The following sequence of Cambridge Catalogues, reaching sucessively fainter objects, is well known as a powerful tool to locate very distant extragalactic sources. They are now considered to be artifacts. The open circles represent the sources of large angular diameter, and in both cases the sizes indicates the flux density of the sources.'' With the advent of satellite astronomy short wavelength surveys became possible. It has influenced observational cosmology significantly. The brightest and most massive galaxies seem to be strong infrared emitters.
Quasars show infrared brightness correlated with their x-ray fluxes.
The infrared background radiation has become important as the short wavelength end of the microwave background.