|AHELP for CIAO 4.2||
CALibration DataBase (CALDB)
The Chandra CALDB is the directory and indexing structure that stores and provides access to all calibration files that are required for standard processing and analysis, provided that they have been put into FITS format. The Chandra X-ray Observatory calibration database is modeled after the HEASARC style CALDB. It is the directory and file structure which stores the calibration data used in standard data (a.k.a. pipeline) processing, as well as for analysis with systems such as CIAO.
When you receive Chandra data products from the pipelines, you are getting data which has been generated using Level 1, 1.5, and 2 calibration files, which reside in the currently-active version of the Chandra CALDB at the Operations Control Center (OCC). When you analyze data, e.g. building a response matrix or ARF, you are using your local version of the Chandra CALDB to access the calibration files. The CALDB provides you with the most recently approved calibration data for distribution to the public, without your having to specify it yourself (unless you so desire).
The Chandra CALDB is often in a state of development, both because of software enhancements and because of calibration updates. The active version of the CALDB used at the OCC will have profound affects on your standard data products (SDP). Similarly, the particular version of the CALDB you have installed on your local system will have profound affects on your analysis results.
Therefore you will be keenly interested in:
- maintaining the most current version of the CALDB for your analysis,
- understanding what version was active at the time your data were processed through the SDP pipelines,
- knowing when updates you need to improve your analysis will be available, and when they are released.
Many CIAO tools can access the CALDB directly by getting the necessary meta from an input file already supplied. Things like the DATE, DETECTOR, and other instrumental configuration parameters are extracted and can be used to query the CALDB directly. The simpliest way is to set the parameter value to "CALDB", eg
The parameters of tools that can access the CALDB directly are noted in their respective help files. Users can also override the parameters that the tool would have picked up from the header. This may be useful to work around some indexing problems, for example finding 'HRC-SI' calibrations which are indexed as 'HRC-S'. The syntax for overriding the various parameters that are used to make the query is
Adjustable parameters in CALDB lookup
|DETECTOR||Change the detector (DETNAM) name|
|START_TIME||Start time of observation (DATE-OBS), HH:MM:SS format|
|STOP_TIME||Stop time of observation (DATE-END), HH:MM:SS format|
|START_DATE||Start DATE of observation (DATE-OBS), YYYY-MM-DD format|
|STOP_DATE||Stop DATE of observation (DATE-END), YYYY-MM-DD format|
|QUERY||Query expression to further restrict search|
acis_process_events evt2.fits gainfile=CALDB ....
The correct gain file for the observation will be selected from the CALDB using the header information in "evt2.fits".
hrc_process_events my_hrc-si.fits tapringfile='CALDB(DETECTOR=HRC-S)' ...
This will load all the necessary meta data from 'my_hrc-si.fits' but will override the DETECTOR, aka DETNAM, to look for HRC-S data since the CALDB doesn't know about the 'HRC-SI' DETECTOR.