Orwell Astronomical Society (Ipswich)
Choosing A First Telescope Or Binoculars
Every amateur astronomer wants to own a telescope - and, usually, the bigger the better! However, in practical terms, there are many factors involved in choosing the most appropriate instrument, not the least of which is cost. This article provides a brief guide for anyone contemplating buying a first telescope or binoculars for astronomical use.
Can you afford to buy a telescope? A first class astronomical telescope will cost at least many hundreds of pounds. However, you can purchase a useful instrument for a few hundred pounds and even for £100 you can obtain an instrument that will provide plenty of scope for observing. If you cannot afford more than a few pounds, rather than waste your money on something unsuitable it's better to save up for something better.
For a starting instrument don't overlook binoculars, which are often cheaper and better value than small telescopes. Binoculars can show you many interesting objects. Other essential equipment for beginning sky watching includes 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 binoculars or small telescope, planisphere and atlas you are ready to learn the key sights of the sky.
Binoculars are the ultimate in compact, simple, portable, easy-to-use equipment, and many experienced observers use them to complement their telescopic observations. Indeed, there are many beautiful sights such as the star fields of the Milky Way, star clusters such as the Pleiades and Hyades, and comets, which can only be truly appreciated in low-power, wide-field binoculars.
Binoculars are usually marked with figures such as 8x40, 7x50, or 10x50. The first figure is the magnification and the second is the aperture of the front lenses in millimetres. For general observing 7x50 or 10x50 binoculars are equally useful. If you find the weight of 50 mm binoculars a problem, opt instead for 40 mm or even 30 mm models.
Avoid binoculars with magnifications greater than 12x, as they will be difficult to hold steady, unless you have a particular need for a specialist pair and can mount them on a tripod. Avoid zoom binoculars too, as they generally have narrow fields of view and poor optics. Good binoculars have coloured coatings on the optics, similar to non-reflective coatings on spectacles, which improve image brightness by increasing the proportion of light transmitted.
Some cheap binoculars employ prisms that are too small, severely limiting 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 in fact they do not. Such instruments are simply opera glasses with a straight-through optical system having a very restricted magnification and field of view.
The choice of binoculars has to strike a balance between light-gathering capacity (determined by the diameter of the object lens), magnification, weight and cost. The subject of weight introduces the question of a tripod: this is almost essential equipment if one wants to do more than a sweep of the sky for a quick look, as even a little hand shaking makes study of detail impossible. If purchasing binoculars, it is worth seriously considering buying a tripod at the same time and having the binocular shop obtain any necessary bracket to mount the binoculars on the tripod. Ensure that the tripod suits your standing height; it is possible to use binoculars from a seated position, but this can place a strain on the back and neck, and it can be awkward to fit the tripod around the chair. Even with a tripod, it can be difficult to view objects directly overhead.
Finally, note that binoculars of course show objects with a true orientation: e.g. Mare Crisium at the top right of the lunar disk. Conversely, telescopes generally reverse the orientation. Thus it may take some mental adjustment to use a telescope after mastering binocular observing. Nevertheless, binoculars have some real advantages, not the least of which is ease of use.
There are two main types of telescope:
To choose the right type of telescope, you need to know the relative advantages and disadvantages of each. Astronomers judge telescopes not by their magnification but by their aperture, i.e. by the diameter of the main lens or mirror. 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. When astronomers refer to a "small" telescope they mean one with a small aperture.
Small telescopes, those with apertures under 75 mm, are always refractors. The smallest refractors, of 50 or 60 mm aperture, will show the Moon's craters and dark lowland "seas" (maria), Saturn's rings, Jupiter's cloud belts and its four main satellites, some attractive double stars, and the brighter nebulae and galaxies. Telescopes with larger apertures are frequently reflectors, since large mirrors can be made more cheaply than large lenses. A larger telescope will show fainter objects and finer details than a smaller telescope.
Although a small refractor may appear a good first buy, there is a very important warning. Many low budget refractors are either poorly made or make misleading claims as to their performance. In the worst cases a telescope may have such poor optics that it is useless for astronomy, even though its finish appears good. Unfortunately, even some leading stores sell telescopes which are of very poor optical quality. One of the main optical failings of cheap refractors is that they rely on simple lenses which produce fringes of false colour around objects, an effect known as chromatic aberration. In order to minimise false colour, the manufacturer reduces the effective aperture of the telescope by means of disc (called a stop) with a central whole in it, positioned some way down the telescope tube. If you look down the objective lens of a telescope and you can see a stop, this should serve as a warning as to the optical quality of the telescope. (Be careful not to confuse a stop with a light baffle, a ring which provides no impediment to the optical path but absorbs stray light rays which would otherwise degrade image quality.) A lens which is corrected for false colour is termed achromatic and it is worth looking for this in a telescope, although it does not of itself guarantee good performance. Mirrors do not suffer from chromatic aberration.
Note that an astronomical refractor will give an inverted (upside-down) image. This is because extra lenses would be needed to bring the image the right way up for terrestrial viewing, and these would introduce additional impairments in the optical path. It makes little difference which way up an astronomical body appears, so astronomers usually accept the basic upside-down view as the price to pay for an optical path without unnecessary impairments. Some refractors are provided with a star diagonal (an encased prism) to turn the image the right way up, but this introduces lateral inversion (reverses left and right); bear this in mind when looking at the Moon and planets.
The magnification of a telescope depends on the eyepiece used. Telescopes usually come with a selection of eyepieces that offer low, medium and high powers. Do not get carried away by advertisements for small telescopes that claim magnifications of many hundreds of times. Too high a magnification will show less detail rather than more, since an over-magnified image is faint and indistinct. A good rule of thumb is to use at most a magnification of twice the aperture in millimetres - see table below. Of course, if the telescope aperture is stopped down, the maximum usable magnification is correspondingly reduced.
The atmosphere 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 poor seeing. No matter how large a telescope you own, from ground level the maximum usable magnification will be about 300x. At higher magnifications, 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 (written f/ratio) is the focal length divided by the 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, observers who like to observe deep sky objects such as nebulae and galaxies generally prefer f/4 to f/6 telescopes.
Conversely, observers who like to view the Moon and planets generally opt for f/7 and above. The following table summarises the key parameters relating to telescope performance:
|Magnitude of Faintest Star||Highest Usable Magnification|
Telescope performance under optimal conditions.
A good mounting is essential if you aim to use a telescope or binoculars for anything beyond casual observing. Department stores 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 observe anything properly, particularly when the wind blows. Also, remember that comfort and ease of use are vital. You will not enjoy using a telescope it you have to kneel down or crane your neck to look through it! A good mounting should fulfil the following criteria:
The simplest type of mounting, used by small refractors, is the altazimuth design. This provides two axes of movement for the telescope: altitude (up and down) and azimuth (left and right). The altazimuth mount is suitable for observing terrestrial objects or for simple stargazing; however, in order to track the motion of a celestial object in the field of view it requires the observer to move the instrument simultaneously about both axes. Larger telescopes generally incorporate an equatorial mount. This again provides two axes of movement: the polar axis is aligned to the north, parallel to the axis of rotation of the Earth, and the declination axis is at right angles to it. In order to track the motion of a celestial object in the field of view it is necessary simply to swivel the telescope around the polar axis. It is relatively straightforward to arrange for a motor to drive a telescope on an equatorial mount so that the telescope tracks continuously, thus facilitating long-exposure astrophotography. An equatorial mount needs to be set up more carefully than an altazimuth mount since it is necessary to align the polar axis to the north celestial pole; it is also generally more expensive.
In recent years, the Dobsonian mount has become increasingly popular as a low-cost, portable alternative to the equatorial. It incorporates a modified altazimuth design and is best suited to reflectors used with low-power eyepieces for wide-angle viewing of the sky when precise tracking is not essential. The following images show examples of each of the mounts among the telescopes at Orwell Park Observatory.
Most small telescopes have so-called slow motion controls, which are gear trains linked to the axes actuated via flexible cables ending on easily grasped drive knobs. The slow motion controls allow the observer to track an object by turning the knobs appropriately. 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 (at high power).
A telescope should have a smaller finder scope attached to its main tube to provide a low-magnification, large field of view to assist with aiming the main instrument. A typical finder has a magnification of 6 and an aperture of 30 mm (described as 6x30). The cheaper finders often have stops in them restricting the working aperture to about 10 mm. They will help locate the brightest objects but are unsuitable for making observations as such.
Eyepieces are the most important accessory you will buy for your telescope! Regardless of how good a telescope's lens or mirror may be, its performance will be seriously degraded if the eyepieces are of poor optical quality. Eyepieces are interchangeable, to provide a range of magnifications. The magnifying power of an eyepiece used with a particular telescope can be found by dividing the focal length of the latter by that of the former. (Usually, the focal length is marked on the barrel of the eyepiece.) The longer the focal length of the eyepiece the lower the magnification but (generally) the larger the field of view. A given eyepiece will provide higher magnification on a telescope of long focal length than one of short focal length. 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 cheapest acceptable eyepiece for amateur use 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.
If possible, make the following checks to test a telescope's quality before purchase:
If the telescope fails any of the above tests, do not buy it!
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 by 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 to prevent a future purchaser suffering the same problem.
Robin Scagell et al