MODIS along-scan direction Line Spread Function (LSF) modeling and verification using the Integration and Alignment Collimator (IAC)

Abstract

The Modulation Transfer Function (MTF) is a standard measure of the spatial quality of an imaging sensor. The MTF is calculated by a normalized Fourier Transform of a Point Spread Function (PSF), which is a two-dimensional function. For simplicity of calculation, a one-dimensional PSF, or Line Spread Function (LSF) is utilized in the along-scan direction. The along-scan direction LSF model is sub-divided into the component level LSFs and the proper LSF is constructed by the design specification information. To construct the modeled LSF, a spatial convolution is performed for the three major components: optical, detector, and integration time LSFs. The optical LSF is calculated using Zemax. The detector LSF is modeled as a rectangular function of the nominal detector size. Similar to the detector LSF, the integration time LSF is modeled as a rectangular function by using the sampling frequency and the integration pulse duration time. The modeling LSF is compared with the measured LSF from a prelaunch ground calibration device, the Integrated Alignment Collimator (IAC). Because the IAC test slit has a width of 0.1 MODIS IFOV, an extra step pulse convolution is added to the final LSF model. The comparison results show an excellent agreement between the modeled and measured LSFs in the spatial domain and MTFs in the frequency domain for selected reflective solar bands (RSB).