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Photometry News (1Jun93)
(from CTIO, NOAO Newsletter No. 34, 1 June 1993)

Unfortunately, it's all bad. Our best phototube appears to have died after a
long and productive life.  The Hamamatsu R943-02 photomultiplier in cold box
71 is undergoing major surgery to see if it can be revived. The tube has been
in use for at least ten years and for the last five has been our most popular
one. It has always been very stable and sensitive.  However, problems began
appearing in February. It showed sudden increases in count rate as well as
high dark counts. After finding a faulty preamplifier, we believed the problem
to be solved. However, subsequent lab tests showed strong hysteresis effects,
i.e. continually increasing count rates upon exposure to a constant light
source (see Newsletter No. 10). A night of engineering on the 1-m in March
confirmed this. At a rate of about 100,000 counts per second (cps), the counts
increased by ~1% in 3 minutes.  Subsequently, the rate of increase diminished,
with a total increase of 1.6% in 10 minutes. This effect decreased as the
intensity of the light source decreased, until no detectable hysteresis was
found for a rate of about 30,000 cps. These tests were performed with a Beta
light source during the day.  At night, data for a large number of standard
stars indicated that the tube had lost about 0.7 magnitudes of sensitivity
compared to all previous observations.  Also, the magnitude zeropoint varied
by several tenths of a magnitude over the night. Interestingly, reductions
showed that accurate COLORS could still be obtained, at least for faint stars.
However, the symptoms were severe enough that the tube has been removed from
further use while it is being overhauled in an effort to restore it.

On the succeeding night, our other Hamamatsu, in cold box 50, was installed
and tested in a similar fashion. Again, evidence for hysteresis was detected,
albeit at a smaller level. Again using the Beta light, at a rate of 85,000 cps,
counts rose by 1% in 3 minutes and by 2.7% in 17 minutes. No increase was
found in 5 minutes at 60,000 cps. A bright star with 120,000 cps showed a 1%
increase in 5 minutes.  However, the reduced photometry of a large number
of standards showed that the magnitude zeropoint was very close to previous
values and rms errors were comfortably small in both magnitude and color.

We are currently contacting Hamamatsu to try and remedy this situation. In the
meantime, cold box 71 is out of commission, while cold box 50 can be used
with caution. We will also install a new Hamamatsu in another cold box.

Doug Geisler

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