Chandra X-Ray Observatory

Replacement of bad target names

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    Details on the replacement of archival "bad" target names can be found here
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Bad target names replacement project

The Chandra "bad" target names replacement project


The target names of Chandra observations are of primary importance to A) characterize the scientific goal of the observations, B) unequivocally locate them in space and time, and C) to make the observations discoverable through the archive interfaces after they become public. Target names that do not satisfy either one of the previous conditions (hereinafter “bad” target names) can negatively affect how observations in the archive are discovered and retrieved and, in turn, jeopardize the optimal scientific exploitation of the legacy of the Chandra mission.

In preparation of the 5-th general reprocessing of the archive (Repro V), the Chandra Data Archive (CDA) team has reviewed the target names of a large majority of archival public observations in the Chandra archive (see Applicability below for details), selected target names requiring enhancements (Selection of bad target names) and determined suitable replacements (Replacement of bad target names).

The target names of affected observations have been replaced with the new target names in the Chandra Observation Catalog (OCat) on 09/15/2020, and across all the VO protocols that serve Chandra observation metadata shortly thereafter. The headers of the Chandra data products containing the target name values in the OBJECT keyword, will be updated as observations are re-processed during Repro V. No other parameter that can be used to identify observations (ObsIDs, target coordinates) has been modified.



All OR (GO, GTO, TOO, DDT, CCT, CAL) archival observations taken through 2016 have been reviewed. Observations taken after 12/31/2016 were not covered as a courtesy towards their observers who might still be working on the data.


Selection of bad target names

Archival observations have been considered for target name replacement if their target names qualify for (at least) one of the following categories:

  • not informative: target names that do not unequivocally determine the identity of the target for being too generic (like "PROTO CLUSTER", "NEWMAGNETAR"), or not descriptive of the actual target but linked to the type of observation (i.e. observations whose target names contain the string "TOO")
  • containing typos, as in the case of "1FLG" instead of "1FGL" for Fermi Large Area Telescope First Source Catalog
  • lacking a substring: many target names do not contain catalog identifiers that are necessary to correctly determine the object that has been observed
  • not resolvable into coordinates, i.e. not associable to sky coordinates by neither the NED and Sesame name resolving services


Target names of observations associated with the same deep fields/surveys/regions, whose current coverage has grown over time through the accumulation of multiple programs, have also been reviewed for naming consistency. It has been verified that all observations of the same field are discoverable with a single search based on the most common and/or resolvable name (when available) of the field/survey/region. When not, the differences in the target names of observations of the same field were reconciled to minimize the number of target name changes needed.


Replacement of bad target names

In total, 5181 distinct observations qualified for target name replacement according to one (or more) of the criteria described above. The determination of the replacement target names for ~2000 of these observations proved not trivial and required manual work, including surveying the linked scientific literature, inspection of the proposal abstract, comparison with existing catalogs of astronomical sources and, in a few cases, contacting the original PIs of the observations. For not obvious cases, the new target name has been selected to reflect as closely as possible the original intention of the observer. Other criteria that were applied to determine the replacement target names are described below:

  • When an observation is selected for target name replacement, all other ObsIDs with same target name have been reviewed (this rule only applies to compact sources, non transients sources);
  • When a section of a target name that - globally - does not resolve into coordinates, is resolvable, the target name has not been modified;
  • Parts of the original target names to be replaced, containing valuable information about the targets have been retained in the new target name (for example, "cold front" for observations of a cluster of galaxies);
  • While "resolvable" new target names have been prioritized over "not resolvable" designations for observations with “bad” target names, non-resolvable new target names deemed to be sufficiently recognizable, based on their common usage in the literature, have been used as replacements (provided that their observations are discoverable through "string-based" searches).


The final list of Observation IDentifiers (ObsIDs) for Chandra archival observations whose target names have been replaced, the old target name and the new target names can be downloaded as a file in ASCII and FITS formats. The number of distinct new target names used to replace “bad” target names is 2212, because in many cases the same target names are associated with multiple observations; 604 ObsIDs whose target names have been replaced are calibration (CAL) observations.


Additional resources

In order to resolve into sky coordinates, target names need to follow naming conventions agreed upon by the International Astronomical Union (IAU). Chandra archive users can find useful information about the syntactic rules for acceptable designations of astronomical sources here. More detailed information about “subcomponents” in the names of astronomical sources are available at this link. A web-based resource dictionary of the nomenclature of celestial objects is hosted by CDS at this link.

This page maintained by the Chandra Data Archive (

Last modified: 2020-09-14

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