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Last modified: 13 September 2018

URL: http://cxc.harvard.edu/csc/caveats.html

Caveats and Limitations



While the Chandra Source Catalog (CSC) is capable of supporting many diverse scientific investigations, users should carefully consider the limitations of the CSC when assessing the efficacy of the catalog for their particular line of inquiry.

  1. Catalog Data:
    1. General
    2. Specific Caveats and Limitations applicable through CSC release 2
    3. Specific Caveats and Limitations for certain ObsIds
    4. Flux Correcting Very Bright Sources
    5. Known issues and limitations with the current catalog view (CSC2)[Updated] (13 Sep 2018)
  2. Catalog Interfaces
    1. CSCview
    2. ADQL
    3. The contributed scripts
[NOTE]
Access to version 2 results and data

Users of the catalog current database view (i.e. those users accessing the version 2 catalog) - as opposed to an official catalog release view - should be aware that the contents of the database are subject to revision as processing progresses, and a small fraction of sources present in the catalog database may not meet all of the catalog release quality assurance requirements.


Catalog Data

General

In particular, users of the CSC should be aware that there may be fundamental and significant selection effects that restrict the source content of the catalog and which therefore may limit scientific studies that require an unbiased source sample. The CSC is constructed from pointed observations obtained using the Chandra X-ray Observatory; it is not an all-sky catalog, and does not include sources detected to a uniform depth. Users should also be aware that the detection of sources with spatial extent more than ~30 arcsec is done with a separate algorithm from that for compact sources.

Note that for some sources in observations with short exposures, there are zero counts in the local background region for some bands. In these cases, the source flux is not computed.

Specific Caveats and Limitations applicable through CSC release 2

Specific Caveats and Limitations for certain ObsIds

Flux Correcting Very Bright Sources

For some very bright sources, such as 2CXO J150842.1+332802, the method used to correct the energy flux in the source aperture for background contributions, using energy flux in the background aperture, may fail, resulting in NaN's for flux_aper or flux_aper90. Other flux quantities, such as photflux_aper, or src_rate_aper, are unaffected.

Known issues and limitations with the current catalog view (CSC2)

The following issues will be addressed before the full CSC 2.0 release.

17 July 2018

The limiting-sensitivity maps - available via CSCview - are not valid for CSC 2 only, and should not be used.

18 July 2018

Some Master Source energy flux columns in CSCview are empty even though the source significance is high. This happens for sources with more than 10 contributing ObsIDs.

7 August 2018

For a small number of master sources (6,648), the aperture photometry values for the master source best estimate, master average, and detect stack average may be either missing or in error. Bayesian blocks data products for these sources may also be in error. The cause is two-fold:

  • a bug in the algorithm for combining data from ten or more ObsIDs
  • a bug in the algorithm for combining data from any number of ObsIDs, if the intensity-space of the probability distributions for individual ObsIDs did not overlap sufficiently
7 August 2018

For a number of master sources (30,874) aperture photometry values for individual observations (ObsIDs) are also in error, due to errors in computing the encircled energy fractions in the source apertures.

13 September 2018

The upper and lower limits on the spectral-fit parameters - such as powlaw_gamma_hilim and powlaw_gamma_lolim - are incorrect.

13 September 2018

The extent flag and the Source Extent parameters are not reliable for sources identified as extended.


Catalog Interfaces

CSCview

The software-requirements page contains current instructions for downloading and running the CSCview java application.

Interoperability with other tools

ADQL

Metadata returned from a query

The metadata returned from an ADQL query is incomplete, as it is missing the units and any UCD terms for most columns.

The contributed scripts

The search_csc and obsid_search_csc scripts currently do not support the CSC 2 database, and so will only return values from the CSC 1.1 release.