Calibration Products and Process

Calibration Products and Process

Last Revised: 1999-01-27 Note: This web page is out-of-date and useful for general concepts only.
See the CXC CALDB page for current implementation details.



Please send any comments and updates to:

Overview of Calibration Process

The purpose of this document is to describe the various "calibration products" that are involved in the HETGS calibration and their path into the ASCDS software. This section provides a general overview to the products and the process. The next section provides more specific descriptions of the types of calibration products. Details of the Calibration Product Directories, the CALDB ARD Directories, and the ASCDS software are provided elsewhere.

Calibration Data is the starting point for all calibration activities. The data can be analyzed to produce specific results, or Calibration Measurement Products.

Calibration scientists then carry out "synthesis activites" to examine, understand, and cross-check the various Calibration Measurement Products to produce Fundamental Calibration Products that distill the instruments' properties.

The Fundamental products are used to calculate more useful and usefully formatted products, generally in a relatively automatic fashion. This results in a set of well-defined Calibration Interface Products that faithfully describe the instrument and can be used by others (e.g., for data analysis, ray-trace simulation, proposal planning, etc.).

Because calibration analysis spans multiple groups, web sites, and data sets it is useful to "capture" the calibration products in an explicit set of files on disk, this is the Calibration Products Directory, $CALDBaxafcal.

One of the main goals of calibration activities is to be able to provide accurate values for the data that are needed by the ASC Data System for data analysis - these data are called Analysis Reference Data (ARDs). Some of the ARDs are created directly from the calibration products, some are created from existing ARDs, and some have their origin outside of the calibration group activities.

The ARDs are also stored on disk as explicit data files in CALDB compliant format in the $CALDBaxafdata directory. These files represent the "official" calibration values. ASCDS tools may access the values in the CALDB ARDs through several means (indicated on the diagram below): (1) through a set of ard_lib functions, (2) by directly accessing the CALDB files, or (3) by accessing another file which has been updated to agree with the CALDB ARD values.

A graphical expression of these ideas is given below:

Cal Interface Products sketch

Definitions: Types of "Calibration Products"

Calibration Data

These are raw data taken at sub-assembly, at XRCF, in flight. They are just data (counts, spot location, event file, etc.). Of course they are the basis of the calibration. Error estimates are an important ingredient of the data.

Calibration Measurement Products

These products cover many levels of complexity and represent the results when the raw data have been "analyzed" to create something, for example:

There are lots of these! And their meaning and organization and synthesis can require a lot of human knowledge. Many results may be ammenable to database storage, others may require memos to describe and define them. Again, error estimates are produced for each result as well.

Fundamental Calibration Products

The wealth of calibration measurement results must be sythesized, resolved (e.g., synchrotron witness vs. reflectivity lab vs. XRCF effective area predictions), distilled, and captured (along with error bars) into some finite comprehensive well-defined set of data that serves to model the component.

For example, for the HETG, there will be a fundamental set of data at the grating facet level that will predict/describe the instrument when combined with similar data/models for the other system components. Initially "filled" with sub-assembly data, we need to develop ways to "perturb" these fundamental products to have a best agreemement with sub-assembly, XRCF, and eventually flight calibration data.

Or, for the HRMA, the Fundamental Calibration Product is the combination of the SAOSAC models of figure plus scattering plus Ir reflectivity models--these make up the most fundamental HRMA model and any "tweaking" to it will probably happen in this context.

Calibration Interface Products

The "problem" with these fundamental calibration products is that they are usually not what we want for many applications. So, for example, the facet-by-facet efficiencies are turned into shell-by-shell efficiencies that are combined with mirror shell-by-shell reflectivity and areas and combined with ACIS-S chip quantum efficiencies and geometry to finally produce the HETGS First-order effective area vs energy.

"Calibration Interface Products" are well-defined, generally useful, products that are derrived from the fundamental calibration products (often from more than a single component). Requiring an understanding of the fundamental calibration products, these interface products are best generated under Cal and IPI guidance and/or s/w.

Analysis Reference Data

The last set of data that we generally put under the term "calibration products" are the very specific parameters and files that the ASC data system analysis software will use. These are dictated by the analysis algorithms (is a 2D PSF matrix desired? is an EE curve needed?) and by the analysis software formats and standards (FITS file? ASCII? binary? compressed? portable?). These formats are dictated by the analysis algorithms and software architecture; i.e., their requirements are produced by SDS. Calibration activities can then help with the "filling-in" of the actual values of the data.

Data Availability

1999-01-27: During this formative period there are three sources for the ARDs and CIPs as listed below. In future, the ARDs will be distributed with the ASCDS and/or through the usual CALDB mechanisms. The CIPs and other Calibration information will be available in TBD way.

On SAO computers:
On MIT computers:


Chandra CALDB Architecture

Chandra Calibration Products Architecture: $CALDBaxafcal

HETG Calibration Products Architecture: $CALDBhetgcal