One day per month will be set aside for calibration of EPIC and NISTAR. Most of this time will be devoted to EPIC as described here. The EPIC spectrometer will take a series of images (about 50) with the Moon positioned near the center of the CCD. Each image will be displaced a few CCD pixels from the previous image. The displacements are caused by lunar orbital motion between exposures (about 15 seconds apart) and by spacecraft jitter (~10 pixels in 15 seconds). After the central portion of the CCD is characterized with the first 50 exposures, another 50 will be obtained distributed over the entire CCD to relate fully the sensitivity of one pixel to another (flat fielding). This procedure must be repeated for each filter position.
In subsequent months the procedure will be repeated and examined for changes in each pixel and for drifts in groups of pixels. Over time an end to end transmission and sensitivity history will be built for each wavelength band. The result can be interpolated to produce a continuous daily change in calibration during the operation of EPIC. Figures A-2 and A-3 show the geometry of the Moon-spacecraft system and the placement of the lunar images on the CCD during the calibration.
The case shown in Figures A-2 and A-3 is for the minimum number of lunar images needed to produce useful results. The planned number of images per filter is larger. It is expected that the acquisition of the images and their transmission to the ground will take about 8 hours. The limiting factors are the transmission rate of images to the ground and the finite amount of onboard memory for storing images.
The algorithm involves a time consuming solution of optimal minimization of least squares to an over-determined system of equations. The solution for all 10 wavelengths will take 1 to 2 days on the planned Sun or Silicon Graphics workstations.
