Telescope window functions How a telescope responds to features on different angular scales depends upon its beam pattern. Patterns of sky emission can be characterized by a spatial frequency spectrum.
Higher frequencies correspond to features of smaller angular extent.
The telescope responds as a filter, passing only a band of spatial frequencies. The filter pattern is known as the telescope's window function . For CMB purposes we use a spherical harmonic representation. Spatial frequencies are given with the spherical harmonic number, l . The window functions for each Jodrell Bank CMB telescope are given below.