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|Comet Hale-Bopp was spectacular in 1997|
The popular image of a comet is one of a bright head with a long tail trailing behind it. You might remember comet Hale-Bopp during the spring of 1997, for example, which stayed in the evening sky for several weeks and was visible even from town centres. People sometimes make the mistake of expecting a comet to shoot across the sky. However, the only natural objects that shoot across the sky are meteors (shooting stars), which are caused by tiny particles from space being vaporised when they hit our atmosphere. Comets, on the other hand, only move slowly from night to night against the star background.
There are dozens of new comets discovered each year, but most only appear as faint fuzzy blobs when first detected, and often never get any more impressive. At the heart of each comet is what has been called a 'dirty snowball' – a chunk of ice usually just a few kilometres in size, orbiting the Sun. This is the source of the gas which could produce a tail. Being so small compared with a planet, this body, or nucleus, is too faint to be seen during the long intervals when it is far away from the Sun, as comets are generally in very elongated orbits rather than nearly circular ones.
|Top: Nine months before its best appearance in 1997, Hale-Bopp was just a fuzzy blob seen in binoculars. Above: Movement of comet Machholz over 48 minutes as seen with a telescope. (Photos: Robin Scagell)|
However, when it approaches the inner Solar System, the warmth of the Sun causes some of the ice to evaporate from the surface of the comet's nucleus. This forms a small haze or coma around the nucleus and makes the comet easier for astronomers to pick out against the star background.
As the comet gets closer to the Sun more material is ejected, causing the coma to expand and, in some cases, one or more tails will develop. One tail, the plasma tail, will contain the smaller gas molecules, whereas the other will contain heavier dust particles.
In contrast to the small size of the comet nucleus, these tails can be thousands or in some cases millions of kilometres in length. Due to the effect of the solar wind, the plasma tail will always point away from the Sun, though the dust tail may be more curved. Hence, on its way out from the Sun, the comet will travel tail first. The particles of the dust tail gradually spread around the orbit of the comet and, if they eventually encounter the Earth, will be vaporised as the shooting stars mentioned earlier.
But not every comet has a glorious tail. The vast majority remain as tiny, distant fuzzy blobs that don't attract much attention. However, every so often a new comet turns up that comes closer to the Sun than most, and these are the ones that can become bright enough to be seen with binoculars or even the naked eye.
How bright will it be?
The first thing that the media will seize upon about a newly discovered bright comet is its predicted brightness. Rather inevitably, the media headlines will often only mention the most optimistic of the brightness predictions. The brightness measure usually quoted by astronomers is the total magnitude of the head of the comet. This is unfortunate because, although being a useful measure for scientists, it can be misleading for everyone else and can lead to disappointment. Because this 'total' brightness is spread out across a patch of sky, a comet of a particular brightness or magnitude will be less easy to see than a star of the same magnitude.
This can be a particular problem when trying to spot a diffuse comet against a sky background that is hazy and/or suffers from light pollution. You also need to bear in mind that most comets are brightest at around the time of their closest approach to the Sun. At such times you may well find yourself having to locate the comet during twilight and/or low down in hazy skies. Consequently it may not be as spectacular visually as the magnitude predictions might have suggested. Even a bright star is hard to see in the evening twilight, and a comet of the same brightness is more spread out than a star, which is essentially a point of light.
In any case, the behaviour of comets is always difficult to predict. Back in 1973-4, comet Kohoutek was very much over-hyped in the media. Even though it did become a quite bright comet, it wasn’t the 'greatest comet of all time' that some media reports, based on the more optimistic magnitude predictions, had claimed it would be. In contrast, other less hyped comets have unexpectedly flared up in brightness and in some cases have broken up into two or more fragments.
For most comets – which are quite faint – you need a telescope, and really the bigger the better. Comets are very similar to deep-sky objects, particularly elliptical galaxies, in their appearance, with a fairly condensed nucleus and misty outer regions fading into the background. So the darker your skies the better you will be able to see the full extent. But while most comets are very small, one of the much rarer close or great comets can easily be so large that a telescope shows just the central condensation and only binoculars will show the full extent of the tail.
When first observing comets, you will probably be happy to merely locate the comet and follow its motion against the star background from night to night. After a while, you may find it rewarding to sketch the appearance of the comet, noting the direction and length of the tail, how condensed or diffuse the head of the comet is, plus the presence of any other notable features. Using averted vision will make it easier to see the fainter details. It is also useful to mark the positions of nearby stars. Remember, of course, to always record the instrument and magnification used for your observation. As for other types of observing, the more you observe comets, the easier you will find it to recognize detail.
Photographing a comet
If a comet is bright enough to be visible with binoculars, but maybe not easy to see in your sky because of light pollution, it may be possible to photograph it using a digital camera and a short time exposure. You'll need to fix the camera firmly, usually on a tripod, use a telephoto setting pointing at where you know the comet to be, and give a time exposure of maybe 10 or 15 seconds. You'll need to be able to control the camera settings, such as the focus (autofocus is useless where a dark sky is involved) and the sensitivity. Use a setting ISO 1600 or higher. Even many compact cameras will usually allow this, but you'll need to read the instructions. Though the image may be faint on a single exposure, the trick is to add several images together in image editing software such as Photoshop.
Photographically, for faint comets you need the same set-up as for deep-sky photography, that is, an equatorially mounted telescope with some means of auto-guiding. However, while the auto-guiding would prevent deep sky objects from moving in the field of view, you’ll soon find that the comet has moved and you get a trail even if your star images are perfect. Auto-guiding on the comet isn’t usually feasible as the nucleus is not as bright and point-like as a star, so the auto-guider will have trouble. These days, it’s easier just to give short sub-exposures of around 30 seconds to a few minutes and add them afterwards.
Advanced visual observations
More detailed observations will include measures such as the degree of condensation (DC) of the head (coma+nuclear condensation), the orientation and length of the tail and the integrated magnitude (m1). The DC value ranges from 0 = uniformly diffuse, through to 9 = star like. The orientation of the tail is measured as a position angle in degrees from north (000) through east (090). Estimating the magnitude of a comet is challenging, even for experienced observers. Various methods exist, all of which are based around defocusing so that the nearby comparison stars also become fuzzy in appearance.
With more experience, you may wish to submit your observations for use in detailed analyses. Further information regarding the required report content and format can be found at www.icq.eps.harvard.edu/ICQFormat.html.