![]() In this way, a dense net of measurements of the relative angular separations of the stars was progressively built up. As the scans also overlapped `sideways' when the satellite axis of rotation changed on each sweep of the sky, the stars appeared again, but this time compared with other stars. In this way, astronomers obtained several comparisons with different stars. The telescope was continually determining the relative (along-scan) positions of the programme stars which appeared first in the preceding field of view and then in the following field of view due to the rotation of the satellite. However, with rapid computer control, it could be switched to all the programme stars for short intervals of time during their passage across the field, which took about 20 seconds. The detector could only follow the path of one star at a time. The detector had a small sensitive area which covered an area of about 38 arcsec in diameter (projected on the sky). At any one time, some four or five of the selected (or programme) stars were present in the combined fields of view. As the telescope scanned the sky, the starlight was modulated by the slit system, and the modulated light was sampled by an image-dissector-tube detector, at a frequency of 1200 Hz. In this way, the telescope was able to scan the complete celestial sphere several times during its planned mission. At the same time, it was controlled so that there was a continuous slow change of direction of the axis of rotation. The satellite was designed to spin slowly, completing a full revolution in just over two hours. ![]() The satellite swept out great circles over the celestial sphere, and the star images from two fields of view were modulated by a highly regular grid of 2688 transparent parallel slits located at the focal surface and covering an area of 2.5 x 2.5 square centimetres. This achieved both large- and small-field measurements simultaneously. A novel feature of the telescope was the `beam-combining' mirror, which brought the light from the two fields of view, separated by about 58 degrees and each of dimension 0.9 x 0.9 degrees, to a common focal surface. The payload was centred around an optical all-reflective Schmidt telescope. Scientists manufactured the mirror of the Hipparcos telescope so accurately that if it were scaled up to the size of the Atlantic Ocean, the bumps on its surface would be only 10 centimetres high. Its data also helped predict the impact of Comet Shoemaker-Levy with Jupiter in 1994. ![]() The mission discovered that the Milky Way is changing shape. ![]() Hipparcos confirmed Einstein's prediction of the effect of gravity on star light. Hipparcos sent a million million bits of information, radioed to ground stations in Germany, Australia, and the United States, which went into the biggest computation in the history of astronomy. The Tycho 2 Catalogue (2000) brings the total to 2 539 913 stars, and includes 99% of all stars down to magnitude 11, almost 100 000 times fainter than the brightest star, Sirius. An auxiliary star mapper pinpointed many more stars with lesser but still unprecedented accuracy, in the Tycho Catalogue of 1 058 332 stars. Calculations from observations by the main instrument generated the Hipparcos Catalogue of 118 218 stars charted with the highest precision. ![]()
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