Speed gains obtained with a cascaded image converter.
M. A. Tuve, Jr. Ford W. K., J. S. Hall, W. A. Baum · The Astronomical Journal · 1959
A 30-fold gain in speed over conventional direct photography has been 9btained with an R.C.A. two-stage cascaded image converter attached to the Cassegrain focus of the Naval Observatory 40-inch reflector at Flagstaff. Under the operating conditions adopted, the fog level due to dark emission inside the converter was~ much less than that due to the natural night- sky background at an optical focal ratio 0 f/6.8. If the sky background had been absent, the dark emission alone would have limited the~ exposure time to about 20 minutes, which would have been equivalent to a I o-hour exposure with conventional photography. Observations of close double stars having separations as small as 0'.'7 have been attempted with the converter attached to the Lowell 24- inch refractor. A I6-mm motion picture camera was mounted behind the converter in order to record very short exposures in rapid succession. It was thereby possible to catch occasional images which were less blurred by "seeing" than they would have been under the same conditions. with the longer exposures required by conven- tional photography. The effective exposure times with the converter and motion picture camera were 1/20th second or less. Since the dark emission of the converter was unimportant for such short exposures, the operating potential could be increased enough to yield speed gains of about 100-fold. The cascaded converter is an extremely simple device to use. It requires no specially prepared plates, no home-made photocathodes, no external vacuum system, and no liquid-air traps. It requires only a voltage supply and a fast camera lens. This work is being carried out under the Carnegie Institution with the support of the National Science Foundation. Carnegie Institution of Washington Washington, D. C., Lowell Observatory Flagstaff, Aris. and Mt. Wilson and Palomar Observatories Pasadena, California