The Hubble Space Telescope

The Hubble Space Telescope (HST), named after the famous astronomer Edwin Hubble was launched on April 24th 1990 at 12:33:51 UT. It was taken into a circular orbit aboard Space Shuttle Discovery (mission STS-31) and it orbits the Earth at a height of 593 km with a 28.5 degree inclination and an obital period of 97 minutes.

above. A NASA drawing of the HST.

The main problem with Earth-based observations, is that you have to look through the Earth's turbulent atmosphere. The HST avoids this entirely by being situated high above the atmosphere. This is why this 2.4 m Ritchey-Chretien telescope can capture such high resolution images. This has not always been the case though. When Hubble was launched, it had a number of problems, and became known as a "techno-turkey".

Firstly, the HST had stability difficulties. Every time it crossed the day and night terminator, the orbiting observatories solar panals became distorated causing it to lose its pointing accuracy.

above. A NASA drawing of the HST in the Shuttles payload.

The main problem though, was that the satillite could not be focused correctly. No focusing could remove the blob of light found around all of its star images. This was due to an optical defect known as spherical aberration - the primary mirror was two microns too flat near to its edge. This hadn't been noticed before it was launched, because the primary and secondary mirrors had not been tested together. Some of the space telescope's tasks were not affected though, such as ultraviolet work, and photometry.

above. A NASA photograph of astronaughts working on Hubble.

Soon after the HST's first light NASA engineers designed COSTAR (Corrective Optics Space Telescope Axial Replacement) to fix the optical defect. It was made up of a number of small adjustable mirrors that the light would have to reflect off before it entered the telescope's imaging devices. COSTAR was placed inside the telescopes optical train in 1993. It took the team of astronaughts 35 hours to replace the distorted solar panals and to fix COSTAR in place next to the Wide Field Planetary Camera.
NASA released the new images on January 13, 1994 which gave unprecedented detail. The HST continues to be at the forefront of optical astronomy today.

above.The Hubble Space Telescope in orbit.

There are plans to build a new space telescope known as The James Webb Telescope - Formally the NGST (Next Generation Space Telescope). in the near future. It will be placed at the "L2" point - 1.5 million km away on the opposite side of the Earth to the Sun. It will concentrate on cosmology, and the formation of our universe. The HST is likely to remain in operation until at least 2010, when the NGST should be operational. There have been proposals to rase Hubble into a "graveyard orbit", where it can continue to function until it deteriorates, instead of attempting to bring it back to Earth.