The interface for setting parameter values in the main window is quite straight forward. For most parameters, you simply type values into the appropriate text entry box. For others, you select a value from a menu. That's all fine and dandy, but imagine the situation where you have several dozen data groups imported into ATHENA and you decide that you need to change the value of the «rbkg» parameter for every group. It would be extremely tedious to manually change the parameter value for each data group one by one. Fortunately there is a better way.
In this chapter, we will see the various tools ATHENA provides for constraining parameter values across data groups. We will also look in detail at how the «e0» parameter is determined and how parameter defaults are set.