| Description: The PDS3 name for this attribute was CSS:MIN_FOOTPRINT_LINE (with MAP_PROJECTION_TYPE = POINT_PERSPECTIVE), and was defined as "Minimum size of footprints along the line axis". The meaning of this is unclear, but we believe it means the (angular) length along the vertical direction of the smallest footprint (a CIRS detector) on the pixel grid, and have renamed it in PDS4 accordingly as pointperspective_min_footprint_height. The equirectangular projection only includes fields of view which fall on the target body and thus does not preserve spectra taken off the limb of the target (which are important at Saturn or Titan, for example), and also does not preserve the original viewing geometry of the observation. We therefore also generate point perspective projection cubes, which approximate the appearance of the target body in the plane of the sky as seen from the spacecraft, and preserve spectra of the limb regions and the surrounding regions of deep space. The line coordinate is the scaled offset from the body center, in the direction of increasing J2000 Declination, in milliradians. To generate these cubes, the field of view of each spectrum is first expressed as an offset, in milliradians, from the instantaneous direction of the target body center. The sample coordinate is the offset in the direction of decreasing J2000 Right Ascension, and the line coordinate is the offset in the direction of increasing J2000 Declination. To correct for the changing distance to the target during the activity, these offsets are then multiplied by the ratio of the distance to the target body center at the time of the observation to the distance to the target body center at the start of the observation, so that the apparent size of the body is constant with time. The resulting scaled offsets for each field of view are then projected onto the cube grid and averaged as described in Table 4.1 of the CIRS Cube SIS. Fields of view further than 3 times the angular size of the target are not included in the cube, in order to limit the cube size. We have not generated point perspective projections for the ring observations because few ring observations were taken in a way that would make point perspective images useful. CAVEAT USOR: Several examples of CIRS cubes do not appear to correctly identify their min/max footprint width/heights. For example, some using equirectangular projection list their max footprint width as 360 deg, even when there is no wrapping. |