Astigmatism — where it appears
Named by 2 essays across one field — each of them below, with the objects they name alongside it.
Also named here as not a projection — the same set of essays touches all of them, so they are one junction rather than several.
A curved mirror has no eye
A flat mirror is a second camera — reflect the eye in the plane and every line of sight passes through the reflected point, to 2.8e-12 mm. Curve the mirror and the point is gone. Over 20 cm of a mirror ball two metres across, the lines of sight miss their own best-fitting point by 2.8 mm, and by 52.9 mm on a ball half a metre across. What replaces the eye is not a worse eye; it is nothing.
Where the focus went
If the reflected rays do not meet at a point, they meet each other in pairs, and the curve they are all tangent to is what the mirror has instead of a focus. On a sphere of 1.6 m vertex radius it is 184.1 mm long and has a cusp; near the axis it sits at R/2, which is why a small enough spherical mirror passes for a good one. This is the bright shape in the bottom of a coffee cup, computed rather than admired.
Named alongside it
The objects these essays reach for when they reach for this one.
Causticcentre of projectionNot a projectionRay tracingReflectionApertureCuspEnvelopeFocusleast-squares intersectionMirrorParaboloid