Bright Limit for Commanded Fidlight Magnitudes

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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Last modified:12/27/13