Cassegrain arrangement using a parabolic concave primary mirror and convex…

The idea

A Cassegrain reflecting telescope collects light with a large parabolic concave primary mirror. Parallel rays from a distant object reflect to the primary focus, but before reaching it they meet a small convex secondary mirror placed near the focus. The secondary mirror reflects the converging beam back through a hole in the centre of the primary mirror to a final focus behind the primary, where the eyepiece or detector sits. The parabolic primary removes spherical aberration for on-axis parallel rays, while the convex secondary increases the effective focal length and allows a long focal length to be folded into a compact tube. The secondary also causes some obstruction of the incoming light. The arrangement is assessed by explaining the path of rays, the role of each mirror and the advantages of the folded design.

What earns the marks

• States that the primary mirror is concave and parabolic, and that it collects parallel light from a distant object.
• Explains that the convex secondary mirror is placed before the primary focus and reflects the converging beam back through a hole in the primary mirror.
• Describes the final image forming behind the primary mirror, where the eyepiece or detector is placed.
• Explains that the parabolic primary avoids spherical aberration for on-axis parallel rays.
• Explains that the convex secondary increases the effective focal length and folds the optical path so the telescope is compact.
• Recognises that the secondary mirror obstructs part of the incoming light and may introduce diffraction effects.

Common mistakes and how to put them right

• Stating that the secondary mirror is concave.

  Put it right by identifying it as convex, which diverges the converging beam and increases the effective focal length.

• Placing the secondary mirror after the primary focus.

  Put it right by placing it before the primary focus so it intercepts the converging beam.

• Claiming that the final image forms in front of the primary mirror.

  Put it right by stating that the beam passes through a hole in the primary and forms the final image behind it.

• Saying that a parabolic mirror removes all aberrations.

  Put it right by stating that it removes spherical aberration for on-axis parallel rays, while other aberrations such as coma can remain off-axis.

Exam tips

• Draw a clear ray diagram with parallel rays entering, reflecting from the parabolic primary, meeting the convex secondary before the primary focus, then passing through the central hole to the final focus.
• Label the primary focus, the secondary mirror and the final focus, and use arrows to show the direction of travel of the light.
• When explaining advantages, link the folded path to compactness and the convex secondary to increased effective focal length, rather than listing features without reasons.