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let's take two of the rays with the angle of incidence 5 degrees slanted from normal. . As shown in the diagram they converge on a point on the focal plane. Red lines are parallel light with the principal axis. blue lines are 5 degrees off.


(this is not much of an answer) I assume you are talking about the case where incident parallel rays are not parallel to the principal axis, i.e. To my understanding it is not invariant (i.e. the focal point shifts) and it also depends on the shape of the mirror (e.g. parabolic or spherical).

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