Long-Wavelength Infrared Camera Calibration

The LWIR camera on Clementine imaged the lunar surface in thermal infrared emission using a single passband centered at about 8.75 microns. The camera uses a catadioptric lens with a 128 x 128 HgCdTe focal plane array. The FPS was operated at a nominal temperature of 65 K. Wavelength range was controlled by a cold filter, and extended from 8.0 to 9.5 microns.

The field of view of the LWIR was smaller than that of the UV/VIS camera, so that the standard global mapping sequences obtained, essentially, a series of "stripes" that covered about a sixth of the surface area of the Moon in equatorial regions and a larger fraction toward the poles. Continuous coverage was obtained at latitudes poleward of about +/- 80 degrees. Images obtained from the camera with preliminary correction for nonuniformity show that the spatial resolution of the instrument is higher than the UV/VIS, and that at moderate to high latitudes the images show very high correlation with visual images due to the high correlation between temperature and insolation. Thus, the camera serves as a moderate resolution imaging system with greater coverage than the HIRES and higher resolution than the UV/VIS. LWIR images likely will be useful for morphologic studies. In addition, a series of special observations were done to define the "emission phase function" of the surface. These observations were done during the periods prior to and following global mapping; they covered about a dozen different sites, including highlands, mare, Apollo landing sites, and the sunlit parts of polar craters that might contain ice.

The sensitivity of the camera was such that only dayside observations are useful. The noise-equivalent delta-temperature was about 12 K at a brightness temperature of 200 K, 2 K at 300 K, and 1 K at 400 K, for example. Minimal application of calibration data obtained pre-flight and during mission operations has been done to date. In-flight observations to aid in the calibration include observations of space prior to and following each imaging pass on the dayside, observations of space prior to and/or following the emission phase function observations, and observations of sites on the Moon where in-situ temperature measurements were made; these have not been incorporated into data reduction to date. Sequences of images of space were also obtained on two different orbits to help define the space background as a function of focal-plane temperature. A substantial number of pixels in each image are not functional (either dead or saturated). These occur in a consistent pattern that can be readily removed with a pixel mask. Some apparently bad pixels may be correctable once proper gains and offsets are applied. The flat-field response of the LWIR has been examined, although a standard flat field has not been constructed; the response is nearly constant across the field, although some variations are seen, and it does not appear to depend strongly on the focal-plane temperature. The present level of calibration is sufficient for morphology and some types of quantitative analysis of surface roughness. Much additional work is required to produce radiance data calibrated at the 10% level.