A typical small refractor on a simple stand,
with a star diagonal at the eyepiece end.
All Diagrams by John Lewis
EVERY amateur astronomer wants to own a telescope and the bigger the better. But for a starting instrument don't overlook binoculars (see the box), which are often cheaper and better value than small telescopes, and will show you many interesting objects.
Other basics for beginning skywatching include a planisphere (a circular star chart with a mask that rotates to show the stars on view at any particular time) and a simple star atlas. Armed with these and a pair of binoculars, you can learn the basic sky sights with little outlay. And the money will not be wasted the binoculars will always be useful, even if you eventually graduate to a large telescope.
Telescopes are judged by not their magnification but their aperture, i.e. the diameter of the main lens or mirror. When astronomers refer to a 'small' telescope they mean one with a small aperture. The aperture governs how much light the telescope collects and the more light it collects, the more you can see. Hence it is best to get the largest aperture telescope you can afford, whether a refractor or reflector.
The smallest telescopes, i.e. those with apertures under 75 mm or 3 inches, are always refractors. Telescopes with larger apertures are usually reflectors, since large mirrors can be made more cheaply than lenses.
A typical small refractor on a simple stand,
with a star diagonal at the eyepiece end.
All Diagrams by John Lewis
The smallest refractors, of 50 or 60 mm (2 or 2.4 inches) aperture, will show the Moon's craters and dark lowland "seas" (the maria), Saturn's rings, Jupiter's cloud belts and its four main satellites, some attractive double stars, and various nebulae and galaxies. Small or medium-sized reflectors have apertures of 100 mm (4 inches) or larger. They will usually show fainter objects and finer details than small refractors.
Although a small refractor may appear a good first buy, there is one very important warning. Many low-budget refractors are either poorly made or make misleading claims as to their performance. In the worst cases the telescopes may be useless for astronomy, even though their finish may be apparently good. Unfortunately, even some leading stores sell telescopes which are of very poor optical quality.
One of the main failings of simple lenses is that they produce fringes of false colour around objects, an effect known as chromatic aberration. A lens which is corrected for such false colour is termed achromatic so it is worth looking for this in the description, although it still does not guarantee good performance. Mirrors do not suffer from chromatic aberration.
A good rule of thumb is a magnification of twice the aperture in millimetres (or 50 times for each inch of aperture see table below). Of course, if the telescope's aperture is stopped down, the maximum usable magnification is correspondingly reduced.
The atmosphere itself places a limit on the highest magnification you can use, because air currents make the images of stars and planets unsteady, an effect known as seeing. No matter how large a telescope you own, from a normal ground-level site the maximum usable magnification will be about 300. Beyond this, an eyepiece just magnifies the distorting effect of the atmosphere, creating a useless "boiling" image.
Telescopes, like cameras, have f-numbers. The focal length of a telescope is the length of the light path from the main lens or mirror to the eyepiece. The focal ratio (or f/ratio) of a telescope is its focal length divided by its aperture. For example, a telescope of 100 mm aperture with an 800-mm focal length is an f/8 instrument.
Focal length is not a critical consideration, but it does determine what objects an instrument is best suited for observing. For example, f/4 to f/6 telescopes are preferred by people who like to observe deep-sky objects such as nebulae and galaxies. However, others who like to view the Moon and planets opt for f/7 and above.
Telescope performance
Aperture Faintest star Highest
(inches) (mm) (magnitude) usable power
2.4 60 11.6 120
3.1 80 12.2 160
4.0 100 12.7 200
6.0 150 13.6 300
Larger telescopes incorporate an equatorial mount, which needs to be set up more carefully with the polar axis pointing to the north celestial pole, near Polaris. An equatorial mount is more expensive 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.
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.
Department store and mail-order-catalogue refractors often employ notoriously unstable 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 the wind blows. 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 to look through it.
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.
A small reflecting telescope on an equatorial mount.
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.
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 the Erfle and the orthoscopic are more expensive alternatives, offering better optical performance.
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 it 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.
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 table on page 2), 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.
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.
Binoculars contain prisms that fold the light path,
thereby making them more compact than telescopes.
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 50-mm binoculars a problem, go for 40-mm or even 30-mm models.
Avoid binoculars with magnifications greater than 12, which will be difficult to hold steady, unless you have a particular 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 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.
