Chandra absolute astrometric accuracy

Summary

The absolute positional accuracy of source coordinates in Chandra observations is estimated measuring the angular offset between Chandra X-ray source positions and corresponding optical/radio counterpart positions from accurate catalogs .

The overall 90% uncertainty circle of Chandra, based on observations between 2018-03-21 and 2023-03-21 X-ray absolute position has a radius of 1.10 arcsec. The 99% limit on positional accuracy is 1.57 arcsec. The worst case offset is 2.3 arcsec. Performance varies slightly between detectors.

These values apply to sources within 3 arcmin of the aimpoint and with the SIM-Z at the nominal detector value.

Unless otherwise stated, the figures in this page are based on observations between 2018-03-21 and 2023-03-21.

Distribution of aspect offsets for each detector

The four plots below show the cumulative distribution of aspect offsets for observations in the last 5 years. The aspect offset is the distance from X-ray source centroid to the accurately known position of the optical or radio counterpart.

Time history of aspect offsets

The time-history of offsets for recent Chandra observations is shown below.

Offsets After Reprocessing

The previous section shows the distribution of angular offsets of observations currently in the archive. As the alignment is calibrated based on historical data, observations that have been processed already are not updated. The absolute pointing in these observations can improve if the observations are reprocessed with the latest calibration.

This section shows the distribution of angular offsets that would result from reprocessing all observations with the latest calibration.

Distribution of aspect offsets for each detector

The four plots below show the cumulative distribution of aspect offsets for observations in the last 5 years. The aspect offset is the distance from X-ray source centroid to the accurately known position of the optical or radio counterpart.

Time history of aspect offsets

The time-history of offsets for recent Chandra observations is shown below.

Detailed description

The above plots show the offsets between Chandra X-ray source coordinates and the corresponding optical or radio coordinates. The offsets are derived using a dataset comprised of observations from the last 5 years, selected for quality according to the criteria described in this section.

Only observations with the following characteristics are considered:

• Single-OBI observation
• SIM-Z at the nominal detector value.
• obs_mode==POINTING
• If it is an ACIS observation:  readmode==TIMED and  dtycycle==0.
• Not an observation of targets known to confound the source detection.
• Not in any of the following science categories: "SN, SNR, and isolated NS", "Clusters of Galaxies", or "Solar System and Misc". Observations in these categories are likely to confound celldetect and produce too many spurious sources.

During the source detection process, the detected sources fulfill the following requirements:

•  SNR > 5
• For grating observations, sources must be within 0.4 arcmin from the optical axis.
• For imaging observations, sources must be within 3 arcmin from the optical axis.

An X-ray source matches an optical/radio source if the offset is less than 3 arcsec. If a source is matched in more than one catalog, the most accurate catalog is used, in the following order of preference: ICRF2, Tycho2.

How are offsets defined?

The offsets $$(\Delta Y, \Delta Z)$$ are specified in the Chandra body coordinate system, since they reflect drifts in hardware alignments. The origin of this coordinate system is chosen to be the nominal pointing of the observatory for the observation $$(RA_{nom}, DEC_{nom}, ROLL_{nom})$$. At a spacecraft roll angle of 0.0 degrees, $$\Delta Y$$ corresponds to an offset along Right Ascension, and $$\Delta Z$$ corresponds to an offset along Declination. The exact equation is: $\begin{pmatrix} \Delta Y \\ \Delta Z \end{pmatrix} = \begin{pmatrix} \cos(ROLL_{nom}) & -\sin(ROLL_{nom}) \\ \sin(ROLL_{nom}) & \cos(ROLL_{nom}) \end{pmatrix} \begin{pmatrix} \cos(\text{DEC}_{nom}) \Delta RA \\ \Delta \text{DEC} \end{pmatrix}$

Off-axis sources or observations with non-zero SIM offset

It is important to note that observations on ACIS or HRC-S at large off-nominal SIM-Z can suffer additional residual aspect offsets of up to 0.5 arcsec. This is due primarily to distortion of fid light images by the retroreflector collimator in the fiducial transfer system. HRC-I observations at large off-nominal SIM-Z can have residual aspect offsets of up to 3 arcsec due to the fid light distortion as well as a possible rotation of the detector relative to the geometry assumed in processing.

Notes

• For old archival data it should be noted that ACIS observations taken in 1999, and particularly before ~November 1999 could suffer larger offsets. Shortly after launch the alignment drifted rapidly due to outgassing of the ACIS mount tabs, and the calibration does not account for this effect.

Archival notes and announcements

NOTE (2007-May): Chandra observations taken after 2006-Dec-10 and processed on or before 2007-May-14 are subject to a systematic aspect offset of approximately 0.4 arcsec. This is believed to be the result of a changed thermal environment following the procedure to cool the ACA CCD from -15C to -19C that took place between late Nov-2006 and early Jan-2007. New calibration files to correct this offset were installed effective 2007-May-14 and affected observations were reprocessed to remove the offset. Observers analyzing these observations should ensure that the processing date is after 2007-May-14.
NEW ALIGNMENTS RELEASED: 2002-May-02

New alignment files were installed in the CXC processing pipeline on 2002-May-02. These alignments eliminate all currently known systematic aspect offsets. Use the