Robtics | BarlowsA Barlow lens is a diverging lens which increases the effective focal length of an optical system. The practical result is that inserting a Barlow lens magnifies the image. Most barlow lenses are not a single glass element, because that would generate chromatic aberration and spherical aberration if the lens is not aspheric. More common configurations use three or more elements for achromatic correction or apochromatic correction and higher image quality.

A Barlow lens needs to be placed between the telescope and the eyepiece to effectively decrease the eyepiece's focal length by the amount of the Barlow's divergence. Since the magnification provided by a telescope and eyepiece is equal to the telescope's focal length divided by the eyepiece's focal length, this has the effect of increasing the magnification of the image.

Barlow lenses are rated for the amount of magnification they induce. Most commonly, Barlow lenses are 2x or 3x, but adjustable Barlows are also available. The power of an adjustable Barlow lens is changed by adding an extension tube between the Barlow and the eyepiece to increase the magnification.

Barlow lenses can be used visually and photographically to magnify the telescope's image for higher resolution observing or imaging.

Telecentric barlow lenses are not diverging lenses but the light leaves parallel to the optical axis. While not strictly a barlow, the effect of a telecentric barlow is the same with added optical advantages. Powermate by TeleVue is an excellent example of a telecentric barlow and is used for high resolution lunar- and planetary photography.

A common misconception is that higher magnification equates to a higher-quality image. However, in practice, the quality of the image generally depends on the quality of the optics (lenses) and viewing conditions, not on magnification.

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