The HST (Hubble Space Telescope) was first conceived back in the 1940s, designed and built during the 1970s and 1980s, and finally launched into orbit by the
Space Shuttle Discovery on 25 April
The HST (Hubble Space Telescope) was first conceived back in the 1940s, designed and built during the 1970s and 1980s, and finally launched into orbit by the
Space Shuttle Discovery on 25 April 1990. Unfortunately, once up there in its LEO (Low Earth Orbit) of 600km, it was discovered that the main mirror in
this 2.4m reflecting telescope was incorrectly ground. This spherical aberration meant that the mirror was beaming only a small amount of light from an object into a well-focused core of light, the rest of the light was being diffused into a halo around it. This resulted in blurry imaging and in a lack of available light for the other scientific instruments that needed it. A repair and servicing mission in 1993, and another in 1997, fixed these problems and resulted in the eighth wonder off the world.
Hubble has three cameras, two spectrographs and a host of fine guidance sensors. By combining them all above the Earths atmosphere it can produce high-resolution images of astronomical objects of the quality that terrestrial telescopes just cant equal. At the heart of the Hubble is the Optical Telescope Assembly, the Hubble mirror. A standard Cassegrain telescope sees light enter the tube from an object and bounce from a primary to a secondary mirror, the latter sending the light back through a hole in the primary mirror, which focuses onto an imaginary surface, the focal plane in other words. The Hubble uses an adapted version of the Cassegrain telescope, which provides light in the focal plane thats as close to the diffraction limit, as specified by the laws of physics, as possible. The Hubble mirror can concentrate 84 per cent of incoming light into a small dot of encircled light energy on the focal plane, the diffraction limit of the mirror, thus providing the highest quality resolution of images to the on-board scientific instruments.
The instruments carried by Hubble are impressive pieces of technology in themselves, especially the WF/PC (Wide Field and Planetary Camera), which has provided the most visually stunning images of the universe, such as clusters of remote galaxies and near psychedelic imagery of distant nebulae, for example. The WF/PC is mounted behind the main Hubble mirror, perpendicular to the long axis of the telescope, and is not one, but four cameras. An L-shaped trio of CCDs (Charge Coupled Devices) provides a }wide field, while a smaller, high-
resolution planetary camera is tucked into the remaining corner of the square space. A full picture of 1,600 x 1,600 pixels across the CCDs is possible, and objects as faint as 28th magnitude can be seen during long-term exposures.
Yet despite its undoubted success, seeing hot matter draining into a black hole and the light of galaxies 10 billion light years from Earth, for example, Hubble is starting to show its age. After all, it only ever had a 15-year life expectancy, so the repair mission in December 1999 to replace the gyroscopes, computer, fine guidance sensor, assorted data recorders and transmitters and more should come as no surprise. However, all this expensive, manned mission achieved was to patch up an ageing spacecraft - there was no element of upgrade here. This is set to happen in 2001 when an ongoing mission to install a host of new instruments is planned, and should boost the telescopes capabilities by more than 25-fold. The ACS (Advanced Camera for Surveys) will be first on board, enhancing the Hubble deep-field capability from tight focus to wide sky surveys to the same depth. Then in 2003 the Cosmic Origins Spectrograph should come into play, exploring the dark matter that accounts for the bulk of the universes mass.
However, its 2008 that holds the most promise, for it is then that the much talked about NGST (Next Generation Space Telescope) is set to launch. This will be the Hubble replacement, designed from the ground up to detect the very edge of light, and so gaze further back in time than we ever imagined possible, right back to beyond when the first stars and galaxies were formed. Light travels at a finite speed, so the universe is always viewed as it was rather than as it is - the sun is always seen eight minutes old, for example. Hubble can see back as far as 40 per cent of the total age of the universe, or around five billion years ago (compared to about two billion before the telescope was launched).
Hubble has gone right back to between ten and 11 billion years ago using a technique known as gravitational lensing. The telescope captured an Einstein Ring optical effect, caused by a distant light source being directly behind a galaxy as seen from Earth, and by using the galaxy in the middle as a gravitational lens, as predicted by Albert Einstein himself. Since the
middle galaxy bends the rays of light from the object, the distant observer (Hubble) sees a halo with the galactic lens in the centre. The object in this case is said to be another galaxy, and it was the first unbroken Einstein Ring seen. NGST will be based around an 8m mirror, compared to Hubbles 2.4m
mirror, and will provide a light-collecting area that is ten times larger
and optimised to record wavelengths further into infrared than Hubble can muster (because the expansion of the universe moves light from distant objects to the longer redder wavelengths). This will provide the ability to get close to the very beginning of time, as far as when matter unleashed by the Big Bang had cooled but not begun merging together into stars.
NGST isnt without problems of its own, not least the cost as NASA enters
a period of insecurity regarding long-term funding, and technical issues such
as the degree of coldness required to maintain optimum sensitivity for
infrared light. This latter problem is thought to be solvable, ironically, by
an orbit around the sun. Away from the heat of Earth and the moon, shielded
from sunlight itself, this could provide both cold temperature and longevity of the mission. The small matter of funding, limited to around $500 million (a quarter of the cost to build and launch Hubble), is being addressed by planned use of an unmanned and expendable launch vehicle instead of the Space Shuttle. Whether this whittles the cost down enough remains to be seen.