As of the time of preparation of this report, instrument assembly is not completed so that there are no results from end to end testing of the EPIC instrument. However, it is planned that there be extensive absolute radiometric calibration both at room temperature and in thermal and vacuum conditions. The latter is necessary because the CCD is expected to operate at L-1 at temperatures of about 40°C under vacuum conditions, while the telescope and optics will operate at nearly room temperature. The sensitivity of the CCD is temperature dependent, especially at the UV wavelengths. Current relative sensitivity is expressed in terms of CCD quantum efficiency QE (shown in Figure A-1).
The product of the transmission of the telescope, filters, and fractional QE of the CCD essentially determine the sensitivity of the EPIC spectroradiometer system. The next figure of importance is the signal to noise ratio SNR of EPIC. This is determined by the CCD's electron well depth which is equal to 120,000 electrons. If the exposure is adjusted to fill the well to 90%, then the SNR is the square root of the number of electrons, or about 330:1, or about an accuracy of about 0.3%. As determined by the design specifications, the system is supposed to achieve 200:1, or about 0.5% accuracy. The accuracy as stated applies to the brightest scenes over the clouds, with reduced accuracy over dark regions (e.g., clear-sky scenes over oceans). When needed, the accuracy can be increased by combining scenes. That is, if 4 scenes are combined, the accuracy is doubled at the expense of halving the spatial resolution to 16 km. The radiometric calibration will consist of determining the sensitivity of each of the 2048 x 2048 CCD elements using NIST calibrated lamps to illuminate a spherical cavity producing a uniform light field over the entrance aperture of the telescope. Once the radiometric calibration is accomplished, it is necessary to determine any change in end to end sensitivity while EPIC is in flight. For this purpose, views of the Moon will be used to calibrate EPIC.
