The Ginga data analysis software

The LDAS is based around Starlink Hierarchical Data System (HDS) file structures (Warren-Smith & Lawden 1995. There are only three types of data file that can be created within LDAS: a time-series; a pulse height spectrum; and a file containing both energy and time. The files consist of a set of ``header'' parameters which describe the basic observational set-up, and the properties of the data stored within the file, together with the actual scientific data. Data are stored in the form of a data cube, i.e. it is regarded as being three-dimensional (time, spectral and detector axes; the detector axis is often referred to as the ident axis). Any or all of these dimensions can be degenerate (i.e. have only one element). A time series, for example, would have only one spectral channel. The main use of the ident axis is to separate data which comes from different detectors, or detector combinations. For example, data are accumulated from both the ``top-'' and ``mid-layers'' of each LAC detector on aboard Ginga satellite. The top- and mid-layers are regarded as separate detector idents within LDAS, with the even number idents (i.e. 2,4,...,16) being the top-layer and the odd number idents (i.e. 1,3,...,15) being the mid-layer. Usually the ident axis is compressed (sometimes referred to as squished) into the two layers (see section 2.4), and the object ID_MASK preserves information regarding the original detector combination. For example, the top- and mid-layer for the full complement of LAC detectors is given as

qcl>list id_mask
 id_mask(2)
    0101010101010101
    1010101010101010
qcl>

The installation of the Ginga data analysis software is described in section 1.4. To access the software it is necessary to source the start_ginga script. This will set some environment variables (see appendix A) and add the directory containing the software to your PATH. The Ginga data analysis software runs on DEC Alpha machines running the OSF/1 operating system.

The software uses the QX parameter system to communicate parameter values to and from the operating system; the default values are displayed between equal ("=") signs. In addition, parameters can easily be communicated between individual programs. The Leicester Quick Command Language (QCL) is a control language that sits on top of the operating system and provides a more convenient way of controlling the data analysis software. At its simplest, QCL reads commands from the terminal and it is common to string a number of commands together into a procedure file (which must have the extension ".qcl"). Procedures are commonly used to display graphics.

The Ginga data analysis software may be run either directly from your default shell or from QCL. To run QCL simply type qcl. When a command to run a program is issued to QCL it checks whether the program resides in the current directory, failing that it checks whether the program resides on the $XMAIN directory. Procedures are searched for in a similar way. To run a procedure it is necessary to use the "@" or "." command followed by the procedure name (without the extension ".qcl"). Further information about QCL and its command syntax is made available via the WWW.

The following sections describe procedures for sorting the cleaning data, background subtraction, attitude correction, spectral fitting and timing analysis using LDAS. A schematic view of Ginga data analysis appears in Figure 1. In addition, a number of programs are described that will convert HDS files into other formats, such as FITS and ASCII. Finally, a description of each program is made available as a HTML document.