View source for Isoptic

{{TEX|done}}{{MSC|52A10}}

The locus of intersections of tangents to a given curve meeting at a fixed angle; when the fixed angle is a right angle, the locus is an '''orthoptic'''.

The isoptic of a [[parabola]] is a hyperbola; the isoptic of an [[epicycloid]] is an [[epitrochoid]]; the isoptic of a [[hypocycloid]] is a [[hypotrochoid]]; the isoptic of a [[sinusoidal spiral]] is again a sinusoidal spiral; and the isoptic of a [[cycloid]] is again a cycloid.

====References====
* J.D. Lawrence,   "A catalog of special plane curves" , Dover  (1972) ISBN 0-486-60288-5  {{ZBL|0257.50002}}