All source code resides in oyster/source, subdirectories common, idl, and c, as well as the directories for the HDS, NOVAS, FITSIO, and WD code libraries. A Makefile is used to compile the C code, whereas a command file,, is used to compile the scripts. Note that the compilation order in has some significance, as functions in general have to be compiled first, that is before they are used in any procedure. In the GUI source code directory idl, type idl c to start up IDL and have it compile all procedures and functions. The command file will also save the compiled code to oyster.cpr and the common block definitions to oyster.cmb. If you would like to change the way a procedure works in your $\cal OYST\!ER\,$session, make a copy of the source code (call it, for example), modify it, and recompile it using .run For your current session, this compiled new code will replace the previous one. It is a good idea to place modified routines into a file and compile them after startup.