Orbital-science investigation: Part O: regional variations in the magnitude of heiligenschein and causal connections
Approximately 35 reasonably good candidates for specialized photometric studies were found during a thorough examination of the frames exposed by the Apollo 15 metric camera. Of these, the majority was of value in heiligenschein studies (refs. 25-36 to 25-38). A few were of value for limited-interval delineation of the photometric functions of crater walls, wherein it is now known from past Apollo Program studies that younger craters have walls much more Lambertian in reflective properties than those of the standard lunar surface (ref. 25-39). It has now become apparent that some difficulty exists in such crater-wall studies with regard to varying the domain of mathematical definition over which such photometric functions are delineated to encompass much more of phase-angle-brightness-longitude space than has already been carried out Nearer to zero-phase angle, the brightness of the illuminated wall becomes so intense compared with the general luminance of the surrounding lunar terrain that it approaches the saturated region of the D /log E curve of the photograph where D is density and E is exposure. Conversely, for phase angles approaching 180°, most of the illuminated wall is seen so obliquely that the image size is small and reliable photometry is difficult. Also, difficulties inherent in the methodology exist—the descriptive geometry used to extract configurations begins to fail as craters become either too old or are imaged too near a vertical view Nevertheless, some useful candidates for crater-wall studies have been found in the Apollo 15 data, although their analysis is too lengthy for incorporation in this preliminary report.