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The Photomultiplier Zoo: Single-Channel Photometry at CTIO (1Sep95)
(from CTIO, NOAO Newsletter No. 43, September 1995)

Single channel photometry continues to be popular on the small
telescopes at CTIO, both for all-sky photometry and for high-speed
monitoring of variable stars. There were also runs scheduled on the
1.5-m and 4-m telescopes this semester. However, many of the
photomultiplier tubes are aging, and it has been some time since their
performances were measured. We therefore arranged to test our best
tubes (as selected from a lab testing of all our tubes by Ricardo
Gonzalez a year ago) on the telescope, under real observing
conditions. These data will not only allow observers to decide which
tube is best for their project, it will also give them a useful means
of estimating integration times when preparing proposals.

The following relative count rates were measured during two
photometric nights in May 1995 on the Lowell 0.6-m telescope. The
Landolt #3 UBVRI filter set was used for all tubes. 4-5 Graham
E-region standards were measured using each tube. The observed counts
were corrected to an airmass of 1.0 using the standard CTIO extinction
coefficients and were normalized to a standard magnitude of
U=B=V=R=I=10.0 mag. Count rates can be scaled to the 1-m, 1.5-m, and
4-m telescopes by scaling by the ratio of the telescopes' effective
mirror areas (2.34, 5.83, and 40.8, respectively). The count rates are
expressed in kilocounts per second. The response functions for the
most efficient tubes are shown in the accompanying figure.

                           [Figure not included]


     Count Rates at the Lowell 0.6-m for Stars of U=B=V=R=I=10.0 mag

                                 kcounts/sec         ct/sec
 Coldbox   Tube          U      B     V     R     I    Dark  Comment

  48   Varian cell      5.52  5.90  9.84  13.90 14.79   6.2  best throughput
  50   Hamamatsu        4.69  6.38  7.52  12.42  6.28   2.5  UBVRI
  53K  RCA 31034(KPNO)  4.41  4.36  7.31  10.33  4.34   0.8  UBVRI
  58   RCA 31034        3.07  3.16  5.74   8.63  3.86   9.4  UBVRI
  63   S-5 EMI 9781A   10.96 11.07  7.98     -     -    1.7  UBV
  61   S-5 EMI 9781A    7.18  6.37  3.18   0.28  0.00   1.1  UBV
  55   S-20             4.52  4.94  5.41   4.30  0.34   4.1  UBVR
  57   S-20             3.12  3.68  3.82   2.75  0.00   1.2  UBV
  64   S-4              2.99  2.44  1.67   0.40  0.00   1.2  UBV
  69   S-11             1.74  2.84  3.13   0.88  0.00   3.3  UBV

                                 Notes

(1)  The Varian cell traditionally has been used only in the red,
since it is relatively fragile. Use of this tube at bluer wavelengths
is subject to staff approval. Transformations to the standard UBV
system have not been tested.

(2)  In NOAO Newsletter No.34, page 10, the Hamamatsu tube in coldbox
50 was reported to show hysteresis effects at count rates above 60
kc/s. In May 1995, this tube was retested as follows. Low airmass
stars were observed continuously for periods of 10 minutes at the
count rates of 59, 66, 75, 110, 160, and 197 kc/s (the maximum count
rate is 200 kc/s). At the highest count rates, a slight count rate
increase of 0.4% was detected marginally over 10 minutes. Thus, the
tube appears to be sufficiently stable for accurate photometry, even
at high count rates, at least under the conditions of the present
test. Observers are encouraged to perform their own count rate tests
to assure themselves on this point.

(3)  A new RCA tube was donated graciously from Kitt Peak, after the
Mark III photometer was decommissioned (many thanks to Caty
Pilachowski, Jim DeVeny, Bill Binkert, and Clark Enterline for their
help in transferring this tube to CTIO, where it will remain on long
term loan). We designate this tube's coldbox as 53K (note that CTIO
already has a coldbox 53). Our tests show that this tube is
intermediate in sensitivity between our Hamamatsu (coldbox 50) and our
existing RCA (coldbox 58); it thus represents an important addition to
our stable of photomultiplier tubes. An abbreviated hysteresis test
suggests a decline in count rates by about 0.4% over 10 minutes of
continuous observing at count rates of 76 and 189 kc/s. Further tests
are planned.

(4)  Coldbox 63 (S-5 type) was not tested in this run. Instead, count
rates from a science run at the 1-m were scaled to the 0.6-m using the
effective area ratios mentioned above. R and I count rates were not
measured, but are expected to be low, like those of the S-5 tube in
coldbox 61.

Andy Layden, Oscar Saa, Edgardo Cosgrove

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