The basic idea is to determine the position on the detector of bright point sources with well-known celestial coordinates. To concentrate purely on pointing performance, only observations from a single SI were selected. Given this constraint, ACIS-S (predominantly with the HETG) provides the largest dataset. For each observation, the mean CHIPX and CHIPY for source photons was corrected to remove any SIM-Z offset (from the nominal -190.14 mm) or pointing offset. After correction, the mean CHIPX and CHIPY for an observation with no offsets is:
CHIPX = 251.8
CHIPY = 509.5
The plot below shows the corrected offset (arcsec) from the mean for the 17
observations. Recent observations are plotted with an asterisk. The worst
case from this mean is about 2.0 arcsec in each axis, while the rest are
within a 2 arcsec radius of the mean.
The 17 points in the analysis can be statistically characterized following the prescription of the TRW requirements document SE31:
99% radial limit = sqrt( E(DY^2 + DZ^2)/2 ) * 3.035 where E(X) = expectation value of XThe 99% limit for these 17 points is 3.3 arcsec. This can be compared to the requirement of 30 arcsec. Clearly the distribution (and potential large pointing errors) are not gaussian, and this 99% value must be interpreted with caution. A more useful measure is simply the worst radial deviation of 2.8 arcsec. Multiplying this by a safety margin of 3 gives a conservative upper limit of a 9 arcsec radius on the absolute pointing error. This should be used for planning observations which require precise pointing (e.g. an ACIS sub-array observation).
The following table gives the mean location in CHIPX and CHIPY of the source for each observation, the relevant observation parameters, and the corrected offset (DY and DZ) in arcsec. (Y and Z refer to spacecraft coordinates).
Obsid RA Dec Date Grating CHIPX CHIPY SIM_Z OFFSET_Y DY DZ
deg deg pixels pixels mm deg arcsec arcsec
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0106 287.9562 4.9827 1999-09-23 HETG 254.6 511.1 -190.14 0.0000 1.3 0.7
0016 82.1866 -65.4488 1999-10-09 HETG 253.1 511.6 -190.14 0.0000 0.6 1.0
0333 49.9504 41.5116 1999-10-10 HETG 255.8 513.7 -190.14 0.0000 1.9 2.0
0105 250.2312 -53.7513 1999-10-20 HETG 254.9 508.6 -190.14 0.0000 1.5 -0.4
1456 308.1062 40.9576 1999-10-20 HETG 254.1 506.5 -190.14 0.0000 1.1 -1.5
0337 329.7166 -30.2255 1999-10-20 HETG 253.4 511.4 -190.14 0.0000 0.7 0.9
0376 322.3004 -15.6450 1999-11-16 NONE 252.5 507.0 -190.14 0.0000 0.3 -1.2
1481 327.9812 -30.4649 1999-11-20 HETG 252.9 392.0 -187.32 0.0000 0.5 -0.0
0336 327.9812 -30.4649 1999-11-18 HETG 249.9 391.9 -187.32 0.0000 -0.9 -0.0
0457 166.1133 38.2088 1999-11-05 HETG 251.3 349.2 -186.24 0.0000 -0.2 1.0
0489 187.2778 2.0523 2000-01-10 HETG 250.9 383.1 -187.12 0.0000 -0.5 -0.3
0605 51.6474 28.7150 2000-01-12 HETG 248.9 385.3 -187.13 0.0000 -1.4 0.5
0837 214.4983 25.1367 2000-02-05 HETG 248.6 510.2 -190.14 0.0000 -1.6 0.3
0614 202.6950 24.2327 2000-03-27 HETG 290.4 380.8 -187.12 -0.0056 -1.2 -1.4
0640 120.8960 -40.0031 2000-03-28 HETG 290.1 384.1 -187.12 -0.0055 -1.0 0.1
0604 47.0422 40.9556 2000-04-01 HETG 249.1 381.7 -187.12 0.0000 -1.3 -1.0
0610 85.1896 -1.9425 2000-04-08 HETG 332.7 381.6 -187.12 -0.0110 0.0 -1.0
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