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Дата изменения: Thu Nov 25 00:45:46 1999
Дата индексирования: Sun Apr 10 00:20:24 2016
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Introduction

On the afternoon of 19 October 1999, it was discovered by John Briggs that the secondary mirror in the 2.5m SDSS telescope had developed a series of circular cracks around its axis of symmetry. As the telescope had not been in use since the early morning of 16 October 1999, there was a great deal of concern and consternation as to what caused these fractures in the glass. As there had been a dramatic drop in temperature over the weekend, a drop of some 15$^{\circ}$C during a snow storm over the weekend of 16 October 1999, it seemed plausable that the temperature drop was the immediate cause for the fracture. However, what needed to be understood was why the mirror was under enough stress to expose it to the possibility of cracking in the first place. It was decided by the APO Site Manager that the SDSS Observers should undertake an analysis of all available forensic data on hand, as they are the people best equipped for such an analysis: they have intimate day-to-day knowledge of the telescope; are responsible for the overall health and safety of the telescope (on a daily basis); and were responsible for the operation of the telescope up until the morning of 16 October 1999. Also, as the dark run was close in time to the event, there may have been misunderstood and poorly documented behaviour, which in hindsight, might point to important clues about the behaviour and state of the secondary before the fractures appeared.