= Solution
A <Gregorian telescope> has a concave parabolic primary and a concave ellipsoidal secondary. Unlike the <Cassegrain reflector>, the secondary is beyond the primary focus: the rays cross that focus before reaching it. The two foci of the secondary’s <ellipse> are the intermediate focus and final rear focus, so the reflected rays return through the primary hole to the detector.
\Image[/past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2018/iii/paper-338-gregorian.png]
{title=Light paths in a Gregorian telescope}
The real intermediate focus permits a field stop to reject unwanted light, and the two image inversions give an erect image. The rear focus is convenient and the effective <focal length> can be large. The secondary beyond prime focus makes the tube longer than the corresponding <Cassegrain reflector>, often with a larger secondary obstruction. The classical design retains off-axis <coma>, <astigmatism> and <field curvature>, and needs careful alignment.
\b[Parabolic primary → real intermediate focus → concave ellipsoidal secondary → rear focus.]
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