A search for a universal-focus lens has led to a new class of optical elements. These are called axicons. There are many different kinds of axicons but probably the most important one is a glass cone. It may be either transmitting or reflecting. Axicons form a continuous straight line of images from small sources.
One application is in a telescope. The usual spherical objective is replaced by a cone. This axicon telescope is in focus for targets from a foot or so to infinity without the necessity of moving any parts. It can be used to view simultaneously two or more small sources placed along the line of sight.
If a source of light is suitably added to the telescope it becomes an autocollimator. Like ordinary auto-collimators it can be used to determine the perpendicularity of a mirror. In addition, it can simultaneously act as a telescope for a point target which may be an illuminated pinhole in the mirror.
The axicon autocollimator is also a projector which projects a straight line of images into space.
JOHN H. MCLEOD, "The Axicon: A New Type of Optical Element," J. Opt. Soc. Am. 44, 592-592 (1954)
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