Work began on the HST in 1977 and it was launched by the Space Shuttle 'Discovery' on 24 April 1990. It was expected to image objects more sharply than telescopes on Earth and detect fainter sources. Unfortunately, following its launch a defect was discovered in the main optical system, limiting its performance, and it was not until December 1993 that the defective mirror was replaced. A reflecting telescope, the HST's main mirror has a diameter of 2 1/2 metres.
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