Aperture edge scatter calibration of the cavity radiometers for the spaceflight Total Irradiance Monitor

摘要

Aperture area knowledge is a primary calibration in radiometric instruments. Corrections for edge effects, particularly diffraction and scatter, must also be taken into account for high accuracy measurements. The Total Irradiance Monitor (TIM) is a total solar irradiance radiometer on NASA's SORCE mission launched in 2003 and on the NASA/Glory mission launching in 2008. In order to measure irradiance, the TIM instrument measures the total optical power that passes through circular diamond-turned precision apertures. The geometric areas of the 8-mm diameter apertures are measured to approximately 25 parts per million (ppm) at the National Institute of Standards and Technology. Due to scatter and diffraction, not all light that passes through the geometric area of an aperture will enter the radiometer cavity of the instrument, and corrections must be made for these edge effects. Diffraction effects are generally well understood and are calculated from the instrument geometry. Scatter, on the other hand, is dependent on the microscopic edge quality of each individual aperture, and so must be measured. This paper describes the measurement of aperture edge diffraction and scatter for the precision apertures on NASA's Glory/TIM instrument.