ABSOLUTE FIELD OF VIEW in Software

Encoding Code 128 Code Set B in Software ABSOLUTE FIELD OF VIEW

When you look through the eyepiece of a telescope, you see a circular patch of sky Actually, you can see anything within a cone-shaped region whose apex is at the telescope (Fig 17-10) The absolute field of view is the angular diameter q of this cone; q can be specified in degrees, minutes, and/or seconds of arc Sometimes the angular radius is specified instead of the angular diameter

The absolute field of view depends on several factors The magnification of the telescope is important When all other factors are held constant, the absolute field of view is inversely proportional to the magnification If you double the magnification, you cut the absolute field of view in half If you reduce the magnification by a factor of 4, you increase the absolute field of view by a factor of 4 The viewing angle provided by the eyepiece the apparent field of view is important Some types of eyepieces have a wide apparent field, such as 60 degrees or even 90 degrees Others have narrower apparent fields, in some cases less than 30 degrees Galileo s first refracting telescope had an apparent field of view only a few degrees wide Another factor that affects the absolute field of view is the focal ratio or f-ratio: the objective s focal length divided by its actual diameter as measured in the same units A telescope s f-ratio is denoted by writing f, followed by a forward slash, followed by the ratio expressed as a number Thus, for example, if the focal length of the objective is 200 cm and its actual diameter is 20 cm, the f-ratio is f/10 In general, the larger the fratio, the smaller is the maximum apparent field of view that can be

obtained with the telescope Long, narrow telescopes have the smallest maximum apparent fields of view; short, fat ones have the widest maximum fields

The quality of the image obtainable with a telescope depends on some things the astronomer can control, such as the objective diameter, the quality of the optics, and the location of the observatory Other problems are unavoidable Here are some of the factors that affect the performance of telescopes

The greatest useful magnification a telescope can provide is approximately 20 per centimeter of objective diameter, or 50 per inch Visible light has properties of a wave disturbance, and the waves have physical length in space The objective lens or mirror must measure at least a certain number of wavelengths in diameter to provide a given image resolution If you have a telescope that is 10 cm (4 in) across, you can obtain up to approximately 200 of magnification with it Beyond this, you can get things to look bigger, but they also become more blurred, so the resolution of the images you see does not improve

The importance of good optics (that is, high-quality lenses and mirrors) is obvious The best optics can be expensive, and the cheapest optics are usually not so good However, the quality of the optics is not an absolute and direct function of expense It is important to keep the objective lens or mirror, the secondary mirror (if any), and the eyepieces clean and free of condensation In some locations, the formation of dew can be a constant problem Frost can form if the temperature is low enough and the humidity is relatively high Dew and frost not only cloud the optics, but they also accelerate the accumulation of dirt

When a telescope is aimed at an object, you expect it to stay there Vibration transmitted from the ground can cause blurring of the image Swaying produced by the wind can make viewing difficult Either of these effects, when they occur in conjunction with the use of a time-exposure camera attached to a telescope, injure the image quality The mounting that secures the telescope to the base must be physically rugged so that the telescope will not waver once it is aimed at an object The platform on which the entire observatory is placed likewise must be solid The effects of vibration and swaying are exaggerated by high magnification