The UV/IR Block Filter (2") is designed to block unfocused infrared (IR) and ultraviolet (UV) wavelengths from the imaging path. Unlike the human eye, both CCD and CMOS cameras are sensitive to these wavelength ranges, but refractive optical systems cannot bring them to the same focal plane as visible light. As a result, they produce distracting halos and blurred spots in images.
This optical issue is not limited to refractor telescopes. UV/IR block filter is also recommended for reflecting systems when a Barlow lens or coma corrector containing lenses is used in the optical path.