Cascade Image Intensifiers for Astronomical Spectroscopy.

A cascaded image intensifier suitable for use in astronomical spectroscopy has been developed at the Electron Tube Division of RCA for the Carnegie Image Tube Committee. This tube gives a substantial gain in speed over ordinary photographic techniques, it can be used for long exposures, and it is simple and reliable in operation compared with other intensifying devices. We now have two samples of these experimental tubes with both good cathodes and low background. The resolution at the phosphor screen, measured visually, is 40 line pairs per mm, and is uniform over the 38-mm diameter field. With our present relay or transfer lens the resolution is limited to 25 to 30 line pairs per mm on IIa-O plates. The gain in exposure time over baked I Ia-O plates is 30 and 40 in the sense that with the image intensifying system plates are obtained in 10 and ~2' min, respectively that are equal in density to 300-min exposures directly on the IIa-0 plates. The gain in the rate of recording information is determined by the efficiency of the photocathode, by the resolution of the system, by the statistical processes involved in the electron multiplication, and by the granularity of the recorded image. An increase in the electron multiplication would increase the rate of blackening but not the information rate. Spectrograms that demonstrate a useful gain of 10 over direct plates have been obtained with these tubes on the DTM spectrograph ~t Lowell Observatory. Dispersions of 90 and 22 A/mm are available at the photocathode with 3 and 12-in. focal length cameras. The development of these tubes was supported by grants from the National Science Foundation.