Telescopes

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Department
Physics, Engineering Physics and Astronomy
Course
ASTR 101
Professor
Prof.
Semester
Fall

Description
Telescopes • Early telescopes first importantly used by Galileo were refractor telescopes which use a glass lens to focus light of distant objects to a point  A big objective lens sits at the front of the telescope where its diameter determines how much light is collected  An eyepiece magnifies the focused image however nowadays most telescopes are fitted with special instruments or cameras to analyze the data Telescopes Gather Light: • Magnification makes little difference when observing stars as they are too far away  Telescopes are useful because they capture the light and focus light from faint/dim objects  Most important dimension of the telescope is the diameter  Larger diameter = more light (which is why pupils dilate in the dark) • Light-gathering power of a telescope is directly related to the size of the objective lens (location of light gathering)  The area of the objective lens is related to the square of the diameter Optical Telescopes: • Magnification: Not important today however was important with optical telescopes in terms of the eyepiece, as the eyepiece is what magnified the refracted image  Too much magnification would produce blurriness and dimness as the amount of light coming from the object does not change and the atmosphere blurs out surficial details • Resolution: Unlike radio telescopes, building a larger telescope does not compensate for poor resolution as it is the atmosphere that causes blurring • Light Intake: This is why telescopes are built so large; to collect more light  Analogy: If you want to wash your laundry in rain water, you would collect it with large tubs rather than cups Uses of Telescopes: • Surveys: Astronomers generally use telescopes with a fixed program in mind, such as observing how stars are moving around in a nearby galaxy  Astronomers do not sit there looking for stars, they come across them during surveys • Observing technique: Eyepiece is rarely used and most distant objects are observed by computers who accumulate the light over time and produce images  The eye is very unreliable and does not accumulate the light, so faint objects continue to look faint Problems with Refractors: • Chromatic aberration: When light passes through the objective lens, the wavelengths of light are split at different angles and therefore do not have a common focus  Bringing one wavelength of light (ex. Yellow) into focus will cause another to go out of focus (blue)  Can be corrected using cemented doublets; two lenses composed of different materials stuck together with clear cement  Lenses are expensive so more than two is impractical  Does not completely remove chromatic aberration  Longer focal lengths reduce it which is why old refractors were so large • Length: Best images come from thin
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