Lecture 10 - Lecture 10 Lecture 10 Telescope [Reading...

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Lecture 10 1 © Jeffrey Bokor, 2000, all rights reserved Lecture 10 Telescope [Reading assignment: Hecht 5.7.4, 5.7.7] A telescope enlarges the apparent size of a distant object so that the image subtends a larger angle (from the eye) than does the object. The telescope is an afocal system, which means that both the object and image are at infinity. Astronomical telescope Using the lens law for the eyepiece: So . For small angles, , then . The exit pupil is the image of the AS. Define From the diagram, it is clear that h  exit pupil eyepiece objective AS Q at infinity f o s s f e Magnification M ---- = tan h s ----- tan h s --- == 1 s 1 s -- 1 f e --- sf o f e +  1 s 1 f o f e + -------------- + 1 f e = 1 s 1 f e 1 f o f e + f 0 f e f o f e + ----------------------- tan + h + f o f e + ---------------------- tan hf o f e f o f e +  tan tan M f o f e CA o entrance pupil clear aperture = e exit pupil clear aperture =
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Lecture 10 2 © Jeffrey Bokor, 2000, all rights reserved The eye is placed at the exit pupil, so a much larger than 3 mm is not very useful. However, mak- ing it somewhat larger makes it easier to align the eye to the eyepiece. Binoculars may have . Resolution The resolution of the eye is 1 arc min = 60 arc sec. So in a telescope, the eye can resolve objects separated by an angle if Now, the diffraction limit of the telescope can be written as , with in sec. and in inches (for 550nm wavelength). At the diffraction limit, the finest detail in the image has an angular separation of . If this angle is at least 60 sec, the eye can resolve the detail. So, with At this magnification, the diffraction limit and the resolution of the eye are equal. Magnification much
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Lecture 10 - Lecture 10 Lecture 10 Telescope [Reading...

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