Reflecting telescopes (A-level only) Flashcards – AQA A-level Physics (7408)

What are the shapes of the primary and secondary mirrors in a Cassegrain telescope?

A large concave parabolic primary and a smaller convex secondary.

Trace light from a distant object through a Cassegrain telescope to the eyepiece.

1. Parallel rays reflect from the primary towards its focus.
2. Before reaching that focus, they reflect from the secondary back through a central hole in the primary.
3. They converge to form a real image beyond the primary, where it can be viewed through an eyepiece.

What advantage does folding the light path give a Cassegrain telescope?

It allows a long effective focal length within a comparatively short telescope.

What causes chromatic aberration in a lens?

Its refractive index depends on wavelength, so different colours refract by different amounts and focus at different positions.

What effect can chromatic aberration have on an image?

It can produce a blurred image with coloured fringes.

Why does a reflecting mirror not produce chromatic aberration?

It does not focus light by refraction.

What optical element gathers and focuses light in reflecting and refracting telescopes?

A reflector uses a mirror; a refractor uses a lens.

How do mirrors and lenses differ in their susceptibility to chromatic aberration?

Mirrors do not produce chromatic aberration; lenses can exhibit it.

How do a lens and a parabolic primary mirror differ in their susceptibility to spherical aberration?

A lens can exhibit spherical aberration. A parabolic primary mirror avoids it for rays parallel to its axis.

Why are large reflectors generally more practical to build than large refractors?

A large mirror can be supported from behind. A large objective lens is supported at its edge and may deform under its own weight.

What two optical advantages can the practicality of building large reflectors provide?

Potentially greater light-gathering power and resolving power.

What two maintenance tasks can reflecting telescopes require?

Mirror alignment and renewal of reflective coatings.

What effect does the secondary mirror in a Cassegrain telescope have on incoming light?

It obstructs some of the incoming light.