INTRODUCTION TO ASTRONOMY

This article featured in the May 2003 Beginners Magazine

MORE ABOUT TELESCOPES

7. USING AN ASTRONOMICAL TELESCOPE

Last month in article 6 of this series, advice was given for setting up a telescope prior to an observing session. This article suggested setting up as much as hour before the intended start time to allow the telescope to acclimatise and in a position where the intended targets would be visible. Also to make sure all the equipment that might be required is to hand. It was advised that at least 10 minutes should be allowed for the observer's eyes to become dark adjusted.Make sure the mounting is stable and if more comfortable use a stool or chair because if you are uncomfortable then you will not be steady at the eyepiece and observing will be impaired. If it is not possible to sit it may be useful to rest against something, perhaps a small step ladder which can also be used if the eyepiece is high as may be the case with a large Newtonian.

GENERAL OBSERVING HINTS

When finding an object, using a manually pointed telescope, always use a low powered eyepiece, 25mm focal length or longer. This type of eyepiece will give a low magnification, wide field view of the sky. Even though this view is comparatively wide it is still very narrow for locating an object in the sky. As an example if a telescope using a 25mm eyepiece can view the full image of the full Moon, this is only half a degree wide. Therefore only about 1/130000th of the visible sky can be seen through the eyepiece and even less in a shorter focal length eyepiece. For this reason a smaller telescope with a wide field of view is normally mounted to the tube of the main telescope. This small telescope is called the Finder and is used to initially locate the object to be viewed. The telescope will need to be pointed by eye in the direction of the object to be viewed. Then the object is located in the finder and centralised. Some finders are fitted with crosshairs to simplify this task. Once centralised in the finder the object should be visible in the eyepiece of the main telescope. If it is not then the finder may not be correctly aligned. To adjust the alignment it will be necessary to point the main telescope at a bright object that will be easier to find, the Moon is ideal. Centralise the object in the main telescope then use the mounting screws to adjust the aim if the finder. Alignment can also be done during the day by pointing the telescope at an aerial or chimney.

OBSERVING THE MOON

For a beginner to astronomy, the Moon is an excellent place to start. It is large, bright, easy to find and covered in interesting things to see. It may still be necessary to locate the Moon as described above but a seasoned observer may be able to find it straight away without using the finder. The full Moon is most impressive to the naked eye but is probably least rewarding through a telescope. At full Moon the Sun is shining straight down on the surface and casts very little shadow. The best time to see specific features is as the terminator (the line between light and dark) passes over those features. The full Moon is also so bright that it may be uncomfortable to the eye. Filters can be bought to attach to the eyepiece to reduce the brilliance and improve contrast. A cheaper option is to make a cardboard mask to cover the end of the tube. Into this mask a hole of about 75mm can the cut to reduce the amount of light entering the telescope.

Depending on the conditions, high magnifications can be used. First centralise the object or region of the Moon to be observed using a low power eyepiece then carefully replace the eyepiece with a higher magnification (shorter focal length) eyepiece and refocus. The object will appear larger and more detail will be seen. Some things to look out for are obviously craters (more below), mountains, Maria (seas) valleys, rills and canyons.Craters come in many sizes, shapes and form and may even look completely different under differing lighting conditions. Large craters often have a central mountain or peak that may cast a shadow across the relatively smooth floor of the crater. Some of these larger craters may have terraced walls both inside and outside the main rim. There may even be smaller craters on the floor of the large crater or another crater may cut through the wall of a large crater. Some areas of the Moon are more cratered than others. There are large areas that have so many craters that there appear to be no smooth areas at all other areas have almost no craters.

The Moon showing part of the back (taken from a spacecraft)

Maria or seas are not seas at all, they are large areas that have been covered by molten rock in the distant past but later than the main crater forming era. To the naked eye the maria appear as darker patches but through a telescope they are relatively smooth plains with a sprinkling of craters.

Mike Bilton using a Phillips Vesta web camera took the picture above. The picture gives an impression of the view of the Moon through a 150mm (6 inch) telescope using a high magnification eyepiece. A large mare can be seen crossed by a mountain range. In the lower part is a crater with a central peak.

Next month we will look at observing the planets and Messier objects.

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