INTRODUCTION TO ASTRONOMY

This article featured in the June 2003 Beginners Magazine

OBSERVING MESSIER OBJECTS

8. USING AN ASTRONOMICAL TELESCOPE

Last month in article 7 of this series, advice was given on observing the Moon which is always a good object to start with. In this article we will be moving on to observe the planets and Messier objects.

OBSERVING THE PLANETS

Although some of the planets are quite bright they are still very small in the sky and can be quite difficult to locate for the first time. It may be necessary to locate the planet using a star chart such as the one on the back page of this magazine. With the exception of Venus and Jupiter there may be stars in the vicinity that are as bright if not brighter than the planet itself. Planets are not always present in the sky so it is worth checking the charts to see what there is to observe at the time of year. Planets are not fixed in the same position in the sky as stars are, this is why they are called planets which means wanderer.

A low powered eyepiece, 25mm focal length or longer should be used for the initial location. This type of eyepiece will give a low magnification, wide field view of the sky The telescope will first need to be pointed by eye in the direction of the planet to be viewed. The planet should then be located in the finder and centralised. Some finders may be fitted with crosshairs to simplify this task. Once centralised in the finder the planet should be visible in the eyepiece of the main telescope. The planet can then be centralised in the eyepiece and brought into focus by turning the knob on the focusing unit.

In the low power eyepiece the planet will look small and little detail will be seen. To enlarge the image the low power eyepiece can be carefully removed and replaced with a shorter focal length and higher magnification eyepiece such as a 10mm. The shorter the focal length of the eyepiece the higher will be the magnification. Great care must be taken not to move the telescope because at high magnification a small movement will move the planet out of view.

A further increase in magnification can be obtained by inserting a Barlow Lens into the focuser and then fitting the eyepiece into the Barlow Len body. The Barlow lens will typically double the magnification of an eyepiece but some may triple it. Higher power may not always give the best view. If conditions are poor, perhaps because there is a lot of movement in the atmosphere it may be necessary to reduce the power slightly. For this reason many keen astronomers have a range of as many as eight eyepieces with different focal lengths.

The brighter planets such as Venus, Mars, Jupiter and Saturn are quite easy to find but the others are much harder. Mercury is difficult because it is always located close to the Sun and is therefore always close to the horizon at dusk or dawn. Uranus, Neptune and Pluto are difficult to find because they are so faint. Uranus can be viewed using a 150mm reflecting telescope or a 100mm refractor. Neptune is more difficult but still possible. Pluto will probably require a 200mm and a good star chart to find.

OBSERVING MESSIER OBJECTS

Messier objects are clusters of stars, galaxies or clouds of gas and dust known as nebulae. A catalogue compiled by a French astronomer named Charles Messier lists 110 of the brightest of these objects. Messier was a comet hunter in the 18th century who became very annoyed when he kept finding 'fuzzy' objects that looked like comets but were not. To avoid being caught out Messier compiled his catalogue of all these 'fuzzy' objects he did not want to see. These objects are now the objects that most new astronomers seek out after they have checked out the planets. The 110 objects in Messier's catalogue are identified by being prefixed by a capitol M for example M3, M51 and M106. Messier objects can be grouped into three main categories, these are Galaxies, Star Clusters and Nebulae.

Galaxies are huge clouds comprised of millions of stars. Our Sun is one star in our galaxy that we call The Milky Way. All galaxies are located at vast distances from us and are therefore generally quite faint. One exception is M31, known as the Great Spiral Galaxy in the constellation of Andromeda. M31 can be seen any small telescope or even a pair of binoculars. As with all galaxies M31 should be observed using a low power eyepiece because they are faint and defused objects.

Star clusters come in two main types, Open Clusters and Globular Clusters. Open clusters are groups comprised of from ten or so up to a thousand or more stars that have formed in the same area of space within our galaxy. They may be quite close together or more often widely spaced. Most are best seen in a low powered eyepiece but the more closely pack ones may tolerate a little more magnification. Globular Clusters typically host between 10,000 and a million stars in a tight ball. They are located above and below the main spiral form of our galaxy and are almost like mini galaxies themselves. Some like M13 in Hercules and M3 in Canes Venatici (see page 1) are bright enough to be seen in binoculars or even with the naked eye on a good clear night. Globular Clusters are best found using low power but a higher power may be used to resolve the individual stars in the brighter examples.

There are a number of different types of Nebulae some examples are clouds of gas and dust where stars are being formed. The newly forming stars illuminate the clouds and cause them to glow. M42 in Orion is the brightest example of this type. Others are the remnants of stars which have reached the end of their active life and have blown off their outer layers. These are known as Planetary Nebulae because they looked like planets to early astronomers. M57 The Ring Nebula in Lyra looks like a tiny smoke ring and is typical of this type. Others are the remains of stars that have exploded in what is known as a Supernova, M1 in Taurus is one example.

All nebulae with the exception of M42 are faint and need low power to locate. A low power eyepiece will effectively concentrate the light into a smaller area so it will appear brighter. Once the eye has become used to the shape and form of the nebula a higher power eyepiece may be used to bring out some of the finer detail. This is particularly so with the brighter Planetary Nebulae but too much magnification will spread the available light over too wide an area and it may become too faint too make out any more detail.

In summary do not be too quick to increase the magnification, allow your eye and brain to become familiar with the object in view. It may require up to ten minutes for your senses to work out all the detail in a particular view. It is surprising how much more detail is discernable after a few minutes of concentration. When it is felt that no more detail is coming to your notice then experiment with increasing magnification. If no more detail is visible don't be afraid to go back to a lower power eyepiece.

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