Slit function measurement optical stimulus

For the calibration of OMI a slit function measurement stimulus has been developed. The slit function is the monochromatic image of the entrance slit of the spectrometer on the detector. Accurate knowledge of this slit function for all wavelengths and field angles is very important both for the spectral calibration and for the DOAS retrieval algorithm. Determination of this function with a spectral line source is inaccurate, because of the detector resolution and incomplete, because of the limited number of discrete spectral lines, that are available. For accurate and complete measurement of the slit function an echelle monochromator has been developed, that offers a number of spectral lines, that can be scanned with a small wavelength step over the entire spectral range of OMI. The spectral bandwidth of these lines is about 0.1 x the spectral resolution of OMI and the wavelength step during scanning is even smaller. The wavelength scanning is performed by accurate rotation of the echelle. In this paper the scientific background of the slit function measurement, the stimulus and first OMI slit function calibration measurements are described.