ABSTRACT
A window is used in an optical instrument primarily as a transparent interface between the internal components and the outside environment. Usually, it is a plane-parallel plate of optical glass, fused silica, plastic, or crystalline material that allows the desired radiation to pass through with minimal effect on intensity and image quality but excludes dirt, moisture, and other contaminants and, in some cases, maintains a positive or negative pressure differential between the internal and external atmospheres. For infrared (IR) applications, the window must not radiate due to its temperature in a manner that interferes with the system’s function. The front apertures of some telescopes, such as Schmidt systems, are closed with windows that have one or both surfaces contoured so as to correct aberrations. These contours are generally aspheric and do not depart drastically from a nominal flat or spherically curved shape. Meniscus-shaped windows, such as those used on a Maksutov system, are called shells. A deep shell is called a dome. The latter may be spherical or shaped to conform to that of its surround.
