1. Good Day Sunshine

Ground-based optical observing

The Sun has been observed with optical telescopes for centuries, gathering all the visible colors of sunlight. These telescopes are used to resolve spatial details that we cannot detect with the unaided eye. Since the Sun is a quarter of a million times closer to us than the next nearest star, it permits a level of detailed examination that is not possible on any other star.

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There are two types of optical telescopes, the refractor and the reflector (Fig. 1.15), and both of them are used to observe the Sun. Never look directly at the Sun through either kind of telescope; it can cause permanent damage to your eyesight. The safest way to observe the Sun with a telescope is to project the Sunís image on a surface and view it there.

In a refractor, sunlight is bent by refraction at the curved surface of a lens, called an objective, toward a focal point where the different rays of light meet (Fig. 1.15). If we place a detector at the focal point, in the plane parallel to the lens, we can record an image of the Sun. The distance from the lens to the focal point is called the focal length, which determines the overall size of the solar image. The critical thing is the diameter, or aperture, of the light-gathering lens. The larger the aperture, the more light is gathered and the finer the detail that can be seen.

The other type of telescope, the reflector, uses a concave mirror with a parabolic shape to gather and focus the sunlight. The diameter of this primary mirror determines the telescopeís resolution and light gather ability. The prime focus is back in the path of the incoming sunlight, so secondary mirrors are sometimes used to reflect the light to another place of observation. There are three types of secondary mirrors called the Cassegrain, Coudť, and Newtonian mirrors, that can focus light to different locations (Fig. 1.15).

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The McMath solar telescope is a fine example of a modern high-resolution instrument (Fig. 1.16). It has a mirror at the top, called a heliostat, that follows the Sun, and sends a beam of sunlight into the side of a mountain to another mirror that takes the beam and focuses it, forming an image (Fig. 1.17). With a focal length of 82 meters, the Sunís image is about 0.77 meters across. The tube that encases the telescope is kept at a cold temperature by pumping water through external tubes around it. This reduces air turbulence inside the tube, resulting in a sharper image.

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At first sight you would think there is plenty of light coming from the Sun. Because of its closeness, it is a hundred billion times brighter than any other star. But you obtain much less light if just a narrow section of the solar spectrum is isolated and observed for short times, as with a spectroheliograph (Fig. 1.18). Large lenses or mirrors might then be needed to obtain a strong signal, permitting detailed studies of spectral components such as absorption or emission lines.

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A special type of optical telescope is the coronagraph (Fig. 1.19), first developed by the French astronomer Bernard Lyot around 1930. It produces an artificial eclipse of the Sun by means of an occulting disk inside the telescope, blocking out the intense glare of the photosphere. This permits the detection of the faint light of the corona around the perimeter of the occulting disk. Special precautions must be taken to reduce instrumental scattered and diffracted light.

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Copyright 2010, Professor Kenneth R. Lang, Tufts University