9.1.2 Reflecting telescopes

Cassegrain Reflecting Telescope

• This design features a concave primary mirror with a long focal length and a convex secondary mirror in the centre.
• Light is reflected off the primary mirror towards the secondary mirror, which then directs it back through a hole in the primary mirror to an eyepiece lens.
• The convex secondary mirror allows the Cassegrain telescope to have a shorter physical length compared to other telescopes with a similar focal length, making it compact.

Newtonian Reflecting Telescope

• A simpler design, featuring a concave primary mirror and a plane mirror placed at a 45-degree angle to the incoming light.
• Light from the primary mirror reflects off the plane mirror and is directed to an eyepiece on the side of the telescope.
• This design allows for ease of construction and is commonly used in amateur astronomy.

Mirror Coatings

• Mirrors in reflecting telescopes are typically coated with a very thin (often less than $25$ nm thick) layer of aluminium or silver to make them highly reflective and reduce image distortions.
• Thin coatings minimise imperfections and improve image quality.

Optical Aberrations and Solutions

1. Chromatic Aberration

• Occurs when different colours (wavelengths) of light are focused at different points because of refraction. Blue light refracts more than red light, causing an image to have coloured fringes.
• Since reflecting telescopes use mirrors rather than lenses to focus light, chromatic aberration has little effect, as mirrors reflect all colours equally. However, chromatic aberration may occur in the eyepiece lens and can be corrected with an achromatic doublet.

2. Spherical Aberration

• Caused by the curvature of a lens or mirror, where light rays at the edge focus at different points than rays passing near the centre. This results in blurred or distorted images.
• Reflecting telescopes use parabolic mirrors instead of spherical ones to eliminate spherical aberration, as parabolic mirrors focus all incoming light rays to a single point.

3. Achromatic Doublet

• An optical solution to minimise both spherical and chromatic aberrations in lenses.
• It consists of a convex lens made from crown glass and a concave lens made from flint glass. These are cemented together to focus all wavelengths of light at the same point.

Advantages and Disadvantages of Reflecting vs. Refracting Telescopes

Disadvantages of Refracting Telescopes	Advantages of Reflecting Telescopes
Glass lenses must be pure and free from defects, which is difficult to achieve for large lenses.	Mirrors with very thin coatings provide excellent image quality.
Large lenses can bend and distort under their weight.	Mirrors are less affected by bending since they are supported from behind.
Chromatic and spherical aberrations affect lenses significantly.	Parabolic mirrors eliminate spherical aberration, and chromatic aberration is minimal.
Refracting telescopes are often heavy and challenging to manoeuvre.	Reflecting telescopes are easier to handle and can be made with larger mirrors.
Large lenses need to be supported only at the edges.	Primary mirrors can be supported from behind, making them more structurally stable.