internal reflections remain visible at or below the 1% level. Of particular note are regions of low sensitivity observed in the flatfield of the WAC VIO filter. These are likely due to imperfections in the Lumogen coating added to the CCD to boost UV sensitivity. This pattern of low sensitivity is not seen in any other filter combination. As with many of the ground calibration measurements, the light transfer measurements were taken at three different optics temperatures, +5, -10, and +25 degrees Celsius, representing the nominal value, lower operating threshold, and upper operating threshold of the instrument respectively. These measurements showed significant variation in flatfields taken at different temperatures, the cause for which remains unknown. Since the vast majority of ISS data is taken near the nominal operating temperature, the +5 degree flatfields will be used for calibration in almost all cases. The in_flight.pdf document included in the Calibration volume document/ directory details the strategy for in-flight measurements of the flatfield. In theory, are obtainable during close flybys of relatively featureless targets like Titan; however, in practice, the large number of images required to average over low-frequency spatial variations, combined with the large number of filter combinations required 81 in all make this difficult to achieve. Limited in-flight analysis has been performed to – – identify flatfield changes since launch, and are documented in West et. al. (2010). One dust ring seen in flight images in all NAC filter combinations has been characterized using data from Venus flyby. Additionally, a correction for the NAC has been derived from Titan images, and accounts “mottling” for low-level spatial variations seen after SCLK=1444733393. Both of these corrections are performed by CISSCAL using data files contained in the calib/dustring/ subdirectory. Shutter ISS exposure times are controlled by the shutter mechanism as described in Section 2.3 of this User's Guide. The actual exposure time differs from the commanded time by a constant offset as well
internal reflections remain visible at or below the 1% level. Of particular note are regions of low sensitivity observed in the flatfield of the WAC VIO filter. These are likely due to imperfections in the Lumogen coating added to the CCD to boost UV sensitivity. This pattern of low sensitivity is not seen in any other filter combination. As with many of the ground calibration measurements, the light transfer measurements were taken at three different optics temperatures, +5, -10, and +25 degrees Celsius, representing the nominal value, lower operating threshold, and upper operating threshold of the instrument respectively. These measurements showed significant variation in flatfields taken at different temperatures, the cause for which remains unknown. Since the vast majority of ISS data is taken near the nominal operating temperature, the +5 degree flatfields will be used for calibration in almost all cases. The in_flight.pdf document included in the Calibration volume document/ directory details the strategy for in-flight measurements of the flatfield. In theory, are obtainable during close flybys of relatively featureless targets like Titan; however, in practice, the large number of images required to average over low-frequency spatial variations, combined with the large number of filter combinations required – 81 in all – make this difficult to achieve. Limited in-flight analysis has been performed to identify flatfield changes since launch, and are documented in West et. al. (2010). One dust ring seen in flight images in all NAC filter combinations has been characterized using data from Venus flyby. Additionally, a “mottling” correction for the NAC has been derived from Titan images, and accounts for low-level spatial variations seen after SCLK=1444733393. Both of these corrections are performed by CISSCAL using data files contained in the calib/dustring/ subdirectory. Shutter ISS exposure times are controlled by the shutter mechanism as described in Section 2.3 of this User's Guide. The actual exposure time differs from the commanded time by a constant offset as well