ABSOLUTE BEGINNERS

This article featured in the April 2002 Beginners Magazine

USING AN ASTRONOMICAL TELESCOPE

When setting up the telescope make sure the mounting is stable. If it does wobble it may have a stone or other object under one of the feet. If you are going to view at night set the telescope up just before it gets dark. There are two reasons for this, first it is easier if you can see what you are doing, second if the telescope has been kept indoors or in a garage or shed, it may take an hour or so to acclimatise (cool down) before it will perform at its best.

Directly the telescope is set up remove the tube cover to allow air to flow through the Tube and around the mirrors. Do not fit the eyepiece until you are going to use the telescope because its lens may get covered with dew. Leave the Finder Cap on until you want to use it or it to will have its lens covered with dew. Dew can of course be wiped off with a cloth or tissue.

Some hints on positioning the Telescope. Use the patio or path they are comfortable and simple for casual viewing but there will be less air turbulence on the grass when you want to do more serious observing. The best position would be on a small path or on the edge of the patio near the grass. The patio stones or walls get warmed during the day and give off their heat in the cooler night air so try to avoid viewing across close paved areas or walls. Rising heat will cause the image to shimmer like the road on a hot day. Obviously try to set up away from trees or buildings but this may not be possible so set up in the best place to view your intended target, you can always move to another position later. If you are viewing low objects sit on a chair or something, you will be more steady and a lot more comfortable.

To start viewing allow about five minutes for your eyes to get used to the dark. This period can be used to familiarise yourself with the sky and work out where everything is. Try to turn off all lights around you. If there is a street light bothering you it may be possible to erect a screen around yourself using canes, step ladders, washing poles, string and old sheets, curtains, towels or even news papers. Even lights which appeared dim, when you first began your session, seem to get very bright when your eyes are fully adjusted to the dark.

It will be necessary to find the target object using the smaller telescope, called the 'Finder' which is normally fitted to the main telescope tube. Once the target is located in the centre of the finder it should be visible in the eyepiece of the main telescope. A low power eyepiece should be used first. A low power eyepiece is one with a longer focal length such as a 20mm, 25mm or even 35mm or the longest available. Once located using the low power eyepiece a higher power eyepiece can be substituted. A 10mm or shorter is classified as a high power but may be as short as 6mm or even 4mm. The magnifcation being usd can be calculated by dividing the focal length of the main optic the mirror or lens by the focal length of the eyepiece being used. For example a 10 mm eyepiece used with a 6 inch (150mm) telescope with a focal ratio of f8 which will have a focal length of 6 x 8 = 48 inches (150 x 8 = 1200mm). Therefore the magnification will be 1200 divided by 10 =120x.

Remember a very high power is not always best because it will also magnify the air movement and the effect of any mist in the air. Try your low power and high power to see which is best for the conditions and the object beeing observed.

OBSERVING THE STARS

Stars seen through any telescope will only appear as points of light. Larger telescopes will make stars appear brighter and will enable more to be seen because they can concentrate more light from the stars into the eye of the observer. If stars appear bigger it will be because either the telescope is out of focus or air movement is distorting the image. The air currents could be inside the tube because the telescope had not had time to cool down. Air movement in the atmosphere or heat rising from buildings, walls or patios can also be a problem. The brightest stars do appear to look larger but this is only because of air movement. These bright stars sometimes appear to burn and flash different colours especially if they are low in the sky as they a viewed through more atmosphere and the lower atmosphere is more Turbulent.

When looking at stars, a low power eyepiece should be used, this gives a wider field of view and makes it easier to find a particular object. If you are looking for a group of stars, use the low power to locate the group then change the eyepiece for a higher power to resolve the group into individual stars. Although we cannot magnify the image of a star there are many different types and many different groupings and formations, there follows a few things to look out for. Stars vary in colour ranging from orange through true white to blue. Colour is caused by the temperature of the surface of the star. Our Sun is classified as a yellow dwarf and is very much an average mid range star with a surface temperature of about 6400 degrees C. See the Absolute Beginners article on star colours.

GROUPS OF STARS

Our Sun is a single star but many stars, probably about half of all stars have companions. It is thought that most stars form in groups from huge clouds of gas and dust. We can see this happening in the Constellation of Orion in the winter sky. When the stars first begin shining they are generally very powerful and blow away all the gas and dust left over from the original cloud leaving the group of new stars. The group called The Pleiades in the constellation of Taurus is one such group which still has traces of the gas surrounding the stars. Over millions of years these groups may break up as the stars drift apart. Lets just consider some types of groups.

DOUBLE AND MULTIPLE STARS

Many stars can be seen to be doubles, some with the naked eye, some only wheb using a telescope. These stars are actually associated, that is they are revolving around each other just as the Earth and Moon do only much further apart. It is most likely they formed at the same time in the same gas cloud. Some stars like Mizar, the middle star in the handle of the plough (Ursa Major), has a naked eye companion, Alcor. When viewed through a telescope another star can be seen close to Mizar making this a triple system. There is another type of double where the stars are not associated but are just in the same line of sight as viewed from Earth. Some are actually further apart from each other than the first one is from us. The target should first be found using the Finder Scope and a low power eyepiece. Once located a higher power can be used to separate the stars.

OPEN CLUSTERS

These are groups of stars probably formed in groups in the same cloud of gas and dust. There are many of these Open Clusters with numbers of stars varying between a few tens to a few thousand. The Pleiades in the constellation of Taurus is the brightest such Open Cluster group which has six (or to the keen sighted seven) bright naked eye stars. The group may actually contain up to 500 fainter stars. The Pleiades occupies such a large area of sky that it will not fit into the field of view of most 150mm telescopes, even with a low power eyepiece. Most Open Cluster however are much smaller and may need a higher power to resolve into individual stars.

GLOBULAR CLUSTERS

Globular Clusters may in some cases look similar to Open Clusters but they are in fact very different. Open Clusters are formed within our Milky Way Galaxy (More about Galaxies in Ablolute Beginners Galaxies). Globular Clusters are comprised of very old stars and are found in a halo around the main part of the galaxy. There are about 100 known and they are comprised of between a few thousand and a few million stars. The brightest from our country is in the Constellation of Hercules and is known as M13. A low power eyepiece should be used to locate the cluster and some may be bright enough to take a higher magnification.

GALAXIES (See also Galaxies)

All the stars we can see in the sky even using our 150mm telescopes are in our galaxy, called the Milky Way. The Milky Way is a spiral galaxy which if we could see it from above would look like a Catherine Wheel or a whirlpool. There is a central ball of stars from which curved arms comprised of millions of stars extend. Some galaxies can be seen with small telescopes but with large powerful telescopes countless millions can be seen stretching out in all directions, to the limits of our largest instruments. To view most galaxies at least a 6 inch telescope will be required and a low power eyepiece. Some brighter galaxies such as the Great Andromeda Spiral Galaxy M31 can be seen using a smaller telescope and even binoculars. In most amateur's telescopes individual stars cannot be seen only a faint misty patch of light.

ASTRONOMICAL TELESCOPES

A telescope is an optical instrument designed to gather light from a distant source, focus that light into an image and then magnify that image. There are basically two types of telescope, REFRACTORS AND REFLECTORS.

THE REFRACTING TELESCOPE

The Refracting telescope consists of a primary light gathering lens which focuses the gathered light into an image. This image is then magnified using a small microscope called the eyepiece.

Almost all refracting telescopes are totally enclosed in a sealed tube. Refracting telescopes have a number of advantages over reflecting telescopes. Firstly dust is prevented from contaminating the parts of the instrument which would be awkward to clean. Secondly the volume of air enclosed by the tube has less turbulence to distort the image. There is no secondary mirror in the light path so the instrument can use the full aperture of incoming light. For these reasons the average refracting telescope will perform about one and a quarter times better than an equivalent sized reflecting telescope. There are however two disadvantages. As light passes through the glass lens a small proportion of that light is absorbed by the glass. Also a certain amount of colour distortion is produced, this is known as "false colour". To a large extent this can be rectified by the use of a compound lens. That is one made of two or more lenses of glass with different refractive indexes which correct the false colour produced by each other. Each lens will however absorb a small amount of light and more lenses means more lost light. Most refractors have a focal ratio of around about f.15 which will necessitate a long tube. For instance a 150mm f.15 lens would require a tube in the order of 2300mm long which can cause considerable mounting problems. Because refractor lenses are made from selected bubble and flaw free glass blooms and have at least four surfaces to be ground and polished., they are much more expensive than reflector optic glasses of a similar light grasp. Large reflecting telescopes are therefore usually well beyond the means of most amateur astronomers.

THE REFLECTING TELESCOPE

The reflecting telescope has a mirror which is normally a glass plate ground concaved. The concaved surface is coated with a thin film of Aluminium. This mirror is used to gather the incoming light instead of the lens which the refracting telescope has. There are a variety of reflector configurations but the basic layout is as follows. The concave mirror is positioned at the bottom of an open top tube. Light is allowed to enter the top of the tube and pass down on to the mirror surface. The light is reflected back up the tube on a converging path. Before the point at which the reflected light forms an image a second small flat mirror is positioned in the path to reflect the light out of the tube and into an eyepiece where the image formed may be observed.

EQUATORIAL MOUNT
THE AUTHORS 14 INCH HOME MADE DOBSONIAN

Most amateur astronomers choose a reflecting telescope because it is possible to become the owner of a quite large telescope for a quite modest cost, compared to a refractor. As the light gathered does not pass through the primary optic there is no false colour so it is an ideal instrument for observing the colour of stars or planets. Being a generally 'short' focal length, normally f4.5 to f8, a reflector represents a 'fast' system for photography and a wide field of view for rich field study, that is star clusters and nebulae. This shorter focal length also allows for a wide choice of magnification powers from very low to quite high, depending on the eyepiece used. Reflectors do however require more maintenance than refractors, due to the open tube. The system must also be regularly checked for alignment, which is however a fairly simple operation. The primary mirror and secondary will also need re-coating about once every five years. This is not expensive but the instrument will be out of action for a week or two while the work is being done.

The type of mounting shown above is an EQUATORIAL MOUNT which will be required is serious photographt is to be undertaken. The equatorial allows stars to be tracked across the sky using only one axis, to compensate for the rotation of Earth. The Dobsonian mounting system is a simplified type of Altazimuth mounting used for reflecting telescopes. The telescope tube is supported in a box section with a trunnion bearing. The box section is in turn mounted on a simple turn table. This type of mounting is simple, cheap and easy to set up and favoured by many amateurs.

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