A-Level · Physics · AQA · Mark scheme decoded
AQA A-Level Physics: Cassegrain Reflectors and Aberrations in Telescopes — mark scheme explained
The short answer
In the realm of astrophysics, telescopes play a crucial role in observing distant celestial objects. One of the most advanced designs is the Cassegrain reflector, which uses a parabolic concave primary mirror and a convex secondary mirror to focus light.
The question
Draw a ray diagram showing the path of light rays through a Cassegrain reflector, from the primary mirror to the eyepiece. [Paraphrased for study — not reproduced from any exam paper.]
Mark scheme, decoded
What each mark is really for — in plain English — and the wording trap that loses it.
- S1
1. Draw a large parabolic concave primary mirror at the bottom of your diagram.
- S2
2. Draw a smaller convex secondary mirror above and in front of the primary mirror.
- S3
3. Draw light rays entering the telescope from the top, striking the primary mirror.
- S4
4. Show the reflected rays converging, intercepted by the convex secondary mirror before they reach the primary focus.
- S5
5. Draw the convex secondary mirror reflecting these still-converging rays back through a hole in the primary mirror to a focus behind it.
- S6
6. Show the final path of the rays to the eyepiece or camera.
Model answer
Worked through, with each step tagged to the mark it earns.
- S1
1. Draw a large parabolic concave primary mirror at the bottom of your diagram.
- S2
2. Draw a smaller convex secondary mirror above and in front of the primary mirror.
- S3
3. Draw light rays entering the telescope from the top, striking the primary mirror.
- S4
4. Show the reflected rays converging, intercepted by the convex secondary mirror before they reach the primary focus.
- S5
5. Draw the convex secondary mirror reflecting these still-converging rays back through a hole in the primary mirror to a focus behind it.
- S6
6. Show the final path of the rays to the eyepiece or camera.
Final answer: A correctly drawn ray diagram showing the path of light rays from the primary mirror, through the secondary mirror, and to the eyepiece.
Common mistakes
- Confusing the primary and secondary mirrors in a Cassegrain reflector. — Reinforce that the primary mirror is parabolic and concave, while the secondary mirror is convex.
- Forgetting to mention chromatic aberration when comparing reflectors and refractors. — Emphasize that reflecting telescopes do not suffer from chromatic aberration, which is a significant advantage over refractors.
- Incorrectly stating that spherical mirrors can correct spherical aberration. — Clarify that only parabolic mirrors, not spherical ones, can correct spherical aberration by focusing all light rays to a single point.
- Failing to explain why reflectors are more compact than refractors. — Explain that the design of reflecting telescopes, particularly the Cassegrain arrangement, allows for a shorter tube length by reflecting light back through a hole in the primary mirror.
- Not mentioning the need for periodic realignment and cleaning of reflectors. — Highlight that reflecting telescopes require periodic realignment and cleaning of the mirrors to maintain optimal performance.
- Confusing achromatic lenses with parabolic mirrors in terms of correcting aberrations. — Clarify that achromatic lenses are used in refractors to reduce chromatic aberration, while parabolic mirrors are used in reflectors to correct spherical aberration.
Where the marks go
- Full worked solution (all marking points)4 marks