Details
Description
Telescope objective lenses are not corrected in the infrared and ultraviolet range of the spectrum (the exception being reflecting telescopes without corrector plates). Unlike the naked eye the CCD chip of a camera is sensitive to the near IR spectrum and as a consequence contrast and sharpness would suffer. For this reason you should cut off the IR and UV light whenever there are any glass elements between the camera and the subject matter.
The IR blocking filter is an interference filter with a steep, precisely controlled filter curve that has a sharp cutoff at 690nm, yet it transmits all the visible red wavelengths including the important 656nm H-alpha emission line. For the naked eye the filter seems fully transparent since the human eye doesn't perceive the IR and UV light.