Telescope Terminology

OBJECTIVE - The objective is the front lens of a telescope. The measurement listed for objec- tive lenses is the diameter of the lens. A larger objective allows more light to enter a telescope and provides a brighter image. The objective diameter is also sometimes referred to as the aperature of a telescope.

FOCAL LENGTH - The focal length of a telescope is the distance from the point where light enters a telescope (the objective) to the point where the image is in focus. In telescopes with the same size objective, a longer focal length will provide higher magnification and a smaller field of view.

MAGNIFICATION - The magnification of a telescope is determined by the relationship between the focal length of the telescope and the focal length of the eyepiece used. The greater the difference in focal lengths, the greater the magnification. A telescope has a maximum use- ful magnification of about 60 times the diameter of the objective in inches. Magnification beyond the maximum useful magnification will provide dim, low-contrast images.

FOCAL RATIO - The focal ratio of a telescope describes the ratio between the focal length and objective size of a telescope. Visually, the smaller the focal ratio (also called f-stop) of a telescope, the wider the field of view. Photographically, the lower the f-stop, the shorter the exposure time needed to capture an object on film.

LIMITING MAGNITUDE - The limiting magnitude of a telescope describes the faintest object you can see with a telescope. The magnitude of a star describes its brightness. The larger the magnitude of an object, the fainter it appears to be. The brightest stars have a magnitude of 0 or less.

RESOLVING POWER - The resolving power, or Dawes’ Limit, of a telescope is the ability to view closely spaced objects through a telescope. The resolving power of a telescope is measured in seconds of arc. The smaller the resolving power, the better you will be able to separate binary stars when viewing through your telescope.

ABERRATION - Aberrations are degradations in image which may occur due to optical system design or improper alignment of optical system components. The most common types of aberration are chromatic aberration, spherical abberation, coma, astigmatism, and field curvature.

COLLIMATION - Collimation is the alignment of optical components within an optical system. Improper collimation will distort an image and may result in abberations present in the im- age. Most reflector telescopes have collimation adjustments which can be made in order to reduce aberrations and image distortion.