflet distributed during the 1996 UK National Astronomy Week, c/o the Jodrell Bank Science Centre, Macclesfield, Cheshire, UK. It was written by Steve Tidey of the Association for Astronomy Education. Additional and updated material was written by Derek Haselden. Please feel free to copy and distribute copies of this document but please acknowledge the authors. Also, please do not alter the content without the permission of the authors.

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© Derek Haselden & Solent Amateur Astronomers 2005