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
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
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.
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
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
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.