INSTRUMENTS

ASTRAL TELESCOPE

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OTHER STARS IN THE SOLAR SYSTEM

In our solar system there are not only nine planets with their natural satellites as well as the Sun and the Moon. There are also asteroids (wandering rocks of varying dimensions from a few centimeters to a few kilometres) and comets. Asteroids, observable in any case, have stellar aspects. They appear as points of light and therefore a fair amount of experience is necessary, as well as a good astronomy almanac in order to find them in the sky. They do not, however, inspire a great degree of enthusiasm.
Comets,
Small bodies made up of ice, ammonia and other volatile components can be spectacular when they come near to the sun and, because of the radiation and energetic particles emitted by the sun they develop an enormous tail and a corresponding tail of substances that evaporate. The tail can become long and bright and may be observed from earth. The comets can move with predictable regularity around the sun (periodic comets) or else can be discovered at the moment when they draw near to the sun. Comets can be observed only at a low level of magnification because of their large angular extension. In fact, magnification is in inverse proportion to the extension of the field of vision in the finder. In other words, the more you magnify, the less you have in your field of vision. Since the comets which have long tails are objects which have substantial extensions, it will be necessary to observe these with a telescope of short focal length, or else with an ocular of very long focal length.

THE STARS
During a perfect moonless night, in the high mountain regions or on an out-of-the-way island which is not polluted or lit up by artificial light, it seems that you can see, with the naked eye, millions and millions of stars, whereas it you could count them, there would, in fact, be only 3600 at most. If, for example, we take any particular area of the sky in which we count let us say 20 stars visible to the naked eye, and then we aim the telescope, furnished with an ocular of 20mm, at that same area of sky, we will notice that with the ocular we can count a far greater number of stars. Why is this? Because the greater the "eye" (or, in this case the objective lens) the greater the number of weaker stars we can see, and the number of weaker stars is enormous. The luminosity of the stars is expressed in "magnitude", a scale that starts at zero or first magnitude. These stars are the most luminous and therefore most visible in the sky (apart from the Sun, the Moon and the greater number of planers visible to the naked eye). The stars of secondary magnitude are two or more rimes weaker than those of first magnitude and so on down the scale. The weakest stars visible to the naked eye of a young person with no sight problems are of the sixth magnitude (6a), that is 100 times weaker than those of first magnitude. A telescope with an objective of 114 mm in diameter for example will let you see stars up to a magnitude of 12a, various millions of stars.
Stars appear in the telescope as points of light and it is impossible to "enlarge" these whatever instrument you use. The main problem confronting the enthusiast just starting out is that of recognizing the constellations which are stars aligned in perspective, and which were named by our forebears on the basis of their geometric forms. These forms inspired the imagination of those early astronomers, and names of mythological figures appear in constellations to complete the picture. The sky is where logic gives place to divinity. To help you to recognize the main constellations, at the end of this manual we have included a series of simple maps which permit a rapid familiarisation with the sky in all seasons of the year. This is useful in that it will help you to place within the frame of your telescope some particularly famous and interesting celestial objects.
Aim your telescope in the direction of one of the brightest stars. You will immediately notice that it has a color. Ail the stars have a color that could be white, navy blue, blue, green, yellow, orange or red with any of these colors' variations. The color is evident, however, only in the brightest of the stars, because out eye loses chromatic sensitivity when the object at which we are looking is under a certain minimum degree of brightness.
Let us take a few examples: in the winter sky, the dominant constellation is Orion, and it's brightest stars are Betelgeuse and Rigel, respectively red and blue. To the southeast of Orion you will find the Greater Dog, where the brightest star in the sky shines: Sirius, which is a whitish blue star.
To the north of Orion, you will find the zodiacal constellation of Gemini. The brightest of its stars are Castor and Pollux (the twins). These are, respectively, blue and orange.
To the west of Gemini is Taurus. The brightest of its stars is Aldebaran (in Arabic meaning the bull's eye) and this is a reddish-orange color.
In the spring sky, the brightest stars are Regulus of Leo, which is white, Arthur of Boote's constellation. This is a clear orange. Spica, in Virgo, is a bluish white.
In the summer sky, dominated by the constellations of the Summer Triangle; Lyra, Cygnus and Aquila we can observe the blue Vegan the white Deneb and the yellow Altair.
If we point at Albireo, in the constellation of Cygnus we shall see something remarkable.
With the naked eye it seems like all the others, while with the telescope we can see it as a double star. A quick observation of the brightest stars in the sky, done with a telescope used at high magnification, will allow us to see that double stars are rather common, even if Albireo, beta of Cygnus, is distinguishable for the difference in color between its two components, one orange and the other dark blue. The two stars are 20 seconds of an arc at a distance one from the other (obviously their angular distance is not the real distance, which is expressed in linear measurements.) N.B. Angles are measured in degrees, which are divided into 60 minutes of an arc, divisible in tutu into 60 seconds of an arc. A second of an arc is, therefore, the smallest of angular distances, 1/3600 degrees).
Another famous double star is Mizar, the zeta of the Great Bear (Ursa Major). The two components are of identical brilliance. They are whitish blue and the distance between them is about 14" (a second of an arc).
Among the characteristics of telescopes listed on the catalogue, you will find resolution power, which is the capacity of an optic system to reveal the details of the object under observation. This depends principally on the diameter of the objective and can be brought into relief by trying to make single a double star.

An approximate formula for the determination of the theoretical resolution power of an objective is as follows:

A telescope with an objective of 60mm, can, in theory, make single a binary star the components of which have a distance between them of 2" of an arc. In reality, this value of resolution power is optimistic and can be reached only with a perfect objective, which is perfectly in alignment, used at a high level of magnification, with a perfectly calm sky and with both stars in the binary system of equal splendor. That is to say, in conditions almost impossible to satisfy contemporaneously.