Telescopes & Constellations

Why do we need telescopes?

Stars and distant objects are too faint and too far away to study with the naked eye. A telescope collects far more light than the human eye and magnifies images so we can study them in detail. Telescopes have been fundamental to almost every major discovery in astronomy.

Types of telescopes

Optical telescopes

Collect and focus visible light using lenses (refracting) or mirrors (reflecting). Used by amateur astronomers and at observatories. Example: SALT.

Radio telescopes

Collect radio waves emitted by stars, galaxies and other sources. Work day and night, through clouds. Example: MeerKAT / SKA.

Space telescopes

Above Earth's atmosphere — no distortion from air, clouds or light pollution. Can detect UV, X-ray and infrared. Example: Hubble Space Telescope.

Infrared telescopes

Detect infrared (heat) radiation — can see through dust clouds and study very distant galaxies. Example: James Webb Space Telescope.

SALT — Southern African Large Telescope

SALT is located in Sutherland in the Northern Cape, South Africa. It is the largest single optical telescope in the Southern Hemisphere:

Primary mirror: 11 × 9.8 m (hexagonal array of 91 smaller mirrors)
Completed: 2005
Run by the South African Astronomical Observatory (SAAO)
Observes stars, galaxies, quasars, exoplanets
International partnership between South Africa and universities worldwide

MeerKAT and the SKA

The Square Kilometre Array (SKA) is the world's largest radio telescope — a global project with its core sites in South Africa (Karoo) and Australia:

MeerKAT: 64-dish precursor radio telescope in the Karoo; completed 2018. Already among the world's most sensitive radio telescopes.
SKA: will ultimately link thousands of antennas across two continents with a combined collecting area of over 1 square kilometre
Purpose: map hydrogen gas across the universe; detect pulsars; search for signs of life (SETI); study the early universe

South Africa's science pride: SALT and MeerKAT/SKA represent South Africa's global leadership in astronomy. The Karoo's low radio-frequency interference and clear skies make it ideal for these instruments.

The Hubble Space Telescope

Launched in 1990 by NASA, the Hubble Space Telescope orbits Earth at about 547 km altitude. Because it is above the atmosphere, it captures extraordinarily sharp images with no atmospheric blurring:

Primary mirror: 2.4 m
Has revealed the age of the universe (~13.8 billion years)
Discovered that the expansion of the universe is accelerating (dark energy)
Provided images of galaxies billions of light years away

Constellations

A constellation is a group of stars that appear to form a pattern in the sky as seen from Earth. There are 88 officially recognised constellations.

Stars in a constellation are not necessarily close to each other in space — they just appear close from our viewpoint
Different constellations are visible from the Northern and Southern Hemispheres
Constellations appear to move across the sky during the night (Earth rotates) and change with the seasons (Earth orbits the Sun)

Southern Hemisphere constellations:

Crux (Southern Cross) — the most recognisable constellation visible from South Africa; its long axis points toward the South Celestial Pole
Scorpius — easily visible in winter; a large curved constellation representing a scorpion
Orion — visible in summer; one of the most recognised constellations globally
Centaurus — contains Alpha Centauri, the nearest star system

Indigenous astronomical knowledge

Many African cultures have rich traditions of astronomical observation. The San people of southern Africa identified and named many constellations, using star positions to guide seasonal activities such as hunting and planting. For example, the Pleiades star cluster (Selemela) traditionally signals the time to plant crops.

Write your answers in the spaces below. Use full sentences.

1. Explain the difference between an optical telescope and a radio telescope. Give one example of each and state where in South Africa it is located.

2. Describe SALT. Where is it located, what does it observe, and why is it significant for South Africa?

3. What is the SKA and what is its scientific purpose? How does MeerKAT relate to the SKA?

4. Explain why the Hubble Space Telescope can produce clearer images than a telescope of the same size on Earth's surface.

5. What is a constellation? Describe the Southern Cross (Crux) and explain how it is useful for navigation in the Southern Hemisphere.

6. Discuss the astronomical knowledge of the San people. Give one example of how a star or star cluster was used in daily life.

7. The table below compares three telescopes mentioned in this lesson.

Telescope	Type	Primary mirror / dish size	Location
SALT	Optical (ground)	11 m × 9.8 m mirror	Sutherland, South Africa
Hubble	Optical/UV (space)	2.4 m mirror	Earth orbit
MeerKAT	Radio	64 dishes, 13.5 m each	Karoo, South Africa

(a) SALT's primary mirror is about 11 m across. Calculate roughly how many times wider this is than Hubble's 2.4 m mirror. (1)
(b) A wider mirror collects more light. Despite SALT collecting far more light per instant than Hubble, explain why Hubble can still take clearer images of very faint, distant objects. (2)
(c) MeerKAT uses 64 separate dishes working together instead of one giant dish. Suggest one advantage of this design over a single huge dish of the same total collecting area. (1)