Reflector Telescopes

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In the illustration, light enters the telescope (right) and is reflected by a mirror called the objective or primary mirror in the back end of the tube. Light is then reflected to another, diagonal or seconday mirror, which is place at a 45 degree angle at the centerline of the primary mirror in front of the eyepiece. The light from the secondary is directed out the side of the tube to the eyepiece.

The primary mirror is a concave mirror that causes all of the light to converge at one point. Note that the point at which the light converges is right in front of the eyepiece. This will allow the eyepiece to magnify the image reflected by the primary. The secondary mirror is a perfect flat mirror that is only there to direct the light to the eyepiece without changing the cone of light.

The primary mirror only reflects the light to the proper location for the eyepiece to work The larger the primary, (The diameter of the mirror is called its aperture) the more light it will reflect. This means that the viewer looking through the eyepiece can see dimmer objects and bright objects will look brighter. Since the objects are brighter, you can use more magnification with larger primary mirrors. A rule of thumb is to not use more than about 50X per inch of aperture. (see OPTICS for a talk on magnification).

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