FGS Instrument Report No. 25
1 November 1993
Correcting FGS Counts for Deadtime in the PMTs
M. G. Lattanzi and L. G. Ta
1 Summary
In our calibration of the FGS photometric system, we are using stars in
the magnitude range V = [8.3,13.8]. As shown in this report, magnitudes
brighter than V = 9.5 mag (taken through the Clear Filter; F583W) contribute an error of more than 0.05 mag if proper account for deadtime in the
detector photomultipliers (PMTs) is not allowed for.
The procedure to remove the eect of deadtime is given in x 2. Since the
deadtime constant is based on pre-launch tests (no in-ight calibrations have
been executed nor are any planned for the foreseeable future), the eect of
uncertainties in the deadtime constant provided have to be addressed; see
x 3.
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2 Deadtime Correction
The formula to correct from measured counts CM per unit of integration
time to "true" counts CT is (Astrometry Handbook 1986)
CT =
1
(1)
CM [1 , CM (TD=TI )] ;
where TD is the deadtime constant (285 nanoseconds; Reed 1993) and TI is
the integration unit time (0.025 sec for the FGS). Eq. (1) is expressed on a
magnitude scale simply by taking the common logarithm of the ratio CT =CM .
Fig. 1(a) shows the amount of correction (in magnitudes) owing to the
PMT deadtime which needs to be applied to the instrumental magnitudes
(as derived from the measured counts) as a function of visual (Johnson)
magnitude. The absolute throughput (counts) is that for the F583W (Clear)
lter. For example, for our standard single star Upgren 69 (V = 9.6), the
deadtime correction amounts to 0.09 mag.
3 Accuracy of the Correction
The deadtime constant given in the previous section is based on prelaunch measurements. There has been no in-ight calibration of the deadtime
constant either during Orbital Verication or Science Verication. In Eq. (1),
the measured counts are known to the limit of the photon noise, while TI is a
given number. Thus, an error in the adopted value for the deadtime constant
will result in an error in the predicted correction derived from Eq. (1). This
error is (in dex)
(2)
E (m) = [2:5 Log e 10(0:4m) CM (TD =TI )]E (TD)=TD ;
2
where E (m) is the error of the correction m as derived from Eq. (1) (after
conversion to a magnitude scale) and E (TD ) is the error of the deadtime
constant. In Fig. 1(b) is shown a plot of Eq. (2) as a function of magnitude
for an assumed error in the deadtime constant of 25% (worst case).
From the two gures we conclude that the deadtime correction is important enough to be routinely removed from the data. The contribution from
the assumed uncertainty in the deadtime constant is several times less than
the sought for correction [the actual errors being probably smaller than what
is depicted in Fig. 1(b)].
4 References
L. Abramowich-Reed 1993, private communication.
L. Abramowich-Reed and C. Ftaclas, 16 Sep., 1993, Bright Object
Observations with the FGS, EM: MOSES 1024.
HST Astrometry Operations Handbook 1 Oct., 1986, SMO-1040 pp. 3-62.
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