There are many filters that determine which data sets are included or excluded from each release of the Chandra Source Catalog.
Public ACIS timed-exposure mode observations in FAINT, VFAINT, or FAINT_BIAS exposure modes are included.
The following ACIS observations are excluded:
- any other exposure modes (including GRADED)
- ACIS continuous-clocking mode observations
- ACIS subarrays with <= 128 rows (there are too few rows to guarantee identifying source-free-regions for the streak map background algorithm).
Public HRC-I observations are included in Release 1.1.
HRC-S is excluded from Release 1, due to background features that get detected as false sources with the existing algorithms.
- Other exclusions
All grating observations are excluded.
Moving target (solar system) observations are excluded.
There are also a number of criteria that an observation interval (OBI) must meet in order to be selected for catalog processing:
The data set must have a good Verification and Validation (V&V) status. During V&V, the products are checked by CXC scientists to ensure data quality.
The data must have been processed through at least DS 7.6.7 (Reprocessing III for earlier data). This ensures that the aspect solution is good, as well as making sure that the data products required by the catalog processing are available.
Observations must pass extended source checks.
Observations containing highly extended sources that may be detected incorrectly are excluded from catalog processing. If the extended emission is restricted to a single ACIS CCD, then in most cases the data from the remaining CCDs are included in the catalog. (For example, if 6 chips are on and there is a bright supernova remnant on a single chip, then data from the 5 remaining chips is processed.) A list of ObsIDs with dropped chips is available here.
Since the largest wavdetect scale used for source detection is ~30 arcsec, sources with apparent extents greater than this are either not detected, or may be detected incorrectly as multiple close sources. Such observations are candidates for exclusion through the extended source checks. Some point sources with off-axis angles >20 arcmin may be lost because of the size of the PSF there; the characterization of the catalog completeness calibrates this effect statistically.
Note that sources in the vicinity of bright, crowded fields, (or near bright, extended emission that has not been excluded), may be incomplete in Release 1 of the catalog. 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.