Lunar libration region L4 photometry
Photometry of the lunar libration region L4 obtained in double umbra during the lunar orbital flight of Apollo 15 is reported. A four-minute photographic exposure of the region centered at celestial coordinates 23 h 14 m, -1.8 deg was made using a 35-mm camera at f/1.2 in the command module pilot's window. The film had been calibrated to the sun's surface brightness, and scene brightness information was converted to digital form, followed by high-speed digital processing with vignetting removal to obtain absolute photometric values. Results reveal an elliptical brightness region 16 x 12 deg in extent with an absolute brightness of 3.3 x 10 to the -14th sun surface brightness units. The mass of the scattering material responsible for the effect is estimated to be between 6 x 10 to the 18th and 2 x 10 to the 7th g.