Invited Article: Deep Impact instrument calibration

Calibration of NASA's Deep Impact spacecraft instruments allows reliable scientific interpretation of the images and spectra returned from comet Tempel 1. Calibrations of the four onboard remote sensing imaging instruments have been performed in the areas of geometric calibration, spatial resolution, spectral resolution, and radiometric response. Error sources such as noise (random, coherent, encoding, data compression), detector readout artifacts, scattered light, and radiation interactions have been quantified. The point spread functions (PSFs) of the medium resolution instrument and its twin impactor targeting sensor are near the theoretical minimum [ ∼ 1.7 pixels full width at half maximum (FWHM)]. However, the high resolution instrument camera was found to be out of focus with a PSF FWHM of ∼ 9 pixels . The charge coupled device (CCD) read noise is ∼ 1 DN. Electrical cross-talk between the CCD detector quadrants is correctable to < 2 DN. The IR spectrometer response nonlinearity is correctable to ∼ 1 % . Spectrometer read noise is ∼ 2 DN. The variation in zero-exposure signal level with time and spectrometer temperature is not fully characterized; currently corrections are good to ∼ 10 DN at best. Wavelength mapping onto the detector is known within 1 pixel ; spectral lines have a FWHM of ∼ 2 pixels . About 1% of the IR detector pixels behave badly and remain uncalibrated. The spectrometer exhibits a faint ghost image from reflection off a beamsplitter. Instrument absolute radiometric calibration accuracies were determined generally to < 10 % using star imaging. Flat-field calibration reduces pixel-to-pixel response differences to ∼ 0.5 % for the cameras and < 2 % for the spectrometer. A standard calibration image processing pipeline is used to produce archival image files for analysis by researchers.