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(usually major) axis. If no "siz2" is supplied, you just get a circle of the specified size. If both a "siz2" and a "pa" (position angle) field are given, then the ellipse is tilted at that angle. (Take a look at the UCAS Galaxies in CD_DATA2.TDF for an example of how this works.) 20a: Defining your own custom .TDF symbols Using a pre-defined type is easy. But you can also create your own symbols. To do this, you need to alter the 'type' line in a .TDF. An example is: ~dtype sc15;e0,0,32;c1;E-20,-20,12;f3;-10,-5;10,-5;0,10;c2;m-45,0;l45,0;c14; The 's' stands for 'symbol', and tells Guide you aren't using the usual pre-defined symbol types. Following are commands separated by semicolons: ~dc15; means set color 15 (white) ~de0,0,32; means draw a 32-unit circle centered at (0,0); ~dc1; means set color 1 (green) ~dE-20,-20,12; means draw a 12-unit _filled_ circle at (-20,-20); ~df3;10,15;30,15;20,30; means draw a 3-point _filled_ object (a triangle) ~d connecting (10,15) to (30,15) to (20,30) ~dc2; means set color 2 (brown) ~dm-45,0 means move to (-45,0); ~dl45,0 means draw a line connecting to (45,0); ~dc14; means set color 14 (light gray) The symbols are scaled just like the standard, predefined types; large objects are drawn with scaled-up symbols, just as large (for example) globular clusters are drawn with larger circles. The unit of measurement in the above commands is 1/32 of an object radius. For example, the 32-unit circle drawn above would exactly match the size of the object. By setting the color to light gray at the end, we make sure that the label for this object is in light gray. Here's a more practical example. Suppose you want to show a catalog of gamma ray burst events with radiation symbols: three triangles in light blue, with orange dots in the center. ~dtype sc3;f6;-32,0;32,0;16,26;-16,-26;16,-26;-16,26;c2;E0,0,15;c14; The three triangles are drawn as one six-point fill (that's the 'f6;' part). Then the color is set to 2 (orange) and a dot is drawn. Finally, the color is reset to 14 (light gray) for the label. The 'align' keyword is the sum of a number for the horizontal alignment (0 for left, 1 for center, 2 for right) and a number for the vertical alignment (0 for top, 16 for center, 32 for bottom). By
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