The brightening of fiducial lights from magnitude 7.0 to
magnitude 6.5 or brighter has been suggested as a means of
mitigating the effects of the ACIS-I fid #6 anomaly (see Tom's
analysis report here). Given the bright limit of the OBC at
5.797 magnitudes and a pixel saturation rate of 32767
counts/readout, it is desirable to define a practical upper
limit for driving the fidlights.
OBC magnitudes and per pixel counts were obtained from level 0
products for all science observations from 2004. This gives 700
individual fidlight data sets. Statistics were compiled over
single observations for OBC magnitudes. Statistics for pixel counts
were calculated over the observation duration and over only the
central four pixels of the 8x8 array. Data were not grouped
according to science instrument or observation duration.
The plot below shows the distribution of the median, 5th and
95th percentile number of counts per readout for individual
observations. Most importantly, we see that the 95th percentile
does not exceed 10500 counts per readout. The 95th percentile
distribution is centered at 7000 c/r.
If the width of the distribution is independent of magnitude, then we can extrapolate a safe upper limit for driving the fidlights without saturation. A 7th magnitude fid provides a maximum 95th percentile of 10500 c/r, so then a magnitude of 5.76 would be the faintest fid to produce a significant number of saturated readouts. The next plot shows the statistics for the OBC magnitude over an observation. The median OBC magnitude is never more than a tenth of a magnitude brighter than the commanded magnitude. The distribution of median magnitude is centered at 7.15.
This suggests that the fidlights can safely be commanded up to
magnitude 5.9 with no operational impact, though any brightening
of the fid should be done gradually as an extra margin of
safety. Also, a stepped brightening will give us a measure of
the effectiveness of brightening.
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