Chandra X-Ray Observatory
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Last modified: 7 April 2014

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.

Catalog Data

General
Specific Caveats and Limitations applicable through CSC release 1
Specific Caveats and Limitations for certain ObsIds
Astrometric Errors of Sources in Some HRC-I Observations

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. Furthermore, the first release of the catalog includes only point and compact sources, with observed (i.e., un-deconvolved) spatial extents < ~30 arcsec. Sources larger than this will not be detected with the current CSC algorithms. In addition, observations of fields containing highly extended sources have been excluded from the catalog. In most cases if the extended emission is restricted to a single ACIS CCD, then the data from the remaining CCDs is included in the catalog.

Users of catalog current database views (as opposed to an official catalog release view) should also 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.

Specific Caveats and Limitations applicable through CSC release 1

  • WARNING The catalog source position error 95% confidence level ellipse radii (err_ellipse_r0 and err_ellipse_r1) do not include the systematic component of the astrometric error. This component is 0.16 arcsec (1 σ, ± 0.01 arcsec), derived by comparison of the positions of CSC Release 1.0 and SDSS DR7 source pairs (see Determining the Astrometric Error in CSC Source Positions). To compute the total absolute astrometric error, the following formula may be used:

err_ellipse_r0[total] = 2.4477467 * √(0.1669041 * (err_ellipse_r0[catalog])2 + 0.0256)

where err_ellipse_r0[catalog] is the value tabulated in Release 1.0 of the CSC. A similar equation can be used to compute err_ellipse_r1(total).
The systematic component of the astrometric error will be included in the tabulated values starting with Release 1.1 of the catalog.
  • In release 1.0 of the catalog, the var_flag property in the Master Sources Table was not correctly populated. This error was fixed in release 1.0.1, which preceded the current release 1.1. Catalog users who extracted this property from release 1.0 should consider re-running their queries against one of the newer releases.
  • We have identified an error in variability determination for a subset of sources near chip edges. This will affect sources that dither across multiple CCDs, where one or more of the CCDs are off or are otherwise excluded (see the "Observation Selection" page for a description of the criteria by which chips are excluded, as well as for the list of ObsIDs with dropped chips). Events from the off/excluded CCDs are not included in the source lightcurve nor in any other source properties; however, the fractional area correction (calculated with the dither_region tool) will still include areas from the off/excluded CCDs when applying the variability tests. This can lead to a false variability signal from all three variability tests (Kuiper, Kolmogorov-Smirnov, and Gregory-Loredo). This variability will not necessarily be on the dither time scale, therefore the dither_warning_flag may be set to FALSE and thus not serve as an indicator of this problem. Likewise, the multi_chip_code flag may be set to 0 if all but one CCD is off/excluded. Thus, highly variable sources with a non-zero edge_code should be treated with caution when assessing variability properties.
  • The background is modeled using a single low spatial frequency component, with the addition of a high spatial frequency "streak map" for ACIS observations. The detectability of sources may be compromised in crowded regions near bright sources (or near bright, extended emission that has not been excluded) where the background has a strong spatial dependence. The emission contributes to the background detection annulus around each source and increases the background variance, hence reducing the significance of the source detection. An example of this problem can be seen near the center of Obsid 6420; see the science study "CSC Release 1: Missing sources in crowded fields" for further details.
  • All position angles recorded in the Source Observations Table are defined relative to the tangent plane projection of the individual observation. The 0 degree position angle reference is defined to be parallel to the true North direction at the location of the tangent plane reference point, rather than relative to local true North.
  • The source position error ellipse is replaced by a source position error circle in this release. This limitation will be lifted in a future release of the catalog.
  • The deconvolved source extent is determined using a circular Gaussian parameterization in this release, rather than a rotated elliptical Gaussian parameterization. This limitation will be lifted in a future release of the catalog.
  • The mean chip coordinates (chipx, chipy) of a source are computed without including the effect of the mean dy, dz, dtheta offsets from the aspect solution. For some observations these offsets are of order 15 arcsec (~30 ACIS pixels) in both dy and dz, so the mean chip coordinates may be uncertain by this amount.
  • In release 1.1 of the catalog, the area_aper90 and area_aper90bkg values associated with a subset of the sources are incorrect. This bug applies only to those sources whose properties were derived from blocked images; i.e., those having a blocking factor of greater than 1, where 1 represents the highest spatial resolution.

    The area_aper90 and area_aper90bkg values represent the areas of intersection of a source region and its associated background region with the field of view of the observation, respectively. This area is equivalent to the number of pixels in the relevant portion of the associated source region image; the blocking factor of this image, if higher than 1, was not properly taken into account when deriving the area_aper90* values.

    In order to determine if your source has been affected by this bug, check the SRCRES header keyword of the source region file, reg3.fits, associated with your source in the catalog:

    ciao% dmkeypar reg3.fits SRCRES echo+
    1
    

    If the SRCRES value is not equal to 1, then the area_aper90* values for that source are incorrect in release 1.1. The correct area_aper90* value can be determined by dividing the incorrect area by the square of the SRCRES factor.

  • [New] The model independent flux_aper values are computed from the events' observed energy. In narrow energy bands, especially the ultra-soft (0.3-0.5keV) band, the spectral response can change rapidly with energy. This means a small change in observed energy from an event's true energy (as described by the RMF) can result in large changes in the flux. If there are few events this can result in large, undetermined systematic errors. In the other energy bands where spectral response generally changes slowly with energy this is less of a problem. It also becomes less of a problem when there are a large number of events as the Gaussian-like RMF is well sampled and the uncertainty balances out.

Specific Caveats and Limitations for certain ObsIds

  • The catalog has no sensitivity on the ACIS readout streak associated with the very piled-up (cratered) bright source on ObsId 5511.
  • [Updated] Some ACIS observations were performed with energy filters applied. Depending on the particular observation, this could remove all or most of the sensitivity for certain CSC energy bands. These data will have NULL values for some of the flux properties and possibly the hardness ratios. More details can be found in the ACIS event energy threshold caveat

  • [New] The ACIS Response Matrix Files (RMF) for the following Observations Id's have an error in the EBOUNDS extension that will prevent the files from being loaded into Sherpa.

    DataErr: RMF 'acisf00307_000N001_r0001_rmf3.fits.gz' is incompatible with PHA dataset 'acisf00307_000N001_r0001_pha3.fits.gz'
    

    Users will need to re-create the RMF files for these observsations using the CIAO tool mkrmf.

    The list of OBS_IDs is: 307, 386, 1769, 1770, 1771, 1773, 1777, 1778, 1779, 1780, 1781, and 1782.

Astrometric Errors of Sources in Some HRC-I Observations

Because of recently identified inadequacies in the fiducial light calibration for HRC-I observations with very-off-nominal SIM positions (primarily calibration observations), the positions of some CSC sources detected in some HRC-I observations may be in error by more than the stated error ellipse dimensions.

The affected obsids are:

1261      6546      6553      8342      8349      8356
6540      6547      6554      8343      8350      8357
6541      6548      6555      8344      8351	  8358
6542      6549      6557      8345      8352	  8359
6543      6550      6558      8346      8353      9684
6544      6551      7414      8347      8354	  9685
6545      6552      8341      8348      8355      

The actual master source and source observation position errors likely exceed the tabulated error ellipse radius for the following sources:

CXO J051629.3+455714
CXO J123536.6-395433
CXO J123548.9-395024
CXO J123548.9-395316
CXO J123556.2-395014
CXO J123558.5-395941
CXO J123600.5-395215
CXO J123645.2-394802
CXO J123651.7-395520
CXO J123656.6-395516

The actual source observation position errors (but not the master source positions errors) likely exceed the tabulated error ellipse radius for the following sources and observations:

CXO J033647.2+003516   obsid 1261
CXO J051641.3+455135   obsids 6541, 6543, 6544, 6545, 6546, 6547,
                              6548, 6549, 6550, 6552, 6553, 6555,
                              6557, 8350, 8352, 8353, 8355, 8356,
                              8357, 8358, 8359
CXO J051641.4+455950   obsids 6540, 6541, 6542, 6543, 6544, 6545,
                              6546, 6547, 6548, 6549, 6550, 6551,
                              6552, 6553, 6554, 6555, 6557, 6558,
                              8343, 8344, 8345, 8346, 8347, 8348,
                              8349, 8350, 8351, 8352, 8353, 8354,
                              8355, 8356, 8357, 8358, 8359
CXO J051724.0+455016   obsids 6540, 6541, 6542, 6543, 6545, 6547,
                              6549, 6550, 6551, 6552, 6553, 6554,
                              6555, 6557, 6558, 8355, 8356, 8357,
                              8358, 8359
CXO J220840.8+454432   obsids 9684, 9685

Catalog Interfaces

Data Scope

The Virtual Astronomical Observatory (VAO) Data Scope service currently fails to return results from the CSC VO Cone Search. We understand the problem and are working on a solution.

CSCview

Java support

CSCView is a java application that requires signed applets. The commerical version of Java supplied by Oracle meets this requirement. Some versions of open source java virtual machines commonly installed on Linux distributions, eg IcedTea/OpenJDK, do not support signed applets. Users should install the Oracle version.

Platform-specific issues: Macintosh
  • The recent versions of Java from Apple, have been found to cause problems with Java applets in both the Google Chrome and Firefox web browsers. As a result, Mac users may find that the CSCview Java applet does not run properly in these browsers. Users are encouraged to run CSCview in the Safari web browser. You can learn more about this issue on the Apple Support site.
  • Some Mac users find that they are not able to use their mouse cursor to drag and drop source properties among the various interactive fields of the CSCview Query tab in order to build a query; the "+" and "-" buttons at the top of each of the fields may be used, instead, to add or remove entries. Users may also do the following to work around this issue and enable dragging and dropping with a mouse:

1. Launch CSCview.

2. Click on the CSCview window, then put the mouse somewhere in the window, such as on top of the Result Set.

3. Hit the Shift and Command keys; a thin bar should appear called "CSCview" just under the one called "Chandra Source Catalog Search - CSCview".

4. While still holding the Shift and Command keys, grab the thin bar and move it slightly; now, the program should be in this new window. The application name at the top of the Mac will be "Java Applet - cda.harvard.edu". Dragging and dropping should work in this version of CSCview.

Web browser issues: Firefox

Some Firefox users may not be able to access the 'Crossmatch -> User Table -> Local File' pop-up window for loading a file of source positions for a Crossmatch query; the work-around is to select 'Crossmatch -> User Table' to activate the Crossmatch pop-up menu, and then use the down arrow and Return key to select the 'Local File' menu item.

Interoperability with other tools
  • The CSCview Send feature has been found to be incompatible with the Catalog Tool in DS9 version 6.0, but should work with versions 6.1 and onward.
  • Users are warned that there is a limit to the amount of data which may be imported into CSCview from a remote, SAMP-connected client (such as Topcat), using the crossmatch User table --> Received Table option in order to perform a crossmatch of this data against the CSC. This limit is 5000 rows in a table of data being imported, e.g., a table containing a full catalog of astronomical data to be crossmatched against the CSC. This means that the number of source matches reported by CSCview will be incorrect, as only the first 5000 rows of the input table will have been used in the query. It is important to note that when an incoming table of data is truncated by CSCview, no warning is issued.

Last modified: 7 April 2014
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