Barlow lens

The Barlow lens is a named after its inventor Peter Barlow lens increases the focal length and thus the magnification of a telescope or microscope. It is inserted in front of the eyepiece in the focuser. The Barlow lens is a negative lens. Usual double Barlow lenses or triple the effective focal length of a lens.

The Barlow lens is located at the end of a long tube, called the spacer tube. The length of the spacer tube, together with the focal length of the lens determines the magnification effect. The actual lens body can be separated from the spacer tube in most cases and are bolted directly to the side of the field of an eyepiece for example. However, this reduces the effect of magnification, for example, 2-fold to 1.4 -fold.

Some eyepieces have installed as standard a Barlow lens, for example, into the receptacle. In such eyepieces ( or others who have installed another burning far -changing element ) should be no need to use another Barlow lens, otherwise the image quality suffers heavily. Also there are often mechanical problems, since the front lens elements lie close to the thread, and could affect the Barlow element.

Shapely Lense

The opposite of a Barlow lens is a so-called Shapely Lense (after Harlow Shapley ), which shortens the effective focal length while increasing the light intensity. The Shapely Lense is a converging lens. Commercially, both Barlow and Shapely Lense with details such as, 2x 'or' x 0.55 ' offered. This means that the focal length is to be doubled or multiplied by the factor 0.55. The value is only true with certain other eyepiece or photo adapters. Both types of lens may be used for both the visual observation and for photography. In photography, applies to the magnification of the image compared to the same lens without Barlow lens:

In this case, f is the focal length of the lens and a is the distance between ( Barlow ) lens and image plane. In a Barlow lens ( diverging lens, f 1 <0) results in v> and thus an extension of the effective focal length of the optical system as a whole. At a Shapely Lense ( converging lens f > 0) results in v < 1, Thus, a shortening of the effective focal length of the entire system.

  • Optical Telescope Technology
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Peter Barlow (mathematician)
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