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Entrance Window

Michelle uses an entrance window made of Potassium Bromide (KBr). KBr has a transmission of ~90% across the mid-infrared. Since KBr is hygroscopic, all contact between water with the window must be minimized. This includes dew that may form on the window while observing under high humidity conditions. The humidity limit for observing with Michelle at Gemini is a relative humidity of 80%. During its use at UKIRT, a protective "sacrificial window" system was implemented to help prevent damage to the cryostat window that serves as the vacuum seal between the dewar and the atmosphere.