11 Browsing and selecting with an X display

 11.1 Statistics computed for individual columns
 11.2 Restarting xcatview after a crash

xcatview is a powerful and flexible catalogue browser. However, it can only be used from a terminal (or workstation console) capable of displaying X output. Before starting xcatview you should ensure that your terminal (or console) is configured to receive X output. Then simply type:


and follow the ensuing dialogue boxes. Copious on-line help is available within xcatview. To obtain it simply click on the ‘Help’ button; every dialogue box in xcatview contains a ‘Help’ button.

In addition to accessing local catalogues xcatview provides some limited facilities to access remote catalogues held on-line at various astronomical data centres and archives around the world. These facilities provide the same functionality as the application catremote and are described in greater detail in Section 25. Obviously they will only be available if the computer on which CURSA is running has appropriate network connections (which will usually be the case at a normal Starlink node).

xcatview provides the following facilities:

A tutorial example of using xcatview to select stars which meet specified criteria from a catalogue (a ‘recipe’ in the jargon of cookbooks) is included in SC/6: The CCD Photometric Calibration Cookbook[22].

11.1 Statistics computed for individual columns

Statistics can be computed for one or more individual columns. They can be computed from either all the rows in the catalogue or just the subset of rows comprising a selection which has been created previously. Obviously, only non-null rows are used in the calculations. Statistics can be displayed for columns of any data type, though for CHARACTER and LOGICAL columns the only quantity which can be determined is the number of non-null rows.

For each chosen column its name, data type and the number of non-null rows (that is, the number of rows used in the calculation) are displayed and the statistics listed in Table 5 are computed. Though all these quantities are standard statistics there is a remarkable amount of muddle and confusion over their definitions, with textbooks giving divers differing formulæ. For completeness, and to avoid any possible ambiguity, the definitions used in xcatview and catview are given below. These formulæ follow the CRC Standard Mathematical Tables[4] except for the definition of skewness which is taken from Wall[30].

Total range
First quartile
Third quartile
Interquartile range
Mode (approximate)
Standard deviation

Table 5: Statistics computed for columns

In the following the set of rows for which statistics are computed is called the ‘current selection’ and it contains n non-null rows. xi is the value of the column for the ith non-null row in the current selection. The definitions of the various statistics are then as follows.

11.2 Restarting xcatview after a crash

Occasionally, due to some misadventure, xcatview might crash. In this eventuality some temporary files can be left in existence; these must be deleted before xcatview can be used again. The files will be in subdirectory adam of your top-level directory (unless you have explicitly assigned this directory to be elsewhere). The files have names beginning with catview and xcatview, for example:


Simply delete these files and xcatview can then be started as usual.