Reflecting rather than refracting
A reflecting telescope uses mirrors to collect and redirect light from distant astronomical objects. At each reflecting surface, the angle of incidence equals the angle of reflection.
In a Cassegrain design, the mirrors send the beam back through the telescope before it reaches the eyepiece. This arrangement allows the instrument to be shorter than a refractor requiring a similarly long focal length.
Main components
1
Primary mirror
- Large, concave and parabolic.
- Receives the nearly parallel rays arriving from the distant source.
- Turns those rays back towards its focal region.
- Has an opening at its centre for the returning beam.
2
Secondary mirror
- Smaller than the primary.
- Has a convex reflecting surface.
- Meets the converging beam before the rays reach the primary focus.
- Returns the light towards the central opening in the primary.
3
Eyepiece
- Sits behind the primary mirror.
- Receives the light after it has travelled through the central opening.
- The mirror system forms a real, magnified image for viewing with the eyepiece lens.
The reflecting layer itself can be extremely thin and deposited on a supporting backing. A large reflector therefore does not require a thick, transparent glass element through which the radiation must pass.
The incoming rays are treated as parallel because an astronomical source is vastly farther away than the dimensions of the telescope.