3.9.1.3
Reflecting Telescopes
Astrophysics | AQA A-Level Physics
Key Definitions
Reflecting telescope (reflector): A telescope that uses parabolic (curved) mirrors to collect and focus light from distant objects.
Cassegrain telescope: The most common type of reflecting telescope, which uses a large concave primary mirror and a smaller convex secondary mirror to direct light through an aperture in the primary mirror to an eyepiece.
Cassegrain telescope: The most common type of reflecting telescope, which uses a large concave primary mirror and a smaller convex secondary mirror to direct light through an aperture in the primary mirror to an eyepiece.
How the Cassegrain reflector works
- Reflecting telescopes use the law of reflection (angle of incidence equals angle of reflection) rather than refraction.
- Instead of two converging lenses, a reflector uses a primary mirror and a secondary mirror.
- The primary mirrorThe large concave parabolic mirror at the back of the telescope. It collects incoming parallel light and reflects it toward a focal point in front of the secondary mirror. is large and concave. Incident light reflects towards a focal point which is behind the secondary mirror.
- The secondary mirrorA smaller convex mirror positioned in front of the primary mirror. It reflects the converging light back through the aperture (hole) in the primary mirror towards the eyepiece. is smaller and convex. It reflects the light again, directing it through an aperture in the primary mirror towards an eyepiece lens.
Key features of the Cassegrain ray diagram
- The rays enter the telescope parallel to the principal axis (because the astronomical objects are so far away).
- The curvature of the two mirrors does not have to be the same.
- Crucially, the rays do not cross before the secondary mirror. They only cross in the aperture of the primary mirror.
- Shading or lines on the mirror indicate the non-reflective side.
- The primary mirror must look like one continuous parabola, not two separate mirrors.
Drawing the ray diagram
- Step 1: Draw and label a principal axis, a large concave primary mirror (with an aperture gap in the centre), and a smaller convex secondary mirror.
- Step 2: Draw two rays entering parallel to the principal axis.
- Step 3: Draw the reflection of the rays from the primary mirror converging towards the secondary mirror.
- Step 4: Draw the reflection from the secondary mirror, directing the rays through the aperture towards the eyepiece.
- The key part is that the rays must not cross until they reach the aperture.
Common Mistake
A common misconception is that the secondary mirror blocks the central portion of the image. It does slightly reduce the amount of light collected, but it does not cause a hole in the image. Light rays from a distant source are parallel, so even light that would have hit the centre of the primary mirror still reaches the secondary mirror and contributes to the image. Also, never draw the secondary mirror as concave or as a straight line: it must be convex.