OYSTER procedures are called from within the IDL environment. This is done from either the IDL command line by typing in the appropriate commands, or by invoking a widget, which provides buttons to click on for specific action. The latter method is less flexible but has the advantage of not requiring you to memorize all the commands and their parameters. Widgets provide an easy way to set parameter values in common blocks, which is how most of the information is passed between functions. You may use both operation modes simultaneously. Note that widget buttons which bring up another widget are labeled in all caps, whereas the other type either displays a sub-menu (indicated by the arrow) or executes a procedure upon selection. At the command level, you manipulate data using IDL. This was one of the reasons to base OYSTER on a language like IDL or PV-WAVE. Note that neither language is case sensitive. An understanding of the language is very helpful when reducing data with OYSTER.
Procedures usually confirm proper completion of the task with a message terminated with a period. In-progress messages are not terminated. Error messages usually contain the name of the procedure in which the error occurred. In case of a crash inside a procedure under PV-WAVE, click on the message window to make it active, then try to return to successively higher levels by typing .return until a .con command causes IDL to resume. In this case the widgets, if previously displayed, will be updated. Note that in a crash situation, IDL is in a debugging mode with operation stopped at the offending line in the code. You can enquire about variables using the info,variable,/structure command. Under IDL, just type retall and you will be returned to the main level.