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, FAINT_BIAS, or GRADED exposure modes are included.
The following ACIS observations are excluded:
- any other exposure modes
- ACIS continuous-clocking mode observations
HRC-I observations are included in Release 2.0.
HRC-S observations are excluded from Release 2.0.
- Other exclusions
- All grating observations are excluded.
- Moving target (solar system) observations are excluded.
- Observations obtained with with defocus <-0.1 mm or >+0.2 mm are excluded.
- Observations obtained with with sim_z_offset <-20.0 mm or >+20.0 mm 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 8.4.2. This makes sure that the data products required by the catalog processing are available.
Note that sources in the vicinity of bright, crowded fields, (or near bright, extended emission that has not been excluded), may be incomplete. The emission contributes to the background detection annulus around each source and increases the background variance, hence reducing the significance of the source detection. Performance for this issue is improved in Release 2 compared to release 1 but users should still exercise caution.
Each observation interval (OBI) chosen by the observation selection process is checked to see if it overlaps with other such intervals such that their pointing directions are within 60″ of each other. Such co-aligned observations are grouped into 'stacks'. For more information see the Catalog Organization section.