Infrared (IR) light is invisible to the human eye. For purely visual observation, the fact that most telescopes are not designed to focus IR light does not matter. However, the situation is quite different when a camera is used! Unlike the human eye, a camera sensor is sensitive to light in the infrared part of the spectrum. Consequently, the telescope's lack of focus in this spectral range compromises image sharpness; the image—which is actually sharp in visible light—is degraded by the superimposed infrared component. This problem can be easily solved by blocking the infrared light—which is precisely what an IR-cut filter is designed to do.
