Terminator Photography in Oblique Illumination for Lunar Topographic Work

Abstract

Advantages to lunar topography of ground-based photographs taken under oblique illumination — especially in the penumbral zone of the lunar surface illuminated by the only partly risen Sun — are discussed. It is pointed out that such photographs require, in general, exposure times from 10 to 100 times longer than would be conventionally regarded as “normal”; and advantages of such “over-exposed” ciné photography in minimising by rapid projection the film-grain noise are stressed.

Illustrative examples of such photography underline the advantages of this method. In particular, a comparison of two photographs of the crater Ptolemy — one taken with the 43-inch reflector of the Observatoire du Pic-du-Midi, the other with the FB camera of Ranger 9 from an altitude of 1406 kms above the lunar surface — discloses that while the linear resolution of the Ranger camera at this altitude exceeds that of the 43-inch telescope more than ten times, the vertical resolution of the ground-based but oblique-illumination photograph exceeds that secured from the Ranger to an almost equal extent.

It is pointed out that photographs taken from terrestrial observatories of the lunar surface illuminated by the partly risen Sun can furnish vertical relief data that are generally unobtainable from spacecraft unless the latter, while overflying the terminator, can secure sufficiently short exposures at the light level prevailing in the penumbral zone.

Work sponsored by the Aeronautical Chart and Information Center under Contract AF 61 (052)-829 with the Department of Astronomy, University of Manchester, through the European Office of Aerospace Research, U.S. Air Force.