Joys of a small Telescope

JOYS OF A SMALL TELESCOPE
A note by Eamon Henry; 10 July 2008

Introduction:Using a small telescope is an acquired taste. A definite advantage is secured, if the first period of viewing is in rural “pitch-dark” conditions. Nebulae and planets can be found and enjoyed, such as the great nebula in Orion, much more easily than under the lit-up conditions of our urban areas.
What follows covers a few obvious themes. First considered are the main features of a “refracting” telescope, in which one views directly the object of interest. By contrast, in a “reflecting” telescope one looks into the side of the telescope tube to see a mirror-reflection image. Next, we list some usual problems encountered in using small refracting telescopes. As an easy first object, the Moon is discussed. Less easy, but of great interest, are a few Solar planets, viewable even in the glare of city lighting. Finally, one may ask what pleasure is to found from such viewing, even in freezing cold winter nights out of doors.
A Small Refracting Telescope Described:
A refracting telescope essentially comprises four parts: 1) a hollow opaque tube with a lens (the “object lens”) at the front; 2) a second much smaller lens (the “eyepiece lens”) fitted in at the back of the hollow tube; 3)a stand of three or four legs to support the tube; 4) a viewfinder (or “finderscope”) attached to the side of the tube. This viewfinder is a small telescope (of magnifying power say five-fold) to find the required typically small star-like object in a larger field of view. Centering this object in the viewfinder cross-hairs should make it visible within the field of view of the main telescope itself, if the viewfinder has been properly aligned in its attachment to the main tube.
Such a small “toy” telescope requires to be kept on viewing-target by hand, whereas more expensive models have electric motors to keep them pointing at the target, by adjusting for the Earth’s rotation, which in some 15 or 20 seconds can move the target-point right across the field of view and out of sight. Other features of the small “toy” type are that it can be rotated horizontally upon its stand, and screws allow elevation of the tube to point upwards as required, with possible further screw fine-tuning of this direction. Also, the eyepiece lens can be moved forward or backwards so as to adjust focusing on the clearest possible image view. This last aspect is rather important.
A small refracting “toy” telescope of Japanese make could cost up to 500 Euros nowadays. The typical main tube is up to one metre long, with the object lens of diameter 6 centimetres having a “focal length” of 90 centimetres (cm). The eyepiece lens can vary, but a sensible lens for this particular telescope is a lens of focal length 1.25 cm. The “power of magnification” is given by the ratio of these two focal lengths, that is 90/1.25, which is 72 for this particular combination. In other words, this lens combination will magnify any object 72 times, which is quite respectable, and works wonder for the Moon. An eyepiece of shorter focal length will give larger magnification, at the cost of a much weaker image of worse quality. Patrick Moore’s book Exploring the Night Sky with Binoculars (1986, Cambridge University Press) gives a very helpful background to the user of a “toy” telescope. Patrick advises a maximum magnification of 50 times per inch of object lens diameter – thus a 6 cm object lens should not be used to magnify an object beyond about 118 times ( 50X6/2.54).
Once assembled properly, the telescope should be left in one piece, except for manipulating the legs for storage indoors. The legs need to be based on a firm level surface, during times of viewing. They are typically connected together by a few light chains, to allow maximum distance apart when supporting the telescope during use.
Some Common Problems:
Several problems are commonly experienced by viewers using the small telescopes described above. Some of the more obvious ones are listed as a) to e) following:
a) A comfortable field of view is possible for the tube pointing between say 10 degrees and 45 degrees above the horizon. Pointing higher involves bending back or other similar neck strain by the viewer. The most comfortable position is to be seated on a small chair behind the telescope legs (typically of wood or plastic and some four feet long) and thus have some comfortable flexibility of shoulders and neck, in following the required tube direction.
b) Getting used to the image being inverted. Thus, a local view of a tree-top shows the top of the tree at the bottom of the picture seen through the telescope.
c) Earth’s rotation causes the target point to move across the field of view from right to left (usually). The inversion effect give the opposite of what we see as the Sun moving across the sky from left to right during the course of the day.
d) Clouds or haze make viewing impossible. Typically the best conditions occur during a clear frosty night, but even here some dew can form on the object lens during a lengthy session out of doors. The lens can be dried off, of course, given that the dew has been recognized.
e) Glare and general brightness, caused by urban street lights and traffic, destroy all delicate features of objects being viewed. The Moon is so strong that it is not seriously affected. Likewise, our four nearest planets have light strong enough to give their outline shapes. But gas-clouds (“nebulae”) are mostly eliminated from possible viewing. By contrast, in a situation of rural “pitch-dark” viewing, the planet Jupiter is surrounded by a globe of light, with possible star-like points indicating its four largest satellites. And gas-clouds such as the great nebula in Orion and the Crab Nebula are clearly visible, with some features showing.

The Moon as an Easy First Object:
Moore’s book has “Lunar Landscapes” as its Chapter 9, giving both photographic and verbal description of the Moon throughout its four-week cycle. The Moon is so large and bright that one could view it from indoors through a clean glass window. A magnification of 72 times gives some remarkable views of craters and cracks. Being large implies that the viewfinder is not needed, unlike the case of a planet. Also, one can get some experience of adjusting for the object moving slowly across the field of view, so as to get back again to the top right point of starting a second pass, and so on. This helps towards similar adjustment for a small object like a planet.
Planets Venus, Mars, Jupiter and Saturn:
Venus is the easiest to recognize, as the “morning star” and the “evening star”. It is usually very bright, and appears as a clear orange-coloured half-moon shape, which can vary in size. Mars is small but strongly red in colour, and will show only an oval red disk. Jupiter gives a very bright and steady light if visible in the late evening, and shows (in urban conditions) an oval-shaped yellow-green globe. Saturn also is rather bright if visible in the late evening, and in the telescope looks silvery-grey, with suggestion of partial ring as well as globe under magnification 72 times. All of these planets, of course, on average pass over us as much during our day-time as during our night-time, so that viewing them after sunset between say 6.00p.m.(winter time) and midnight is possible only in some years during October-March. Regarding Mars, Jupiter and Saturn, if recognized at say 6.00p.m.fairly low-down in the eastern sky, they gradually progress at same time 6.00p.m. from east to west over the following six months.
What Pleasure is to be Had in Viewing:
A few thoughts may give the basis for satisfaction through these kinds of telescope viewings, even with a “toy” instrument.
No telescopes were available, as far as is known, before about 1600 A.D. Even the ancient brilliant Greeks were not able to invent a telescope, simple though it is as comprising two lenses in a tube. The best-known names of earliest viewers, who adapted telescope designs for their own purposes, are Galileo Galilei (1564-1642) and Isaac Newton (1642-1727). Thus, with at least 70-fold magnification we are seeing the heavenly bodies in a way that nobody could see them before 1600 A.D. The Moon excepted, things above generally could only be seen as points of light. Comets, of course, showed large on occasion.
There is also great satisfaction in being able to look “here and now” at some bright object in the sky, rather than depending on someone else’s pictures. As a final advice, in suburban conditions, one should find a dark alley or the centre of a large open space in a park, to improve viewing. Getting away from direct glare of lighting helps considerably towards a satisfactory viewing result.