= Solution
In a <classical Cassegrain reflector>, a concave parabolic primary sends light toward an intermediate focus, but a convex hyperbolic secondary intercepts it before it reaches that focus. The secondary returns the beam through a central hole in the primary to a <focal plane> behind the primary. The two relevant foci of the secondary’s <hyperbola> are the primary’s would-be focus and the final focus.
\Image[/past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2018/iii/paper-338-cassegrain.png]
{title=Light paths in a classical Cassegrain reflector}
The secondary magnifies the effective <focal length>, giving a compact tube and convenient rear-mounted instruments. The classical conics correct on-axis <spherical aberration>, but not the off-axis <coma>, <astigmatism> or <field curvature>. There is secondary obscuration, <diffraction> from its supports, and sensitivity to mirror alignment. A long effective <focal length> is useful for a small angular image scale per detector pixel, but yields a small field for a fixed detector size.
\b[Parabolic primary → convex hyperbolic secondary before prime focus → rear focus.]
Back to article page
