Header Keywords

The CALDB is modeled after the HEASARC CALDB and is assembled based on the mandatory calibration file keywords prescribed by that group. At the end of Chandra's lifetime, the CALDB will be ingested into the HEASARC CALDB structure, where all the calibration databases for previous HEASARC missions are maintained.

The FITS header keyword structure specifies the subdirectory into which each file is to be placed, and provides the indexing information by which to select the file for use in processing and analysis. It is the function of the header specification and index to provide a unique specification for each and every active file, so that only one file and extension are returned for a sufficient indexing specification.

Conventions have been established for Chandra header keywords used by the CALDB; they are outlined in the ASC FITS Designer's Guide (PS, 40pp). Further details are also available from the CALDB Header Specifications reference document.

Keyword Specifications

The list of required keywords for Chandra CALDB files is given in Table 1. The keywords with a "Y" in the Index column are identical to those in the HEASARC list and are included in the index files.

Notes for Table 1:

1. By convention, the TELESCOP keyword must be set to CHANDRA, although the conventions allow AXAF as well. It was necessary to choose one value for use in the CALDB.

2. When not set to NONE, the GRATING keyword is carried to the index using a CBDn0001 keyword such as CBD30001 = 'GRATING(HETG)'. This is done so that the value may be selected for an index search.

3. The GRATTYPE keyword allows distinction between HEG and MEG. It is indexed with a CBD keyword as well, such as CBD40001 = 'GRATTYPE(MEG)'.

The process of implementing the CALDB involved coordinating Chandra FITS keyword standards with the indexing requirements of the Chandra CALDB within the guidelines provided by the HEASARC. Additions to the HEASARC standards were necessary, most commonly in the area of calibration file types, as many more exist for Chandra than for any previous mission. A number of new CALDB keyword values specific to Chandra were invented; these may be used by future missions with similar calibration requirements.

Codenames

The codename (CCNM0001 keyword) value indicates the type of calibration data that is stored in the FITS file; Table 2 lists the allowed values.