Preflight Spectral Radiance Infrared Calibration Facility

Abstract

Preflight calibration of space-based observation systems (SOBS) is carried out by means of standard sources with known spectral radiance. There are no difficulties in preflight calibration of SOBS within the visible spectral range. The main problem here lies in achieving sufficiently high uniformity of spectral radiance across the radiating aperture of a large-area source. Standard blackbody radiance sources with the temperature that is measured and with the calculated emissivity are used for calibration of SOBS in the infrared (IR) spectral range. The emissivity of sources having an aperture as large as 500 mm cannot be calculated accurately enough, and they have to be measured. It is quite challenging to conduct the measurements in a vacuum chamber simulating the low earth orbit environment in a broad temperature range. A spectral radiance calibration facility for preflight calibration of SOBS which is based on using a large-area blackbody with a diameter of 500 mm and an operational temperature range from $-60~^{\circ }\mathrm{C}$ to $150~^{\circ }\mathrm{C}$ is presented. The facility includes a gallium fixed-point blackbody, a variable temperature blackbody with a temperature range from $-60\,^{\circ }\mathrm{C}$ to $150\,^{\circ }\mathrm{C}$, a reference liquid nitrogen-cooled blackbody located in the vacuum chamber, and a Fourier transform IR spectrometer (FT-IR) used as a comparator. Radiation from the different sources is fed, in sequence, into the comparator by means of a custom-made optomechanical system located in the vacuum chamber. Operation of the calibration facility is described. Characteristics and specifications of the sources are shown.