Buying a telescope as a gift

A telescope is a popular gift idea for Christmas or birthdays, but buying one can present real problems for prospective purchasers. They may be parents wanting an instrument for a bright young child, or a husband or wife wanting to surprise a partner who has expressed interest in the heavens. Either way, it means the person buying the gift is not an astronomer. So they are buying "in the dark" before the scope itself ever sees a night sky. One major problem is that the telescope will almost certainly have a nice shiny finish to it but the appearance will give you little clue about the really important matter of its quality and how it performs. Our own Amazon stores in the UK, USA, Canada and France, show telescopes picked for quality and value at a range of prices to suit most pockets. But here are some tips from Skymania to help a buyer through the purchasing minefield wherever they buy. The first thing to say is that a telescope is a serious scientific tool and you should be suspicious of buying anything that looks too cheap to be true. Telescopes are certainly excellent value at the moment, with so many being imported from China, but don't expect to pick one up for next to nothing. Avoid the dirt cheap plasticky telescopes that appear for just a few notes in some catalogues and department stores, even if you are buying for a child. The impossibly cheap ones may even have plastic lenses, or poor quality optics that will give disappointing views of anything it is turned on. They are usually little more than toys. If your child is serious about stargazing, a toy telescope that disappoints could put them off their new hobby completely and your gift will end up in its box under the bed or on a wardrobe. The impossibly cheap telescope will almost certainly also have simple, unstable mounts or tripods that make the telescope wobble like a jelly when it is touched. The slightest tremble will be magnified greatly when the telescope is looked through, making it impossible to view anything properly. If you are unsure about your child's level of interest and suspect it might be a passing fad, then why not choose a pair of binoculars. They are usually especially good value and can be bought reasonably cheaply due to the scale of manufacture. But if the child ultimately discovers that stargazing is not what rocks their boat, then they can use the binoculars for other pursuits such as bird-watching. Again, there is a good selection of binoculars in our Amazon stores. If you are still set on buying a telescope, then it makes sense to go for a respected brand such as Celestron, Meade or Sky-Watcher, all of whom specialise in making telescopes which include those at the cheaper end of the market. But with any instrument, check out any reviews of it. If you put the name and model number into Google, you may well find reports of its quality and value on astronomers' personal websites or on dedicated review sites such as Cloudy Nights. It may help if you have a friend who is an astronomer. They will probably be delighted to advise on the pitfalls and what to is a quality, useful instrument. If you don't have such a friend, check out specialist astronomy stores that are dotted around our countries. You will find them advertising in the main astronomy magazines and they generally have staff who are happy to help. They almost certainly will know a lot more about telescopes than those who work in the big chains of camera shops. The telescope pictures is the Celestron Firstscope Reflector 114, which is an example of a nice first instrument which has a motor to keep it pointed at the object being observed as the Earth rotates.

Five top targets for your new telescope

You were lucky enough to be given your first telescope and the sky is clear. But what should you look at first when it seems there are so many objects to choose from? There are plenty of targets worth exploring, but for a beginner it is best to turn to some of the brightest and best known first as you get used to using your new instrument. Here are five top targets for a small telescope in January 2008. Mars: The Red Planet is only just past its best, having been closest to the Earth around Christmas 2007. The distance between Mars and the Earth is now steadily increasing again but use this opportunity to glimpse some of the markings on our neighbouring world. A few minutes at even a small telescope will help you to glimpse some of the larger dark regions on Mars, plus the white area around the planet's north pole due to ice and clouds. Mars is easy to find in the sky just now on the borders of Gemini because it shines so brightly like an orangey lantern. Look for it high in the sky as soon as it gets dark. You'll learn more on how to find it and about this fascinating world in our special Mars pages. Mars will gradually fade over the next few months and its features will become much harder to spot on a smaller disk. The sketch of Mars here is by experienced UK amateur astronomer David Graham. Saturn: Once you've viewed Mars, you can look above the eastern horizon in mid-evening to catch another bright planet - and what a jewel this one is. Even a small telescope on a steady mounting will reveal the amazing bright ring system that encircles Saturn. For many, the rings make this the finest object you can view in the sky. The rings are made up of a myriad of particles of rock and ice kept in check by Saturn's moons. If atmospheric conditions are steady, check to see if you can spot a dark gap in the rings called the Cassini Division. Saturn itself is one of the gas giants and unfortunately the cloud tops are a fairly uniform yellow and do not normally offer much, if any, detail for small telescopes to detect, unlike the clearer belts around giant Jupiter. You could also sketch the pattern of stars visible in the telescope's field of view around Saturn and check which are the planet's brighter moons, including its biggest, Titan. Our image, with a simple webcam, resembles a view of Saturn through a small telescope on a night with average atmospheric disturbance, or seeing. The Moon: Don't neglect our own planet's natural satellite when you plan an observing session. There is no other object in the sky that we can see in so much detail. Craters and mountain ranges are easy to spot and you will never run out of things to look at on the lunar surface. What is more, their appearance changes from night to night as the Moon orbits the Earth. Because the Moon has no light of its own and shines thanks to reflected light from the Sun, its features vary according to the direction of sunlight upon them and the shadows they cast. You can see new features come into view or disappear completely into shadow as the phase of the Moon changes during its monthly cycle. Strangely enough, Full Moon, when we see the whole of the lunar disk, is not a good time to observe its fascinating detail. Sunlight then bounces straight down onto the surface and directly back towards us, with no shadows visible. You can still make out the lunar seas, or maria, plus it is a good time to spot the bright "rays" caused by material ejected from some of its younger craters blasted out by relatively recent asteroid impacts. The Moon looks especially brilliant at this time and a special Moon filter which you can fit to the eyepiece may help counter its glare. With such a wealth of features available to see on the Moon, you might like to check out Skymania's special Guide to the Moon. It includes a list of 50 fantastic features which you can print out and tick off as an observing project. The photo of a waning gibbous Moon, by Paul Sutherland, shows how shadows starkly pick out the craters and mountains. The Great Nebula in Orion: The mighty hunter Orion stands proud in the southern sky from the northern hemisphere in January. It is a bright and impressive constellation that actually has some resemblance to the figure of a man, complete with three stars marking the belt around his waist. Look a little below the belt to see a vague line of stars marking his sword. If the sky is dark, the Moon not too bright and you are away from streetlights, you will hopefully be able to make out a bright blur with the unaided eye. Turn your telescope on this blur and you will more clearly see it as a bright luminous patch - a vast cloud of shining gas in which new stars are being born. Don't use too high magnification, but choose a low-power eyepiece giving a wide field of view to see the Orion Nebula at its best. A higher power will show a bright grouping of some of these new stars, called the Trapezium, at the centre. The main nebula is also known as Messier 42, or simply M42, after a French comet hunter who catalogued fuzzy objects in the sky that people might mistake for comets. If you have seen photos in books showing bright billowing clouds in dazzling colours, you may feel disappointed when you view the nebula for yourself. This map will help locate M42. Remember that these photos are usually long exposures taken with large telescopes. You will be lucky to see it as other than a luminous silver, but it is worth spending time and allowing your eyes to take in the view. You may then discern a clear, sharper edged feature called the "fish mouth" and trace the swirls of gas as they extend and fade from the central region. The Pleiades: Here is an easy cluster of bright young stars visible in the constellation of Taurus the bull. You could never mistake them for a comet but Messier included them in his catalogue as M45. They are better known as the Seven Sisters, or Pleiades. Their popular name comes from the number of stars supposedly visible with the unaided eye, but many people can today only count six in our poor skies, although others count more than seven. Binoculars will show vastly more than this and you will be able to count hundreds when you turn a telescope on the region. Once again, use a low-power eyepiece to get a wide field of view and see the cluster in all its glory. Larger telescopes and long exposures with cameras will reveal wisps of gas in the cluster illuminated by the hot blue stars. This map will help you find the Pleiades.

Getting started with a new telescope

There's nothing that compares with your first outing with your telescope - what astronomers refer to as first light. What amazing things you will see and what great discoveries you will make! But often the reality is very different, and people see very little, usually because they are too eager to get going and don't puzzle over the instructions. But here are some steps to get you started, and which will overcome most of the hurdles that people encounter. Take the first step by daylight if at all possible. Step 1 (by daylight) Align the finder. People often wonder why they need a little finder scope attached to the side of the tube. It's there because the magnification of the main telescope makes it very hard to pick out individual objects, particularly in the night sky where there are no big landmarks to get started on. Some finders are small refracting telescopes, while others show you a red dot against the stars when you look from some distance behind them. Both types have adjusters that enable them to be lined up precisely with the main telescope. The secret is to do this job by day, so that you can find a distinctive and distant object easily through the main telescope (but not the Sun, which will blind you). It's easier to focus on daytime objects because there is more to see. Once you have found something with the main scope, adjust the finder so that its crosswires point exactly to the same object. What if...? What if nothing is in focus or visible at all? If you are using a refracting telescope, you may need to use the star diagonal supplied in order to get the telescope to focus. Always use the lowest magnification first (see Choosing the magnification). The focus position for nearby objects is slightly different from that for astronomical objects, but it should be similar, so leave the telescope focused on the most distant object you can observe by day. If nothing is visible, look through the telescope in the light without an eyepiece. You should see a circle of light, but if not try to work out where the obstruction lies - is everything uncapped? Check that the star diagonal if you have one is working properly and that nothing has been dislodged inside it. Look through it in the light - you should see a circle of light with objects mirrored at 90° from the direction you are looking. TIP: If your finder is a small telescope with single adjusters and you are having trouble adjusting it, make sure that the telescope tube is gripped properly by the mounting. If it slops around, insert a sheet of plastic to help as a washer to grip it. Step 2 (at night) Find a bright object (star, planet or the Moon) using the finder and use the lowest magnification on the main scope (see Choosing the magnification). You should see the object in the middle of the main field of view. Even if the view is out of focus, a star will appear as a large disc which you can focus down to a point of light. That's it - you have made your first-light observation. Notice how bright this star appears compared with its appearance in the sky with the unaided eye and through the finder. Get to know the field of view of the finder compared with the main scope by looking at groups of stars. Don't be tempted to increase the magnification until you have had a bit of practice with moving the telescope around and looking at different objects. Choosing the magnification Always begin your observations with the lowest magnification - that is, the eyepiece with the highest number on it, such as 20 mm or 25 mm. This usually gives the widest field of view and the brightest image. Even experienced astronomers do this - rarely will they start using a high magnification. As you increase the magnification (often referred to as power), four things happen. One, the field of view decreases, so an object which was not dead central at low power might now be outside the field of view. Two, if your telescope is not accurately set up and motorised to follow the stars, objects will drift through the field of view more quickly than at low power. Three, the focus position may change, so if there is no bright object in the field of view you may see nothing at all, or just faint discs of light which are the defocused images of faint stars. Four, the images of anything other than stars will get dimmer, so again you may not see anything at first glance. Contribution by Robin Scagell For more details and vast amounts of help with learning how to use your telescope, check out Stargazing with a Telescope by Robin Scagell. Click to buy this essential guide from Amazon in the USA or click here for the UK store.

How to collimate a telescope

First you need to be sure that your telescope needs collimation by testing it. Then you have to adjust the mirrors in the correct order, working from the eyepiece end downwards. What is collimation? Collimation means accurately aligning the optical components of the telescope to give the best image possible. This is usually only needed with reflecting telescopes, such as Newtonians and catadioptrics. Refractors do need to be collimated, but they have simpler designs and the collimation usually stays fixed once they are made. Because the mirrors of reflectors need to be recoated from time to time, it’s usually necessary to provide them with adjustments so they can be brought back into line after recoating. There is no reason why a well-made reflector should not stay properly aligned for a long time, but unfortunately many commercial instruments are out of collimation even when new, and may need to be collimated to give good images. People are often scared that they might make things worse, and are reluctant to meddle, but as even Celestron’s manual points out, the rewards of correcting bad collimation are well worth the effort. And judging by some of the questions I have been sent, collimation is easier to do than to spell! It’s colli-mation, not coll-mination. Is it necessary? Testing your telescope The easiest way to test your telescope for collimation and other defects is to observe a bright star. Do this outside, not through a window, and allow the telescope to cool down to night-time temperature or thermal effects will undoubtedly affect the image quality. With an eyepiece in place, and the star central within the field of view, slowly defocus the star image. As it goes out of focus it will get larger until you see a disc of light, probably containing brighter zones. If you are using a reflector, you will see a circular shadow within the bright disc. This is the shadow of the secondary mirror. In a well-collimated telescope, this should be dead central within the disc (but only when the star itself is centred in the field of view, remember). In extreme cases, the defocused disc is some kind of weird crescent shape, in which case the telescope is badly misaligned. If the circular shadow is not dead central, you will need to collimate the telescope. This is possible while looking at the star, but doing this makes a difficult job much more tricky because as soon as you start to adjust the mirrors, the star moves away from the centre of the field of view. So it is best to carry out the task during daylight, or in a brightly lit area. While you are looking at the defocused star, however, you can check other things. Is the outline of the bright disc completely even and circular? A common fault is to find that there are permanent blobs or streaks at the edge, often arranged at 120º to each other. This is a sign that the mirror is too tight in its cell. It should be held just tightly enough that it won’t move, without putting any strain on it. You may also see other shadows. The legs of the spider assembly will also be visible, and there is not much you can do about these as they are part of the structure of the telescope. There may be a rectangular shadow jutting into the edge of the field of view. This is probably the focusing tube of the eyepiece, and is a sign that the telescope has not been well designed. For every departure from a plain, perfect disc of light, you are losing performance. Check the shape of the disc on either side of focus. If it shows a radically different structure on either side, the mirror is probably badly made. If, as you go through the focus position the disc is slightly elongated in one direction, then the other, there is a fault known as astigmatism somewhere in the system. Neither of these faults can be corrected. Many telescopes I have seen show slight astigmatism, and as long as you can get a good star image it is not too serious. You may find that it is worse with low-power eyepieces than at higher powers. Collimation In a nutshell, to collimate your telescope you start from the eyepiece end and work your way down to the main mirror. You first adjust the secondary mirror so that you can see the main mirror centrally within it, then adjust the main mirror so that you can see the top of the tube and the secondary centrally within that. What follows is this procedure in detail, for the faint-hearted. Step 1 Before you start to collimate your telescope, you need one essential accessory – a dummy eyepiece of sorts, with a central hole. This could be nothing more than a piece of card with a hole punched in it, stuck over the eyepiece draw tube with the hole dead centre, or you could make something a bit more elaborate if you want. 35mm film cannisters are excellent raw materials for this. The size of the central hole is not critical. Its purpose is just to make sure you look right down the centre of the drawtube, so about 5 to 7 mm is fine. Step 2 In conditions bright enough that you can see inside the telescope tube, look through this hole and work out what you can see. When collimating, the key thing is to adjust everything from the outside inwards. The outside edge of your view is the end of the draw tube, and beyond that is the secondary mirror. To start with, ignore anything that the secondary reflects. Its outline should be centrally placed within the outline of the eyepiece tube. If it is not, serious adjustment of the secondary position is called for. In the case of catadioptrics, it is not usually possible to adjust the position of the secondary within the tube, as this is fixed in place. In my experience, even in a Newtonian the secondary is not often grossly misaligned, except perhaps in home-built telescopes, so I will not dwell on the possibility now. Get the hang of the other adjustments first. Step 3 Now look at the reflection in the secondary. You should see the outline of the main mirror somewhere within the outline of the secondary. Ideally it should be central and symmetrical, but if not you will have to realign the secondary. This is possible in both Newtonians and catadioptrics. There should be a central screw or rod that moves it in and out, and there will probably be three screws for adjusting its alignment. On a cat these screws may be hidden by a blanking plate. You may need to slacken off the central screw slightly, which also holds the mirror in position, before you can adjust the outer three. Your aim is to be able to see the outer main mirror dead centre within the outline of the secondary. Make tiny adjustments so you can see which way each screw moves the mirror, and keep adjusting the other two screws so as to keep the mirror fairly well held. If you loosen one, tighten the other two, and vice versa – you don’t want it so loose that it can slop around and ruin your adjustment. Step 4 Once the secondary is properly adjusted you can now look even more centrally, still through your dummy eyepiece. What can you see reflected in the main mirror? Ideally, it should be the top end of the tube, with the secondary central within that. If you can see more of one end of one side of the tube than the other, with the secondary also off-centre, you now need to turn your attention to the three adjusters at the back of the main mirror. On catadioptrics there are no such adjusters available to you, and adjusting the secondary is as far as you can go. On most Newtonians you will find three pairs of screws. One of each pair is a locking screw, while the other is the adjuster. Slightly loosen the locking screws, and by trial and error (made much easier by a helper doing the adjustment while you look through the dummy eyepiece) adjust the other screws to get the secondary central. Again, take up the slack with the other two screws as you adjust one. Eventually you should get everything in line and you will be able to see your own eye looking up at you from the dead centre of the main mirror. Tighten the locking screws, but not to battleship standards. You don’t want to break the casting of the mirror cell. Now everything should be lined up and tight. Check by swinging the telescope around a bit and make sure the alignment is good in all tube positions. If not, one of the mirrors could be loose in its cell which is another matter. In most cases, however, your telescope should now be properly collimated. These instructions are rather long, to allow for most possibilities, but in practice the job is quite simple and should take only a matter of minutes with a bit of practice. The shorter the focal ratio (eg f/4), the harder it is, but the more essential. Tip You may find it hard to judge the centre of the main mirror when looking through the eyepiece. If you can get to the mirror there is a simple trick that might help. Get a small blob of Blu-tack, and by measurement with a ruler stick it dead centre. This sounds drastic, and you might worry about it affecting telescope performance, but the central part of the mirror should always be in the shadow of the secondary anyway and never contributes to the image. An alternative is to stretch two pieces of thread from one side of the tube to the other, close to the main mirror, so as to define the centre of the tube where they cross. Contribution by Robin Scagell

Testing your telescope - Part 3

The previous test involved a bright star. This next test uses the same bright star, but is much more difficult to carry out. If you have a small refractor on a fiddly mounting it may defeat you at first, but keep trying! Knife edge test This is a very similar test to the one which amateur telescope makers use when making mirrors - the Foucault test. In this case, though, you need no elaborate equipment, just a simple knife edge. If doesn't have to be very sharp (indeed, that would be dangerous, particularly in the dark); all that is necessary is that it be straight, with a well defined edge. A small table knife is perfectly adequate. Keep tracking your bright star, but now remove the eyepiece and look into the eyepiece hole. Put your eye very close up and you should see the main mirror or lens filled with light from the star. If you are too far away you will simply see a shaving mirror type image of the star, so you will need to rack the eyepiece tube in somewhat. Having done this, bring your knife edge across the front of your eye, fairly close to it (now you know why it shouldn't be too sharp!). You will see a shadow move across the mirror or lens as it cuts off the light from the star. If you are finding this too difficult, on account of the star moving out of the field of view all the time, practice on a distant streetlight. But the streetlight is not a point source, so it won't be good enough for a proper test, and you must go back to the star once you have practiced cutting off the image. If the knife edge shadow appears to move in the same direction as the knife edge itself, then you are holding it at a point inside the focal point of the telescope. But if the knife edge is outside the focal point, its shadow will actually appear to move in the opposite direction. You must establish where the focus point is by chopping the beam in different places so as to get the shadow moving in one direction, then the other. The first diagram shows why this happens. The light from the star comes to a focus, forming an image, which you normally look at through the eyepiece to magnify it. In order to simplify the diagram, I have just put a straight line for the objective - it could be either a mirror of a lens. If it is a mirror, then of course you will have a secondary mirror as well, but I have left this out for the sake of simplicity. If you chop the beam near to the mirror at point (a), then obviously the shadow will seem to move in the same direction as the knife edge. But if you chop the beam beyond the focus point, it will first cut the rays coming from the opposite side of the mirror, as shown at (b). When you chop the beam at the focus point itself, the shadow should not come from any direction - it should cover the mirror instantly, as it is slicing into the point image of the star and not spread out beam. This is the crucial point for your test - when the knife edge is precisely on the image of the star. If the optics are perfect, practically all the light from the star will be focused at that point, so all the light you can see is cut off. But if there is an imperfection, resulting in some of the light being diverted to one side of the point of light, the knife edge may not cut off all the light, and you will see a bright area remaining after the rest of the light has gone. Alternatively, that bit might be cut off first by the knife edge, and will go dark before the rest. So with the knife edge just at that critical point you get a view of all the hills and valleys on the mirror that shouldn't be there - deviations, that is, from the perfect paraboloidal figure. The second diagram shows how a shadow zone can be caused by a defect in a mirror throwing some of the light to one side of the image instead of into the focal point. This sort of thing can be caused by the mirror being left too long on a polishing machine. The effect has been exaggerated in the diagram. In carrying out this test, you may discover three apparent strain marks evenly spaced around the rim of the mirror. This is almost certainly due to the mirror being gripped too tightly by its clips. A mirror should not be held tightly, but should just be able to move very slightly. Another effect you may see is turbulence moving across the objective. This is very local, and could be caused by, for example, a chimney pot just below the line of sight to the star. Or it could be the result of your own body heat, or the telescope itself not being at air temperature. Test the effect by putting your hand into the line of sight - you'll probably see the heat rising from it. This setup is the basis of the schlieren system used in wind tunnels to reveal air density variations. If you can see turbulence, you must wait until it has subsided before you can test the optics. To analyze the appearance of the mirror further, you'll have to consult a book on mirror making, for the test is very similar to the Foucault test. But in that test you use a pinhole close to the mirror, and aim to produce a certain appearance of shadows on the mirror, whereas when using a star for a test object, you want to see no shadows at all. If you have a mirror which shows a bright rim on once edge using this test, it has what is called a 'turned down edge' which may reduce the contrast of the image. The best way to deal with this, if it is not serious, is simply to mask off the offending bits of the mirror. If you suspect that the secondary mirror of a reflector is at fault, the thing to do is to allow the star image to drift so that you are looking at the main mirror through different bits of the secondary. This again calls for care and practice, so you know what you are looking at. Having done all this, you should now have a good idea of the performance of the telescope. not only will you have tested the optics - but in trying to keep the star in the centre of the field all the time, you'll know just how steady it is mechanically as well! Contribution by Robin Scagell.

Testing your telescope - Part 2

Our previous article described simple checks you can make, including viewing the bright Moon, to show that your telescope's optics are in order. This next test involves pointing at a star. Bright star test Find a good bright star reasonably high up. A first magnitude star is ideal, particularly a white one such as Vega. Train your telescope on it and focus it as carefully as you can: you should be able to get it to a single very tiny disc of light (assuming that you haven't chosen a double star!). Use a moderate magnification, of the same order as the aperture in millimetres (that is, 150 for a 15 cm - 6 inch - instrument). You may have read that a good telescope will show what are called diffraction rings around star images. This is not always the case: it depends on the aperture and the magnification as well as the seeing. A small telescope (say 50mm) will show these delicate rings surrounding star images quite readily if it is any good. However, a larger instrument may not show them unless you see a fairly high magnification and look at a fairly faint star. But if the seeing is bad all you will see is an animated blur, even if the telescope is perfect. So diffraction rings are not a good guide to performance. Once you have found the best focus point, move the eyepiece slightly outwards and inwards, equal amounts on either side of the focus point, and compare the defocused star images. What should happen is that the point of light opens out to a disc, getting larger and fainter as you go further from the focus point. This disc should be evenly illuminated and perfectly circular. In the case of a reflector which has a secondary mirror, you will see the image of this as a shadow in the centre of the disc. The defocused disc should be identical on either side of the focus point. If it isn't there is something wrong. You may find, for instance, that on one side of the focus the disc has a bright edge, which indicates a fault in the mirror or lens. Small deviations from the ideal should not affect the images too much, however. A very common fault which the bright star test will reveal is astigmatism - where the optics have a slightly different focal length in one direction. You will find in this case that the star image is never perfectly round, and that as you move very slightly inside focus it elongates into a short line. The other side of the focus, it turns into a line at right angles to this one instead. If you detect this fault, it's worth checking that the eyepiece isn't the cause. Rotate the eyepiece but not the telescope: if the short line moves as you turn the eyepiece, then it is at fault. Your eyes, too, can produce the same effect and you may have to try rotating your head to check this (it's not easy!). But if the effect is not in the eyepiece or your eye, then the problem lies with the rest of the optics. Incidentally, as you move outside focus you are actually focusing on comparatively nearby points in space - the atmosphere a few hundred metres away from you. You may well see streams of turbulence against the defocused disc, indicating that this may be the cause of the bad seeing. The next test will use the same bright star, but is much more difficult to carry out. Contribution by Robin Scagell.

Testing your telescope - Part 1

Stargazing guides often recommend that you get an expert to check a telescope before you buy it. This may be excellent advice but it can be difficult to do in practice. If you are just starting out, you may not know anyone else interested in astronomy. And even if you do, the chances are that they know little more about telescopes than you do! Then of course there's the biggest problem of all - how do you get your expert to the telescope on a clear night before you've bought it? So here is another way: a series of simple checks which you can carry out for yourself on a clear night, and which should at least tell you how good your instrument is, once you've bought it, plus where any faults lie. No complex test equipment is needed, just plenty of time, including allowing the telescope to settle down to night-time temperatures. Don't take it straight out of a warm room. In the shop Unfortunately, there is no quick way to test an astronomical telescope during the day in the shop. All you can do is to look through it, but there are so many other factors which can affect the image that this tells you little. It may seem poor, yet be ideal for astronomy, or it may seem to give a bright, sharp image by day and dismal images at night. Only if you can see strong, false colour around the edges of objects silhouetted against a bright sky (such as TV aerials or chimney pots) can you be sure that the fault will be worse on astronomical objects. A small amount of false colour is inevitable anyway in all but the most expensive refracting telescopes. A lunar check-up The Moon is an excellent object to begin your checks with, especially when it is high up and bright. The bright lunar limb should appear perfectly sharp and free from false colour under low powers. If you can detect ghost images of the limb, or cannot get it sharp, then there is something definitely wrong. Increased magnification will start to show up any faults, but you can easily be misled by atmospheric turbulence - seeing - which has the effect of blurring the image and sometimes even producing its own double images as air cells act as lenses in their own right. If your instrument fails the Moon test, you will want to find out why. If it passes, then you will still want to test it further. So in the next guide, we will show you how to test on a bright star. Contribution by Robin Scagell.