Caveat: Chandra Aimpoint Shift
Posted: 31 August 2011
Abstract
There is a shift in the Chandra aimpoint of about 8 arcsec, caused by a change in alignment of the aspect camera with respect to the HRMA and focal plane science instruments (ACIS & HRC detectors). All aimpoints are affected, particularly ACIS-S: the direction of the shift moves targets positioned at the ACIS-S default aimpoint closer to the node boundary, increasing the risk of apparent loss of events at that boundary.
There are no permanent science impacts from the aimpoint shift, but users with ACIS-S observations made from 11 July 2011 through 29 August 2011 may need to take special care in the data analysis, as outlined in this document.
All observations taken as of 29 August 2011 will include a Y-offset of +0.15 arcmin to return the aimpoint to its previous location. This updates the default offsets for ACIS-S observations to:
- Y-offset = +0.15'
- Z-offset = -0.25'
The following special processing concerns apply to ACIS-S observations made from 11 July 2011 through 29 August 2011; refer to the list of affected ObsIDs. The V&V report for affected observations has also been updated to include a note about the aimpoint shift.
The observations taken in this period were done at a different point on the detector then expected. While this may have an impact on the science analysis intended by the observer, the data products produced by standard data processing are to the same accuracy as any other observation.
Spatial Analysis
There is no impact on spatial analysis. The coordinates in the aspect solution and event files are correct.
Spectral Analysis
Spectral analyses should be unaffected, except for a slightly lowered effective exposure time if the source was on the node boundary.
Timing Analysis
The events lost into the node boundary may introduce a false dip or period into the resulting lightcurve. Use the dither_region tool to correct a lightcurve for variable exposure induced by dithering across bad pixels, bad columns, and going off-chip.
The Search for Variability in a Source thread shows an example of using dither_region and the effect with and without using it.
The file created by dither_region is used in the dmextract exp parameter, renaming the "fracarea" column to "dtf" on-the-fly:
dmextract infile="evt2.fits[sky=region(src.reg)][bin time=::2000]" \
outfile=lc_corr.fits opt=ltc1
exp="fracarea.fits[cols time,dtf=fracarea]"
Note that this is a first-order correction (just the geometric area of the aperture). A more complete method would be to simulate a PSF with the correct spectral weights for the source and run dither_region with the psffile parameter set. Then run dmextract, using the PSF fraction column to apply the correction instead of the fracarea.
Checking for False Variability
If you are concerned that source variability may be due to the source crossing the node boundary, it is possible to reprocess the data and include all the events that fall on the node boundary.
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Create a new badpixel file that excludes the node boundary, meaning that those events will be included in the event file.
Run acis_build_badpix as shown in Option B: Events on the chip node boundaries of the Customizing an ACIS Bad Pixel File thread.
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Reprocess the level=1 event file with new bad pixel file and create a new level=2 event file by running the chandra_repro script or by following the Reprocessing Data to Create a New Level=2 Event File thread.
Note that the event file with the node events included should not be used for general analysis since those events are not calibrated.
ACIS-S Observations taken from 11 July 2011 through 29 August 2011
obsid object date_obs ----- -------------------------------------- -------- 12740 3C_402 2011-07-12T00:41:40 12343 HD_189733 2011-07-12T03:27:24 12959 NGC_2663 2011-07-12T14:33:53 12990 2XMM_J120405.8+201345 2011-07-13T07:34:41 12689 G1.9+0.3 2011-07-14T15:33:44 12344 HD_189733 2011-07-16T13:47:05 12547 J1718-3825 2011-07-17T07:44:25 13407 G1.9+0.3 2011-07-18T04:52:38 12345 HD_189733 2011-07-18T19:10:04 12979 NGC_0337 2011-07-19T23:07:02 13235 SGR_0418+5729 2011-07-20T02:27:18 12341 HD_189733 2011-07-21T00:24:22 12342 HD_189733 2011-07-23T06:19:56 12938 1RXS_J165739.1-294946 2011-07-23T12:33:04 12921 GRB_110709B 2011-07-23T14:16:10 12468 XTE_J1710-281 2011-07-23T18:59:28 12947 1RXS_J180408.9-342058 2011-07-24T16:15:09 13306 3C445 2011-07-25T07:31:09 12612 SWIFT_J1822.3-1606 2011-07-27T04:25:16 13307 3C445 2011-07-27T20:56:17 12537 PSRJ2017+06 2011-07-30T11:24:34 12891 Abell_1775 2011-07-31T19:19:18 12405 IGR_J17091-3624 2011-08-01T06:59:16 13305 3C445 2011-08-02T03:39:52 12613 SWIFT_J1822.3-1606 2011-08-04T02:48:03 12469 XTE_J1710-281 2011-08-07T10:04:39 12975 NGC_4649 2011-08-08T07:31:12 12338 Gl_124 2011-08-09T08:02:48 14194 3C445 2011-08-09T10:04:57 12541 PSR_J0821-4300 2011-08-11T15:48:34 12867 ESO_490-IG026 2011-08-13T10:47:14 12395 HD_120066 2011-08-14T12:20:54 13296 NGC3921 2011-08-14T19:25:55 13208 Crab 2011-08-15T08:53:02 12467 J2256-10 2011-08-15T10:42:49 12442 MAXIJ1659-152 2011-08-15T19:59:16 13440 W49B 2011-08-18T07:42:36 12331 Oph1622-2405 2011-08-20T16:17:24 13441 W49B 2011-08-21T04:05:57 12669 SN2011ei 2011-08-21T21:25:05 12702 GRB_110731A 2011-08-22T01:16:30 14329 Swift_J1834.9-0846 2011-08-22T15:29:12 13254 Markarian_3 2011-08-26T02:56:58 12803 SDSS_J040148.98-054056.5 2011-08-27T08:38:55 14341 PTF11kly 2011-08-27T10:37:32 12795 Q_0000-263 2011-08-29T08:12:50 12495 1RXS_J042704.5-594854 2011-08-29T11:29:25 12453 NGC_6388 2011-08-29T12:53:08 14337 IGR_J17498-2921 2011-08-29T13:59:59