A calibration target for far-infrared spaceborne applications

We describe progress in the development of a calibration target for use in the EOS-MLS 2.5 THz radiometer on NASA's CHEM-I spacecraft. Although the intended use is as a stable, isothermal black body load at a frequency of 2.5 THz,the design is suitable for use throughout the far-infrared (FIR). A wedge design is used for the target body to enhance the emissivity to desired levels at 2.5 THz. The body is machined from aluminum, giving the best trade between issues such as cost, thermal conductivity, mass, and strength. The target utilizes a white coating to reduce the destabilizing effects of periodic solar illumination. The coating can be made relatively thin to allow accurate temperature measurements of the FIR-absorbing medium. Emissivity of greater than 0.99 is achieved at 2.5 THz, while the solar absorptance is estimated at < 0.5.