Mirror Cooling Policy, per Mick Connelley email 12/17/2015 from: Mike Connelley Date: 12/17/15￼ To: Techgroup￼ The goal of this policy is to balance having a cold mirror when conditions are good but not have the mirror so cold that it fogs over when the weather is bad. The mirror cooling is usually turned on by the TO at the end of each night, left on during the day, and turned off by the TO at the start of the next night. The system currently turns itself off if the humidity in the mirror cell exceeds ~95%. If the mirror is cold and the humidity rises quickly, the mirror can fog over, which should be avoided. Turning off mirror cooling once the humidity in the mirror cell exceeds 95% is likely to be too late since the mirror changes temperature much more slowly than the humidity changes. The staff should use their judgment as to when to turn on and off the mirror cooling since sensors and policies cannot foresee all possible future events. We should err on the side of caution, and turn off mirror cooling if bad weather is likely and live with a warm mirror when the weather clears. Based on these guidelines, the staff should decide whether to turn the mirror cooling on or off, and to communicate their decision. I propose the following policy: For TOs: The TO should use their judgement to decide at the end of the night whether to turn on the mirror cooling, based on the current conditions and the weather forecast. The default is to turn mirror cooling on. Reasons to leave mirror cooling off include, but are not limited to: - high humidity at the end of the night (greater than about 80%...there's nothing special about 80%, it just seems reasonable) - high chance of fog (greater than about 80%) or precipitation in the forecast for the next day - experiencing bad weather or bad weather is imminent - tried to turn on the mirror cooling, but it didn't work Deviations from the default (i.e. leaving the mirror cooling off at the end of the night) should be noted in the troublelog as well as the reason for leaving mirror cooling off. For example, if there was a fault with the HVAC, then it is important to note this so that the day crew can fix it. For daycrew: The default is to leave the mirror cooling on throughout the day, and the TO will turn it off at the start of the night. At the middle of the day, superintendent (or next in charge) should make an evaluation to turn the mirror cooling on or off. This evaluation should use the guidelines above, and consider the current conditions, the weather forecast, and the TOs decision as noted in the troublelog. As with the TOs, deviations from the default should be noted in the trouble log.