Windows

Several types of window material, such as KBr, are highly transmissive at mid-infrared wavelengths and strong enough to serve as a pressure bulkhead, but they have moderate drawbacks when used as windows; e.g., they are easy to scratch and/or hygroscopic, or only transmissive over limited wavelength regions.

To permit the observer to select the appropriate window for a given combination of scientific requirements and observing conditions, three science entrance windows are installed.

The available window materials are:

• ZnSe, which has exceptionally high transmission at 10 microns, but is opaque at 20 microns. It's the best choice for 10 microns' observations.
• KBr, which has excellent transmission at 10 and 20 microns, but degrades under high humidity conditions.
• KRS-5, which is insensitive to moisture and transmits at both 10 and 20 microns, though it is less transparent than both the KBr and ZnSe.

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