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The power of magnification of a telescope is determined by the focal length of the objective lens and the eyepiece lens. If you divide the focal length of the objective lens (refractor) or the mirror (reflector) by the focal length of the eyepiece, the result will be the magnification of the telescope. Numerical application on my Vixen cosmostar: 1) F / f1 = 900mm ÷ 20mm = 45X whereas F: the focal length of the objective mirror. f1: the focal length of the 20mm eyepiece. 2) F / f2 = 900mm ÷ 6mm = 150X whereas F: the focal length of the objective mirror. f2: the focal length of the 6mm eyepiece. The Barlow lens will multiply the magnification by two when set with the eyepiece: Barlow + 20mm eyepiece gives 90X. RESOLVING POWER Two stars are located very close to each other may appear to be a single star as observed by the naked eye, whereas they can be identified separately by a telescope. In the same way, two stars that appear to be a single star as observed by a telescope with a small objective lens can be identified separately by a telescope with a large objective lens. The minimum the distance between two bodies that allows a telescope to identify them separately is referred to as the resolving power. This distance is expressed as an angle, i.e., -'-". Any two bodies with a distance greater than this minimum angle can be identified separately. The smaller the angle value, the better the telescope. Vixen cosmostar resolving power 1.00". CONDENSING POWER How much light can it condense as compared with the human eye? As light enters the eye through the pupil, it may be condensed according to the area of the pupil. In this way, as it enters the telescope through the objective lens, it may be condensed according to the area of the objective lens. Therefore, the condensing power is determined by the formula: area of objective lens ÷ area of pupil . Namely, condensing power = (effective diameter of objective)sq ÷ (pupil diameter)sq. = (effective diameter of objective lens)sq. ÷ 7 sq. Since the human pupil normally opens to 7mm in the dark, the diameter of the pupil is taken as 7mm. The greater the condensing power, the better the telescope. With great condensing power, stars invisible to the naked eye are made observable. Numerical application on my Vixen cosmostar: condensing power = 37.85 sq ÷ 7sq = 265X or Light gathering LIMITING MAGNITUDE Up to which magnitude of stars can it make observable? In the order of descending brightness, stars are classified into -2 magnitudes, -1 magnitude, 1 magnitude, 2 magnitudes and so on forth. For example, Sirius, the brightest star among fixed star, is -1.6 magnitudes, the polestar of 2.1 magnitude, the full moon of -12.5 magnitude, and the sum of -26.7 magnitude. The dimmest stars visible to the human eye are normally of 6 magnitudes. Since telescopes more greatly condense the light coming from stars than does the eye, they can make stars dimmer than the one just mentioned visible. Therefore the larger the telescope, the more stars become observable. When stars of up to 10 magnitude are made observable by a telescope, the limiting magnitude of this telescope is expressed as 10 magnitudes. The larger the limiting magnitude, the better it is. Limiting magnitude of a Vixen cosmostar: 12 magnitudes. THE MOUNTING AS A STAGE Since astronomical telescopes are high in magnification and heavy in weight, it is very difficult to bring a star into the filed of view if this observation is made by supporting telescope by hand. Even if the star is once caught, it will soon go away. The mounting is a stage to support the body tube and has the following three functions: 1) Locate quickly and easily the star to be observed. 2) To prevent the star image from swinging during observation. 3) To move the body tube so as to follow the traveling star. Depending on the method of moving the body tube, the mounting may be classified as the azimuth stage or the equatorial. The mounting stage of a Vixen cosmostar: German equatorial mount. BEST USE OF THE TELESCOPE Since astronomical telescopes are high in magnification and narrow in filed of view, it is rather difficult to catch a particular star among a great number of stars and to follow its movement. The crux of succeeding in the observation is to mater the use of an astronomical telescope. HANDLING Do not handle the telescope violently. In particular, when the body tube is carried, be sure not to bump or drop it. ASSEMBLING If the star is difficult to catch and, when caught, is swinging and hardly observable, this is most often due to a poor assembly of the telescope. 1) Check closely to make sure that the clamp screws of the footing as well as the screws of the stage are kept tight. 2) Adjust the balance of the body tube so that its front and rear parts are equalized in weight. OBSERVATION SITE Since it takes considerable time to observe a star, the telescope should be set at a properly selected site. 1) Select an open site where light is at a minimum and the largest possible celestial area can be seen. 2) Select a flat site where there is no unevenness or stone. Take care not to stumble and damage the telescope. 3) Place such parts as the eyepiece on the triangular plate, or keep them in a small box. ADJUSTMENT OF THE FINDER The finder is a telescope of small size and magnification that is attached to the body tube and functions to catch easily and quickly the star to be observed. Unless the same star is seen by both telescope and finder, it is difficult to catch it in the telescope. 1) With the telescope, a bright neon sign or lamp distant more than 1,000 meters. 2)
Watch the finder, and if the same image seen at the center of the telescope is 3) If the same image is not formed at the center of the field of view, bring the image to the center by moving the adjustment screws attached to the footings of the finder. If one of the screws is loosened, be sure to tighten it and all the others firmly. The finder of a Vixen cosmostar magnifies 6X.
My Telescope in the Lebanese "Daily Star" newspaper HOW TO ADJUST THE FOCUS Although it appears easy to adjust the focus, it is rather laborious in practice until one gets used to it. Focus on many distant image in the daytime and repeat this practice to develop skill in it. 1) Adjust the focus by moving the drawtube back and forth. This may be done by turning the focus-adjusting knob, or by hand in case no knob is attached. 2) Since stars are always moving, focus on a neon sign or lamp over 1,000 meters away. When the image is focused roughly, adjust the focus by moving the drawtube back and forth slightly until the image is made clear. Then without shifting the tube, point it to catch a star, and the focus will normally be found to be correct. 3) First adjust the focus with an eyepiece of lower magnification and then change it to another one having the required magnification. 4) Whenever the piece is changed, readjust the focus by moving the drawtube slightly. BEFORE OBSERVATION Try to focus the telescope. It is advisable to start by using a low powered eyepiece n a distant terrestrial object. The image given by an astronomical telescope will be up side down (inverted) and reversed left and right.
FINALLY The novice astronomer always thinks of power. the power is one of the important factors but it is not the most important quality of the telescope. Resolving power is more important. The magnification is theoretically unlimited, but the practical number of magnifications should not exceed about 60 power per inch of diameter of the objective lens (or mirror). When this ratio is exceeded, the quality of the image produced diminishes rapidly. Accordingly, for best results, it is generally better to employ the lesser powers. This gives the observer the benefit of a larger field of view, a brighter and crisper image, and sharper detail. The high powers should be used only when it is necessary to look at a small area. Experience will teach that moderate power are best for most observations "
With this care, your telescope will be a friend for life" |
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Site Manager:
Alain Khayat |
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