High-precision astrometry and image reconstruction with Chandra

Following are notes on improving the absolute astrometry and image reconstruction of X-ray sources in Chandra observations.  This is based on documents written by Eric Feigelson and the ACIS team, and we gratefully acknowledge their contribution.

Overview

• First read the CXC  memo on astrometry problems  to see if your dataset is affected by offsets introduced by processing errors or use of non-standard fiducial lights.  Also read the CXC Caveat regarding  randomization of event positions by +/- 0.25" which is incorporated into Level 1 processing.  The original positions can be recovered if desired.

 
• The Chandra mission requirement on absolute source position (i.e. celestial location) accuracy is an RMS radial deviation of less than 1.0 arcsec.  Currently, most ACIS observations and many HRC observations exceed this requirement, for reasons explained in the memo on astrometry problems.  However, the problems have been all been understood and addressed, and after all observations are reprocessed (planned to begin May 15, 2000) we expect to meet the 1.0 arcsec celestial location requirement.

 
• The aspect pipeline software design and flight performance (as of February 2000) are described in a paper presented at the SPIE meeting on Astronomical Telescopes and Instrumentation 2000.  The key result for astrometry is that the celestial location accuracy for HRC is currently about 0.6 arcsec (RMS radial offset).  We expect the same performance for ACIS using currently available software and alignment calibrations.

 
• The design and performance of the Chandra aspect flight hardware is described in another SPIE paper.

Improving absolute astrometry

Following is a suggested procedure for bringing an image into the Hipparcos coordinate frame.  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.  The USNO UCAC1 catalogue with 27M stars with V<16 in the southn hemisphere are expected to be released in early 2000;  check here  for updates.

 
• 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.  See description of Tycho columns here .  When printing out Hipparcos/Tycho tables from Netscape Navigator, you must "Save as" in "text" format; otherwise most columns are lost.
• 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.

• If several astrometric counterparts are available, one can quickly find the mean offset in (RA,Dec), or more elaborately one can make a least-squares solution that includes a roll angle correction.  The mean offsets (without a roll correction) can easily be inserted into the FITS header events or image files of interest, using the FTOOL fv:

 
• View the Header of the binary EVENTS table, and find the keywords TCTYPdd (where "dd" is a two-digit number) which have the values "RA--TAN" and "DEC--TAN".
• Use the Edit Insert/Delete menu options to correct the corresponding keywords TCRVLdd.  These keywords give the RA and Dec (degrees) of the central pixel in the physical (detector) image.
• For images (as opposed to event files), examine and edit the CTYPEdd and CRVALdd keywords, respectively.
• Note: Off-axis positions may have additional systematic positional errors of unknown amplitude due to the asymmetric off-axis point spread function.

Statistical uncertainty of source locations

Improved image reconstruction