Telescopes

What is a telescope?

A telescope is an instrument that collects and focuses electromagnetic radiation from distant objects to make them easier to observe.

Why are telescopes needed?

Distant objects are very faint. Telescopes collect much more radiation than the eye, allowing astronomers to observe them.

What is the objective lens?

The objective lens is the large lens or mirror that collects radiation and forms the first real image.

What is the eyepiece lens?

The eyepiece lens magnifies the image formed by the objective lens.

What is a refracting telescope?

A telescope that uses lenses to collect and focus light.

Advantages

• Simple design

• Closed tube

Disadvantages

• Chromatic aberration

• Large lenses difficult to manufacture

What is a reflecting telescope?

A telescope that uses mirrors to collect and focus light.

Advantages

• No chromatic aberration

• Large mirrors easier to support

Disadvantages

• More complex alignment

Refracting telescope in normal adjustment diagram

What is chromatic aberration?

Different wavelengths are refracted by different amounts by a lens.

Chromatic aberration diagram

What is spherical aberration?

Rays passing through the edge of a lens or mirror focus at a different point from rays near the centre.

Spherical aberration diagram

What is the Cassegrain arrangement?

The Cassegrain arrangement is a reflecting telescope design that uses a large concave primary mirror and a small convex secondary mirror.

Cassegrain arrangement diagram

What are the advantages of reflecting telescopes over refracting telescopes?

• No chromatic aberration (mirrors reflect all wavelengths equally)

• Large mirrors are easier and cheaper to make than large lenses

• Mirrors can be supported from behind, preventing sagging

• Greater collecting power because much larger mirrors can be built

• Less light absorption since light does not pass through thick glass

What is collecting power?

Collecting power is a telescope's ability to collect electromagnetic radiation.

Collecting power is proportional to the diameter sqaured

What is resolving power?

Resolving power is the ability to distinguish two close objects as separate. (how much detail you can see)

What limits resolving power?

Diffraction causes light to spread out, limiting a telescope's ability to distinguish close objects.

What is an Airy disc?

An Airy disc is the bright central spot produced when light is diffracted as it passes through a telescope's circular aperture.

What is the Rayleigh criterion?

Two objects are just resolved when the central maximum of one diffraction pattern falls on the first minimum of the other.

What is the Rayleigh criterion equation?

θ = λ​/D

Where:

• θ = minimum angular resolution

• λ = wavelength

• D = diameter of objective

How can resolution be improved?

• Increase objective diameter

• Use shorter wavelengths

What is telescope magnification?

Magnification is the ratio of the angle subtended by the final image to the angle subtended by the object.

What is the magnification equation?

M=f_o/​f_e

Where:

• fo​ = focal length of objective

• fe = focal length of eyepiece

What is a radio telescope?

A radio telescope uses a large dish antenna to detect radio waves from space.

Why are radio telescopes so large?

Radio waves have long wavelengths.

A larger diameter is needed to achieve good resolution.

What is radio interferometry?

Using multiple radio telescopes together to act as one large telescope.

Why do astronomers use non-optical telescopes?

Different astronomical objects emit different wavelengths of electromagnetic radiation.

Observing multiple wavelengths provides more information.

Why are infrared telescopes often placed on mountains or in space?

Water vapour in Earth's atmosphere absorbs infrared radiation.

Why are ultraviolet telescopes usually in space?

Most ultraviolet radiation is absorbed by Earth's atmosphere.

Why must X-ray telescopes be in space?

Earth's atmosphere absorbs X-rays before they reach the ground.

Why must gamma-ray telescopes be in space?

Earth's atmosphere absorbs gamma rays.

What is a CCD?

A Charge-Coupled Device (CCD) is a detector that converts incoming photons into electrical signals.

How does a CCD work?

Photons strike the CCD and release electrons.

The number of electrons produced is proportional to the intensity of the light.

How does a CCD work? 6 marks

• CCD consists of an array of pixels made from silicon.

• Each pixel contains a potential well to store electrons.

• Incoming photons strike the silicon.

• Photons release electrons (photoelectric effect).

• Electrons are collected and stored in the potential wells.

• The number of electrons stored is proportional to the light intensity.

• Charges from each pixel are transferred across the CCD.

• The charges are converted into digital signals.

• A computer processes the signals to produce a digital image.

What is a pixel in a CCD?

A pixel is a small light-sensitive element that stores charge produced by incoming photons.

Why is silicon used in CCDs?

Silicon is used because when photons strike it, they can release electrons from the silicon atoms.

Silicon is a semiconductor.

What is a potential well in a CCD?

A potential well is a region in a CCD pixel where released electrons are collected and stored.

What is quantum efficiency?

Quantum efficiency is the percentage of incident photons that produce electrons.

What is quantum efficiency of the human eye?

1%

What is quantum efficiency of a CCD?

70-80%

What is quantum efficiency of photographic film?

4%

What are the advantages of CCDs over photographic film?

• Higher sensitivity

• Higher quantum efficiency

• Detect fainter objects

• Digital output

• Linear response

• Easy computer analysis

How is a digital image formed using a CCD?

Each pixel stores charge proportional to the intensity of light received.

A computer converts the charges into a digital image.

Ground-based vs space-based telescopes?

Ground-Based

• Cheaper

• Easier maintenance

• Atmospheric distortion

Space-Based

• No atmospheric distortion

• Access to X-ray, UV and gamma-ray observations

• More expensive

Reflecting vs refracting telescopes?

Reflecting

• Uses mirrors

• No chromatic aberration

• Larger apertures possible

Refracting

• Uses lenses

• Chromatic aberration

• More difficult to build large versions