Browsing and selecting with an X display

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:

xcatview

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 . 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:

• list columns in a catalogue,

• list parameters and textual information from a catalogue,

• list new columns computed `on the fly' using an algebraic expression defined in terms of existing columns and parameters. For example, if the catalogue contained columns V and B_V (corresponding to the $V$ magnitude and $B-V$ colour) then the $B$ magnitude could be listed by specifying the expression ` V + B_V'. The syntax for expressions is described in Appendix ,

• fast creation of a subset within a specified range for a sorted column (see Section  for details of how to create a catalogue sorted on a specified column),

• creation of subsets defined by algebraic criteria. For example, if the catalogue again contained columns V and B_V then to find the stars in the catalogue fainter than twelfth magnitude and with a $B-V$ of greater than 0.5 the criteria would be ` V > 12.0 .AND. B_V > 0.5'. Again see Appendix  for the syntax of expressions,

• compute statistics for one or more columns. The statistics are computed from either all the rows in the catalogue or just the subset of rows contained in a previously created selection. The statistics computed are described in detail in Section  below,

• plot a simple scatter-plot from two columns. The scatter-plot can show either all the rows in the catalogue or just the subset of rows contained in a previously created selection,

• plot a histogram from a column. The histogram can be computed from either all the rows in the catalogue or just the subset of rows contained in a previously created selection,

• subsets extracted from the catalogue can be saved as new catalogues. These subsets can include new columns computed from expressions as well as columns present in the original catalogue,

• subsets extracted from the catalogue can be saved in a text file in a form suitable for printing, or in a form suitable for passing to other applications (that is, unencumbered with extraneous annotation).

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].