Alternating current ampllfier for astronomical photometry.
Carl W. Gartlein · The Astronomical Journal · 1949
The National Geographic Society-Cornell University Study of Aurora has used photomultipliers with A.C. amplification methods regularly since 1941. By 1944, results were so promising that it seemed desirable to build the best equipment possible. The new amplifying set-up was completed early in 1945 and modified some since then. The incoming light is chopped at 330 cycles before entering the photomultiplier and the output current fed into a 330 cycle push pull amplifier (Goldberg circuit) having band width of 20 cycles and a maximum gain of one million. The output is rectified, filtered and put through a D.C. impedance matching amplifier into a Leeds and Northrup or Esterline-Angus recorder. Response is linear within a few per cent and constant over periods up to eight hours, even though this is all operated from A.C. It is believed this type of circuit offers advantages over other circuits for astronomical use except in those cases where linearity is the prime requisite or where the photomultiplier tubes will be refrigerated. It would appear that this method allows fainter stars to be measured without refrigeration of the cell. It also allows the use of cells so noisy or having leakage currents so high that they cannot be used on D.C. amplification. One of these amplifiers is now being used by Dr. E. F. Carpenter of the Steward Observatory. Cornell University, Ithaca, N. Y.