Last modified: 7 November 2022


Caveat: Chandra Aimpoint Shift

Posted: 31 August 2011


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:

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.

  1. 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.

  2. 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