11 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:

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:

• 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 A,
• fast creation of a subset within a specified range for a sorted column (see Section 15 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 A 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 11.1 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].

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].

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

• The minimum and maximum are (obviously) simply the smallest and largest values in the current selection and the total range is simply the positive difference between these two values.
• If the column is sorted into ascending order then the $j$th quartile, $Q_j$, is the value of element $j\left(n+1\right)/4$, where $j=1$, 2 or 3. Depending on the value $n$, there may not be an element which corresponds exactly to a given quartile. In this case the value is computed by averaging the two nearest elements.

  The interquartile range is simply the positive difference between $Q_1$and $Q_3$.

• The median is simply the second quartile ($j=2$). The mean has its usual definition: the sum of all the values divided by the number of values.

  The value computed for the mode is not exact. Indeed it is not obvious that the mode is defined for ungrouped data. Rather, the value given is computed from the empirical relation:

  $$mode=mean-3\left(mean-median\right)$$

  (1)

• The standard deviation, $s$, is defined as:

  $$s=\sqrt{\frac{1}{\left(n-1\right)}\sum _{i=1}^n{\left(x_i-mean\right)}^2}$$

  (2)

• The skewness and kurtosis are defined in terms of moments. The $k$th moment, $u_k$, is defined as

  $$u_k=\frac{1}{n}\sum _{i=1}^n{\left(x_i-mean\right)}^k$$

  (3)

  then

  $$skewness=u_3^2/u_2^3$$

  (4)

  and

  $$kurtosis=u_4/u_2^2$$

  (5)

  The expected values for the skewness and kurtosis are:

  • skewness = 0 for a symmetrical distribution,
  • kurtosis = 3 for a normal (or Gaussian) distribution.

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.