Great Paris Refractor coelostat. This sidereal coelostat (siderostat) belonged to a telescope designed for the Paris Universal Exhibition of 1900, a trade and cultural fair. A siderostat is a separate, rotating mirror that constantly reflects a given region of sky into a fixed telescope. This mirror measures 2 metres (79 inches) in diameter, and because it was separate, negated the need for an expensive equatorial mount. The refractor telescope to which it belonged did not survive long after the exhibition, although the main lens (the largest ever at 49.2 inches) can still be seen at the Paris Observatory.