This recipe shows how to import and display a data cube observed with the ROSAT X-ray astronomy
satellite (Röntgensatellit). ROSAT data come in a number of different formats. The example used here
is an Asterix binned dataset of the irregular galaxy M82 (NGC 3034). This format is used by the
Asterix package (see SUN/98). The data cube comprises a grid of 216 by 216 points on the sky, with
a spectrum of 22 points at each position. The example data are available as file
procedure for importing and displaying these data is very similar to the corresponding procedure for
JCMT data described in Section 13.
ast2ndfin the CONVERT package (see SUN/55) performs this task. Proceed as follows. First type:
to make the CONVERT applications available. Then type:
Note that though the input Asterix binned dataset and output NDF are respectively held in files
m82ndf.sdf they are specified to application
ast2ndf without the ‘
It is also worth noting that an Asterix binned dataset is itself almost a standard Starlink NDF.
ast2ndf merely makes the data format completely standard and rearranges the order of the
hdstrace. Simply type:
Again the file name is specified without the ‘
.sdf’ file type. This facility is useful because it lists
the value of much of the auxiliary information contained in the data set.
documented in SUN/102. In addition,
hdstrace and many other standard Starlink
applications, will also work on the original Asterix binned dataset.
ndf2dx, which is part of SX, the Starlink enhancements to DX (see SUN/203). By convention files in the native DX format have file type ‘
.dx’. To convert the entire data cube simply type:
Note that though the file type is not specified for the input NDF file, it should be given for the output native DX format file.
The above example will convert the entire data cube. However, often the useful information will be contained in only a small range of energies. For example, in the example dataset most of the useful information lies between energy steps 5 and 15. It is possible to convert a subset of the NDF corresponding to a given range of energy steps. For example, to convert a subset corresponding to steps 5 through to 15 type:
The syntax to specify a subset of an NDF is to give the bounds of the required region inside
parentheses after the file name. Unfortunately however, by default the Unix shell will attempt to
interpret these parentheses. Thus, in the above example the input file name and NDF subset are
enclosed in single quotes in order to prevent this behaviour and ensure they are
passed correctly to
ndf2dx. The use of ‘escape mechanisms’ of this sort to prevent the
premature interpretation of special characters sent to Starlink applications is discussed in
axes=no option causes
ndf2dx to ignore any axis information present in the input dataset
and write the output DX file with axes consisting of simple pixel numbers. This option may or
may not be appropriate depending on the details of your data. In the present case it leads to a
rosatslice.netis the basic network and
rosatslice.cfgis a ‘configuration file’ which controls some aspects of its behaviour). Start DX (as described in Section 2). Then proceed as follows.
rosatslice.net. The network should now load and appear in the main window. See Section 4 for further details.
m82sub.dxand 10 respectively.) Close the ‘Control Panel’ window.
The network includes a ‘scale bar’ showing how the colour displayed for each pixel in each slice corresponds to the value of the pixel. In this example the value of each pixel is the count rate in counts per second. This network for displaying ROSAT data is virtually identical to the corresponding network for JCMT data (see Section 13 and Figure 15). The only differences are the defaults for the file names and the axis labels. Similarly, the network for generating an iso-surface in JCMT data (see Figure 16) requires only minor cosmetic modifications to display ROSAT data.