Space based Telescopes
Earth based telescopes suffer from the limitations caused by light pollution, atmospheric turbulence and weather systems. Added to this the Earth’s atmosphere absorbs electromagnetic radiations of the shorter wavelengths such as X-rays and gamma rays which are emitted by objects in the universe.
To overcome the limitations of Earth based telescopes highly sensitive telescopes are placed in orbits around the Earth. These space telescopes can get a clearer view of objects in the universe with no restriction of night and day or weather. They can also detect-rays and gamma rays giving a further critical insight into the universe.
Hubble Space Telescope (HST)
The most famous space telescope is the Hubble Space Telescope. Hubble Space Telescope was launched abroad the space shuttle Discovery in 1990. Hubble’s orbit lies 569km above the Earth. It completes one rotation of the Earth in 97 minutes, travelling at a speed of approximately 8 kilometers per second. During its journey Hubble’s mirrors capture light and other radiations and directs them onto its several instruments.
The telescope has a Cassegrain reflector with a primary mirror of diameter 2.4 meters. Although this mirror diameter is smaller than those of Earth based telescopes (these can be up to 10 meters in diameter) its location above the Earth’s atmosphere gives it excellent clarity. The resolution of the images produced is 10 times greater than those from any Earth based telescope.
Along with the telescope Hubble carries other instrumentation. One of these is the Wide Field Camera 3 (WFC3). This allows it to see radiations from three different wavelengths; near ultra violet, visible light and near infra-red, however not at the same time.
X-ray sources in the universe are of great interest to astronomers as they are associated with highly energetic processes involving very high temperatures for e.g. exploding starts, a vast cloud of hot gases in a galaxy cluster . Ordinary mirrors do not work for wavelengths in the x-ray region of the electromagnetic spectrum. This is because x-rays have so much energy that they pass right through standard mirrors rather than being reflected by them.
To capture x-rays they need to be bounced of specially constructed mirrors at a very low angle. This technique is called the grazing incidence and thus x-ray telescopes are also referred to as grazing incidence telescopes. In these telescopes the mirrors are precisely shaped and aligned with their surfaces nearly parallel to incoming x-rays. The x-rays strike the mirrors at grazing angles and are reflected just like pebbles skipping across a pond when thrown at a low angle. The surfaces of the mirrors are coated with a thin film of gold or iridium so that the x-rays are reflected and not absorbed.
The animation below shows the grazing incidence technique of x-ray telescopes.
|X-ray telescopes such as the Chandra telescope system consist of 4 pairs of specially constructed mirrors. They are shaped such that they represent a glass barrel design. This design suits the grazing angle technique required for x-ray telescopes. The incoming x-rays graze the mirrors at very low angles and are reflected onto an x-ray detector. The mirrors are coated with a thin film of gold or in the case of the Chandra telescope a thin film of the highly reflective rare metal, iridium.|
Gamma ray Telescopes
Gamma rays have the shortest wavelengths and most energy of all the waves in the electromagnetic spectrum. Sources of celestial gamma rays are important to astronomers as they are linked to violent events such as supernova explosions and the destruction of atoms. They are also linked to less violent events such as the decay of radioactive material in space.
Gamma rays cannot be captured and reflected by mirrors in the same way as visible light and x-rays. The high energies associated with gamma rays makes them pass right through such devices. Instead, gamma rays are detected by using a specially designed piece of equipment call a scintillation detector. Gamma rays transfer their high energies to the particles in the material of the scintillator. This results in the generation of high energy charged particles which interact with the crystalline structure of the scintillator material producing low energy photons (packets of energy). These photons are collected by the detector and give the intensity of the received gamma rays.