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It has been found that the CCD window is susceptible to condensation if the relative humidity inside the dome exceeds 50%. If this occurs, but the relative humidity outside is less than 90% you may observe but you are recommended to turn on the supply of dry nitrogen gas which flushes over the CCD window. This can be done by adjusting the flow valve on the gas meter which is mounted on the blue autoguider cover of the A and G Box. Set the flow rate so that the ball float can just be seen to be floating indicating that there is a small flow of nitrogen passing over the window.
DO NOT FORGET to turn the nitrogen off if the humidity drops below 50% or at the end of the night.