MODIS spectral response anomaly when using a double monochromator operating in the subtractive mode

摘要

Abstract: The Moderate Resolution Imaging Spectrometer (MODIS) will be one of the primary instruments observing the earth on the Earth Observing System (EOS) scheduled for launch in 1999. The relative spectral response of MODIS is an important part of the radiometric calibration train. A double filter grating monochromator anomalous spectral response was observed in some MODIS bands but not in others. Special tests were made which illustrated a spectral cyclic structure which should not have been present. A computer model was built of the double monochromator that enabled small perturbations to be introduced for the first and second monochromator sections. A parametric study illustrated that the anomalous spectral response behavior could be duplicated by assuming a constant and a variable offset between the first and second monochromator sections. The cause of this was found and a simple solution was implemented. The solution was to have the first monochromator section have a larger spectral slit function than the second section. Selected data will be shown to illustrate the phenomena. !0