= Solution
A <Newtonian reflector> uses a concave primary generated by a <parabola>, followed by a flat secondary inclined at $45^\circ$ to the <optical axis>. Parallel marginal rays converge toward the primary focus; the secondary intercepts that converging beam and folds it sideways to an accessible <focal plane>.
\Image[/past-exam-of-the-mathematics-course-of-the-university-of-cambridge/2018/iii/paper-338-newtonian.png]
{title=Light paths in a <Newtonian reflector>}
The <Newtonian reflector> needs only one powered mirror, so it is relatively simple to fabricate and inexpensive. A parabolic primary has no on-axis <spherical aberration>. Its principal wide-field limitation is <coma>, accompanied by <field curvature> and off-axis <astigmatism>. The diagonal and its supports obstruct the <entrance pupil> and introduce <diffraction>; the tube is long compared with a folded two-powered-mirror design, and heavy instruments at the side focus can be awkward to support. These comments also answer the unheaded design-comparison clause on the next PDF page.
\b[Parabolic primary → flat diagonal → side focus.]
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