A telescope or instrument's emissivity is the degree to which its temperature produces the thermal radiation (and/or thermal noise?) associated with that temperature. It is expressed as a fraction or percentage, i.e., what fraction of the total black body. It is of high interest in far infrared and microwave equipment in creating highly sensitive instruments: the instruments that are often cooled to cryogenic temperatures to improve their sensitivity. A low emissivity "leverages" the benefits of the low temperature.
Emissivity is dependent upon both the black-body temperature under consideration, and the wavelength at which it is measured, and measures of it may be cited or graphed specifying one or both. For example, an instrument may have a specific emissivity value as measured at wavelength λ, when the instrument's temperature is t.