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An Isoptic Curve formed from the locus of Tangents meeting at Right Angles. The orthoptic of a Parabola is its Directrix. The orthoptic of a central Conic was investigated by Monge and is a Circle concentric with the Conic Section. The orthoptic of an Astroid is a Circle.
| Curve | Orthoptic |
| Astroid | Quadrifolium |
| Cardioid | Circle or Limaçon |
| Deltoid | Circle |
| Logarithmic Spiral | equal Logarithmic Spiral |
| Parabola | Directrix |
References
Lawrence, J. D. A Catalog of Special Plane Curves. New York: Dover, pp. 58 and 207, 1972.