Customising the plot

By default catchart plots all the objects in a target list using the same plotting symbol drawn to a constant size in the same colour. Often this effect will not be what you want. Traditionally in astronomical atlases and charts stars are shown as circles whose size varies with their magnitude. Also different symbols and colours may be used to indicate different types of object or different aspects of the same sort of object.

The target lists which catchart might have to plot can come from a wide variety of sources (for example, catremote allows you to retrieve target lists from data centres and archives scattered around the world). All that can be guaranteed about them is that they will contain columns of celestial coordinates. No other assumptions can be made about the other columns which they may contain or how the objects in them should be plotted. It is not even possible to guarantee that the columns will include a magnitude; many non-optical catalogues do not and even if they do it may not be appropriate to plot symbols scaled on the magnitude.

To solve this problem application catchartrn is provided to allow you to prescribe how the objects in a target list are to be plotted; you specify the symbol, size and colour of the plotted objects. These quantities may be constant for all the objects or may be computed for each object, based on the value of other columns for the object (the traditional example is computing the symbol size from the magnitude). catchartrn adds some extra columns and parameters to the target list defining how the objects are to be plotted and catchart automatically uses these. This technique is very flexible and allows a great deal of control over the way objects are plotted. catchartrn itself reads a prescription of how the objects are to be plotted from a simple pre-existing file, the so-called graphics translation file. Example graphics translation files are provided for most of the catalogues in CURSA's default list of remote on-line catalogues (see Table  and Section ). You can either use one of these or prepare your own. Thus, the sequence for preparing a customised finding chart is:

1. obtain or prepare a graphics translation file,

2. run catchartrn to add the extra columns defining how the objects are to be plotted,

3. run catchart to plot the finding chart.

Often you will use the same graphics translation file for different finding charts plotted from the same catalogue, or even from different catalogues. Usually you will need some knowledge of the columns in the target list in order to construct a graphics translation file. For example, you would need to know the name of the column containing magnitude if you wished to scale the symbols on magnitude. You can, of course, examine the target list using xcatview (see Section ), catview (see Section ) or catheader (see Section ).

The following sections describe how to run catchartrn, give a brief, tutorial introduction to the graphics translation file, and finally document the file format in full. Creating a graphics translation file is usually straightforward, particularly if you use one of the examples as a starting point, and the tutorial will probably give enough information to allow you to create your own. You will probably only need to read the full description if you want to create more complex effects.