Cassegrain reflecting telescope

A Cassegrain telescope uses two mirrors to collect light from a distant object and bring it to an image. Its large primary mirror is concave and parabolic; its smaller secondary mirror is convex.

Light from a distant object arrives at the telescope as approximately parallel rays. The primary mirror reflects these rays towards its focus. Before they reach that focus, the secondary mirror intercepts them and reflects them back through a central hole in the primary.

After passing through the hole, the rays converge to form a real image beyond the primary mirror. An eyepiece placed beyond the primary allows this image to be viewed.

Cassegrain telescope: tracing the rays

In a ray diagram, follow the light in this order:

1. Draw parallel incoming rays travelling towards the primary mirror.
2. Trace the rays reflected by the primary towards the secondary mirror.
3. Trace the rays reflected by the secondary back through the hole in the primary.
4. Continue the rays until they converge at the real image, which is viewed through the eyepiece.

The mirrors fold the light path by sending the light back through the telescope. This arrangement gives the telescope a long effective focal length while keeping its physical length comparatively short.

Spherical aberration

A spherical optical surface has the curvature of part of a sphere. When rays parallel to the optical axis meet different parts of such a surface, they may be brought together at different positions along the axis after reflection or refraction. This effect is called spherical aberration.

For example, rays striking a spherical mirror near its edge meet the axis at a different position from rays striking it near the centre. The rays therefore do not all come together at one point. Instead, their spread of focal positions makes the image less sharp.

A parabolic primary mirror has a different curvature. It brings rays parallel to its axis to a common focus, avoiding spherical aberration for those rays.

Chromatic aberration

A lens may not bring all the colours in light to the same focus. This effect is called chromatic aberration. It can produce a blurred image with coloured fringes around its edges.

A lens focuses light by refraction: light changes direction as it passes between air and the lens material. The refractive index of the material depends on the wavelength of the light, so different colours are refracted by different amounts. They consequently have different focal positions rather than meeting at one common focus.

If the image is in focus for one colour, other colours may be out of focus at the same position. Their images do not line up perfectly, producing blur and coloured fringes.

A reflecting mirror focuses light by reflection, not refraction. It therefore does not produce chromatic aberration.

Reflecting and refracting telescope comparison

A telescope collects light from a distant object and brings it to a focus to form an image. A reflecting telescope uses a curved mirror as its main light-gathering element; a refracting telescope uses a lens, called the objective lens.

Aberrations in reflectors and refractors

An aberration occurs when light is not brought to the intended focus, making the image less sharp.

A lens can produce chromatic aberration because it bends different colours of light by different amounts. The colours therefore come to focus at different positions. A mirror reflects different colours without this separation, so it does not produce chromatic aberration.

A lens can also produce spherical aberration, in which rays passing through different regions of the lens do not meet at the same focus. A reflecting telescope with a parabolic primary mirror avoids spherical aberration for incoming rays that are parallel to the mirror’s axis: those rays meet at the same focus.

Size and practical limitations

A large objective lens must be supported at its edge, so it can deform under its own weight. A large mirror can be supported from behind, making it easier to maintain its shape. Large reflecting telescopes are therefore more practical to build than large refracting telescopes.

A larger light-gathering element collects more light, allowing fainter objects to be observed. It also has greater potential resolving power: the ability to distinguish two close objects as separate.

Reflectors have their own practical disadvantages. Their mirrors may need alignment, and their reflective coatings need renewal. In a Cassegrain reflector, a secondary mirror lies in the path of incoming light and obstructs some of it, reducing the light collected.