Proper estimation of exposure times in astronomical spectroscopy is often difficult, largely because the quality of the stellar image is so dependent upon “seeing.” The integrating photometer described here, while not new in principle, functions with satisfactory sensitivity and stability on only two percent of the light transmitted by the slit of the coud6 spectrograph of the 100-inch telescope. When the meter indicates a predetermined number of counts, the finished spectrogram will have the proper density regardless of seeing, slit width, passing clouds, and efficiency of guiding.
HORACE W. BABCOCK, "An Integrating Photometer for Low Light Levels," J. Opt. Soc. Am. 40, 409-411 (1950)