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Mayall 4-m Shutter Failure(1Dec95) (from KPNO, NOAO Newsletter No. 44, December 1995) The gearbox on the 4-meter dome shutter failed in mid-October, and we have been forced to close operations at the 4-meter while we make repairs. The failure occurred during a period of exceptionally clear weather, and we left the dome open for nearly two weeks so that observers could continue to collect data. It was only after the rains became heavy that we closed the dome, and we cannot safely open it again until the repairs are completed. We have now developed a plan and a schedule for the repair. Based on what we know now, the repair will take about eight weeks and we are projecting resuming operations near Christmas. The length of the closure is set by the long time required to fabricate a complete set of gears for the shutter mechanism. The failure was discovered on 18 October. The gear train that operates the 4-m dome shutter was found to be damaged to such an extent that it would be unsafe to continue to operate the shutter on a daily basis. Further use had the potential of leading to complete breakage of the gearbox with possible loss of the shutter off of the end of the rails. We decided to keep the dome open day and night until we were forced to close by bad weather. Finally after ten clear nights and one mostly cloudy, the night of 29/30 October saw the first storm of fall arriving on the tropical jet stream. We stayed open and did not close the shutter for two drizzling rains which came after midnight. In the morning we wrapped a tarp around the Cassegrain cage and stowed the telescope so that the top-end was sheltered under the dome. We felt that we could ride out a few more drizzles and maybe a shower or two. The forecast was for rain showers and then clear weather for five days before the next storm. However, around noon on Monday, 30 October, the storm intensified, and we decided our luck had run out. We turned out to be right. The storm earned an alert on the Weather Channel, which designated it a strong storm with embedded thunderstorms, and that is what we actually got all afternoon. Significant water damage would have resulted had the 4-meter dome not been closed. The cause of the failure of the gears remains unknown and will only be ascertained when the gearbox is removed from the dome and disassembled. Because the origin of the damage may not be in the gearbox, the motors, clutch and brake assemblies will be refurbished during the time we are closed. This whole mechanism is almost 30 years old and, while well maintained, will now be checked over thoroughly. Actual work for removal of the gearbox and other components in the mechanism began as soon as we closed the shutter. A new set of gears has been ordered, with a delivery time estimated to be six weeks. Two additional weeks have been set aside for assembly, testing, and installation of the gearbox. A firm date for reopening will be set when the repairs are further along. The affected observers have been notified and will be kept informed of progress. Sidney C. Wolff
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