In addition to the above packages, most basic image manipulation software (e.g., display,
daophot,
imexamine,
contour) and spectral analysis software (e.g.,
splot,
tables,
specfit,
igi) available in
PyRAF/IRAF/STSDAS can be used on COS data, either directly through the
PyRAF or
IRAF FITS interface or by converting data to another format. Chapter 3 of the
Introduction to HST Data Handbooks includes information about how to display COS images and extracted spectra, as well as how and when to convert data formats, and a description of spectral analysis tasks. We present a brief summary of spectral display and analysis tasks in
Section 5.1.3.
In Chapter 3, we gave detailed discussions of the use of the data reduction pipeline
calcos. This task is contained in the
STSDAS package
hst_calib.hstcos. Other tasks useful for reducing and analyzing COS data are contained in this package as well. A complete listing and brief description of these tasks is given in
Table 5.1. All of these tasks can be run in
PyRAF. Consult the on-line help for each task for more information. Some of these tasks will be discussed in greater detail in the remainder of this chapter.
COS spectral extractions, TIME-TAG data, and most COS reference files are stored in FITS tables. (See
Section 2.4.1 for a description of the structure of the table extension files for spectral extractions and
TIME-TAG data.) Tasks designed to handle this format can be used to examine and extract information from the tables. Here we give specific examples of the use of routines in
ttools to help you get started. A sample output is given after each command.
To use tread to look at the contents of the table:
The tlcol output indicates that some of the columns contain arrays of 16384 elements rather than a single value. For those columns,
tread displays the value of the first element in the array. e.g., the initial wavelength in this
x1d extraction is 1585.58 Angstroms. To plot an entire array, or plot one array against another, you can use a routine that operates on the table data directly:
You may want to change the parameter values in the supporting parameter sets axispar,
dvpar, and
pltpar to adjust the plotting. Alternatively, you may find it easier to work with FITS images. To make FITS image files of the arrays:
Table 5.2 lists some of the more useful
PyRAF/IRAF/STSDAS applications for displaying and analyzing COS spectral data.
Specfit is a powerful spectral fitting routine that provides the user with many options for modelling emission and absorption features, continuum, and extinction. The fitting parameters can be linked to each other (e.g., fixed wavelength ratio or fixed flux ratio for two emission lines) and constrained. Different algorithms can be selected to first explore chi-square space, then rapidly achieve convergence in the deepest minimum. The application is fully described in a help file.
COS spectra in tabular format are very similar to STIS spectra. The following examples illustrate the use of the sgraph task to plot COS FUV and NUV tabular spectra using a row selector and specifying the columns (e.g., wavelength and flux) to plot.
