Procedure for Rapidly Rising Dewpoint Making Condensation on Mirrors

In rare weather circumstances, it sometimes happens that the dewpoint rises rapidly so that water/frost can condense on primary mirrors that are still cold from the previous night's observing. This can damage the aluminum coating when it evaporates. Following Issue #5454, we generate an automatic email when this condition is threatening.

Here's the procedure that you should follow whenever practical if the dewpoint is rising above the primary mirror glass temperature. (It happened in Issue #2499.)

1) Close the dome if it isn't already closed (just to state the obvious)

2) Verify that the two PMC subsystems are running and generating a setpoint above the dewpoint. ECS also needs to be running.

4) Turn on Mirror Ventilation on both sides. It should start warming the two mirrors at a rate of about 1 degC per hour.

5) Stop Mirror Ventilation as the weather clears so that further heating is no longer needed. If observing in imminent, then you may want to continue ventilation to prepare the mirrors for the night.

-- JohnHill - 19 Jan 2010