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High-precision astrometry with Chandra

Following are notes on improving the absolute astrometry of X-ray sources in Chandra observations.  This includes contribution from Eric Feigelson and the ACIS team.

Improving absolute astrometry

Improved celestial location precision is possible for some observations by cross-correlating detected X-ray sources (from celldetect or other detection algorithms) with high-precision optical, IR, or radio catalogs.  This can be used to fine-tune the astrometry to well below the typical 0.6 arcsec performance. This technique has been used to achieve absolute astrometry accurate to +/-0.3" (90% confidence, Sgr A* field), +/-0.15" (Hubble Deep Field), and +/-0.1" (Orion Nebula cluster). Once suitable counterparts have been determined (see below), the procedure for actually updating the Chandra aspect solution and event file data products is described in the thread Correcting Absolute Astrometry with reproject_aspect.

Finding high-precision counterparts

Following is a suggested procedure for finding high-precision counterparts to Chandra sources. Best results are expected from high-S/N sources in the inner portion of the field where the PSF is narrow.

  • Check to see whether any sources are associated with  in the  Hipparcos/Tycho catalog with ~1 mas precision for the ~100,000 Hipparcos stars (V<9) and ~40 mas precision for the ~1M Tycho stars (V<11), and especially the new Tycho-2 catalogue  with ~2.5M stars. 
    • Note 1: Do not use the HST Guide Star Catalog Version 1 with ~20M stars due to its poor precision and pre-Hipparcos reference frame.
    • Note 2:  It may be necessary to look at all columns of these databases; e.g., column 40 of the Tycho catalogues gives the astrometric accuracy of each star.
    • Note 3: The Hipparcos/Tycho-1 catalog is in J1991.25 coordinates, so proper motion correction is potentially important.  Tycho-2 is given in J2000, so proper motion is less of an issue.
  • Check to see whether any sources have counterparts on the all-sky Schmidt photographic plates.  For declinations >-20 deg, search the USNO-A2.0  catalogue of 526M objects from the Palomar Sky Survey, which is based on the Hipparcos frame and has positional precisions around 0.3".  For more southerly declinations, try the ROE/NRL COSMOS catalog of ~500M objects from the ESO/UK Southern Sky survey plates, but note the frame is pre-Hipparcos.
  • Check the new 2-micron all-sky catalogues.  2MASS has >162M objects over half the sky available at  IPAC. The  astrometric accuracy  of these objects a standard deviation of +/- 0.1" with respect to Tycho stars.
  • Though unlikely, check to see whether any source has a counterpart in the FIRST radio survey  covering 15% of the sky.  Currently at 549M, these sources have precisions ranging from 0.05-1 arcsec on the VLBI reference frame (which is precise and nearly identical to the Hipparcos frame).
  • Check to see whether any sources are listed in  NED  or  SIMBAD  databases, but beware that their stated positions come from a variety of sources with various precisions and reference frames.

Statistical uncertainty of source locations

Individual source locations are subject to statistical uncertainties affecting the centroiding algorithm and to the dispersion of photons due to the PSF. This has not been studied thoroughly, but the ACIS team has done a detailed astrometric analysis of 27 ACIS sources with 2MASS/VLA counterparts in the Orion Nebula Cluster (Garmire et al. 2000, AJ submitted, Table 2). From this they estimate 90% confidences of +/-0.5" for sources with ~10 counts, +/-0.2" for 20-50 count sources, and negligible for >100 count sources.

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