"Which Telescope?" - A guide to what to buy for Christmas

Roger Steer, Chairman, Bristol Astronomical Society, on 7 December 2001

In the run-up to Christmas, the variety of telescopes advertised is bewildering. There are so many cheap telescopes in the attics of disappointed would-be astronomers, yet a good telescope need not be much more expensive than the impressive-looking but inferior ones which are hung with tinsel in the shops at this time. This talk was a guide to what and where to buy and how to get the best out of your purchase.

Roger Steer started by explaining that a telescope not only magnifies the object of interest but it also increases light-grasp, making it possible to see things which are too faint for the unaided eye. The pupil of the eye is about 6 mm in diameter when dark-adapted. An objective (the lens nearest to the object of interest) with an aperture of 60 mm diameter is 10 times as big and therefore has 100 times the area of the pupil of the eye. It therefore funnels in 100 times as much light and makes the faintest stars to be seen on a good dark night appear as if they were the brightest seen with the unaided eye. In astronomical terms it would be possible with such a telescope to see stars of magnitude 11 - well beyond the limits of most star-charts. Simply, the larger the aperture, the fainter the stars, galaxies and nebulae that can be seen.

The magnification (diameters) of a telescope is the ratio of the focal lengths of the objective and the eyepiece. However, the maximum one can expect from a telescope (because of the coarse nature of light-waves) is 2x for every mm of objective diameter. Hence, the 60 mm telescope above would theoretically offer a magnification of 120x (if the optics are good!). Cheap telescopes make claims far in excess of this. With the right eyepiece, there is no real limit to the magnification but above 2x per mm no more detail will be seen. All that would happen is that the fuzziness would also be magnified. There is also a minimum useful magnification and that is because of the size of the eye pupil. It is calculated by dividing the aperture in mm by 6; hence, for a 60 mm aperture it is 10x.

A refracting telescope uses a convex lens as its objective and a poor lens produces an image with a coloured fringe (chromatic aberration). Achromatic lenses are designed to correct for this but large ones are very expensive 150 mm refractors are rare in the hands of amateurs. However, they have long focal lengths (f/10 to f/12) and therefore give high magnification with eyepieces that are more comfortable to look through. There are no obstructions between the objective and the eyepiece and the air in the closed tube is steadier; therefore the image is better. For these reasons, they are the best for studying the detail of bright objects such as the Moon and the planets. However, faint objects are not ruled out at all. On the negative side, they require high mountings and, unless a star-diagonal is used, can produce a crick in the neck when looking at things high in the sky. Very large ones need an observatory as, being long, they tremble in the wind.

A reflecting telescope uses a convex mirror as its objective, which has no chromatic aberration. Consequently they are cheaper to produce and telescopes with an aperture of 200-250 mm are common in the amateur world. They tend to be of shorter focal-length (f/6 and f/8), have open tubes which causes air-currents to produce `poor seeing'. Because the light-path is `folded' and the observer looks perpendicular to the direction of the object, cricks in the neck are not common and the mount is short and easier to engineer. However, the obstruction in the light-path by the secondary plain mirror and its supports causes `diffraction fringes' which degrade the image somewhat. For these reasons they are better suited to faint objects like galaxies, nebulae, star-clusters and comets. However, they can be used to study the planets too, though they are less suited to this than the refractor. Reflectors can have very short focal lengths (f/4) and these `Rich Field' telescopes are the best to introduce the beauty and wonder of the heavens.

Some of the advantages of each of these types can be combined in compound telescopes such as the Schmidt-Cassegrains or by Maksutov-Cassegrains. Roger Steer explained how these worked. They are reflecting telescopes with a corrector plate to overcome the aberrations of a spherical mirror. They tend to be small-aperture and, as the light-path is `folded', they are compact and are cheaper than simple refractors. They are better suited to higher magnifications and therefore to planetary study.

The mounting has to be sturdy because vibrations are also magnified by the telescope. This is easier to achieve with reflectors and the compound telescopes. The rotation of the Earth causes the stars to appear to move from east to west so the telescope can be designed to follow this movement. Traditionally this was done using an `equatorial mount' in which one of the axes of movement was able to be adjusted parallel with the axis of the Earth and this was driven slowly from east to west by a geared-down motor. A sturdy equatorial mount presents considerable engineering problems and is thus expensive. It is also better suited to a fixed observing site because the alignment of the axis is time-consuming. The modern compound telescopes described above usually have a cheaper `altazimuth' mount and the telescope is driven correctly by a computer - which also can find faint objects for you! Alternatively, if manual adjustment is easy, a very simple altazimuth mounting will suffice and a simply constructed type of reflecting telescope called a `Dobsonian' (after its original inventor) is a good way of getting a large aperture for low cost. They are also reasonably transported by car to a dark site.

Cheap telescopes are supplied with Huygens (H) or Ramsden (R) eyepieces. These were invented centuries ago and are not good. A telescope which offers these types of eyepiece or does not take an eyepiece with a 1¼ inch barrel is a poor telescope. The best are `orthoscopics' and more expensive ones are `Plössls'. A 2x Barlow lens doubles the effective focal-length of the telescope and will therefore double the magnification of any eyepiece. All that is needed therefore is an 18 mm and a 12.5 mm eyepiece for faint objects and a 2x Barlow lens to double the magnification for planets.

Roger Steer went on to describe good-quality telescopes costing in the region of £250 - £400 and gave details of their makers and suppliers. There is not space enough here to give all the details but a brief list of his recommendations can be obtained from: Richard Phillips, 33 Easton House, Grosvenor Bridge Road, Bath BA1 6BG (telephone: 01225 315762 and e-mail: [email protected]).

All amateurs, even those with the largest telescope, use binoculars while observing and this essential accessory is the best way of starting astronomical observations. Don't be lured by advertisements offering high magnifications and small apertures or zooms (the guides above apply to binoculars as well as all other types of telescope). A pair of 7x50 or 10x50 binoculars (Russian ones are the cheapest and of very high quality) is ideal. Considering the English weather, with a good star atlas (Norton or Tirion) they will provide a challenge for at least a year - which will give you time to save up for that telescope. At the end you can decide whether to go ahead as an amateur astronomer or become a bird-watcher or race-goer!

Richard Phillips