Isoptics of an astroid
Isoptic curves of an astroid
The isoptic curves (i.e. the geometric loci from which an astroid is viewed under a given angle) have been studied in paper by Th. D-P @ Journal of Symbolic Computation. The present applet enables to expore the isoptics for different angles. Pay attention to the different shapes, in particular for theta=90 degrees, and for theta=120 degrees..