Blur from Residual Errors in HRC Event Position Reconstruction
Introduction
The HRC event positions are not telemetered to the ground but must be
reconstructed from data generated by the HRC crossed-grid charge
detector (CGCD) (see
memo HRC Position Logic).
In reconstructing the event positions corrections for
ringing in
amplifiers and
degapping are
applied. The detector positions that result are then combined with
information on the position and motion of the detector behind the
HRMA and with the telescope aspect solution to determine a
celestial location for each event. The quality of the resulting
images is determined by the combination of "blur" from several
sources: the PSF of the HRMA, the aspect solution, the intrinsic
resolution of the HRC detector, and the HRC event position
reconstruction. In this memo I will address the last of these
contributors.
Figure 1 demonstrates the result of event position reconstruction
errors. Using the most recent processing of data from ObsID 1385
(a ~19ks observation of AR Lac), I have selected events from
hrcf01385_000N004_evt1.fits within a 20 pixel radius centered on
the source and plotted the "Sky" X and Y positions versus
time. The wobble that is apparent in the mean position as a
function of time is indicative of an error in either the aspect
reconstruction or the HRC event position reconstruction. Since
this sort of wobble is not observed in ACIS observations, it is
logical to attribute it to dithering over the residual errors in
the determination of HRC event positions.
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Figure 1: Events within 20 pixels of AR Lac: X and Y plotted
versus time.
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HRC Position Errors
Residual errors in the HRC positions are the likely cause of the
apparent wobble observed in Figure 1. We can use the spacecraft
dither as a probe of the position errors as a function of detector
position by using the aspect solution to model the expected
location of the source on the HRC detector. Starting from the
aspect solution file, pcadf055469783N003_asol1.fits, and the
nominal pointing direction, I modeled the expected time history of
the location of AR Lac on the HRC detector (U and V axes)
including the motion of the detector behind the HRMA. Then for
each event I calculated the model position by linear interpolation
of location time history. Plotted in figure 2 is the deviation of
the corrected (de-rung and degapped) event position from the
modeled event position as a function of modeled position for the
HRC U-axis; figure 3 is similar but for the V-axis. There is a
natural width to the deviations which is due to the HRMA, aspect
reconstruction, and intrinsic HRC detector blur but the systematic
wave as a function of modeled position is indicative of errors in
the HRC event position reconstruction. The size of this systematic
error is shown in the over-plotted yellow curve which is the mean
of the observed deviation as a function of modeled position.
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Figure 2: Observed deviations from modeled event position on
the HRC U-axis. The yellow curve is the average
deviation at each modeled position.
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Figure 3: Observed deviations from modeled event position on
the HRC V-axis. The yellow curve is the average
deviation at each modeled position.
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Figure 4 shows a plot of the histograms of the mean deviations. The
additional blur caused by residual errors HRC event position
reconstruction could be as large as 3-4 pixels or
0.4-0.5 arcsec. The actual additional blur that is observed will
depend on how these distributions are sampled.
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Figure 4: Histograms of the deviations from modeled event
positions for the U-axis (left) and the V-axis
(right).
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Conclusions
The fact that there is a systematic trend in the deviations from
modeled position as a function of modeled positions demonstrates
that HRC event position reconstruction is a significant
contributor to the overall PSF. It may be possible to reduce its
contribution through improvements to the ringing or degapping
corrections but this will require further study.
Back to my memos page
Michael Juda
email Mike Juda
Last modified: Wed Nov 21 09:21:55 EST 2001