A fit combines a BornAgain simulation, a user-defined residual function, and a Python optimizer. BornAgain also provides helpers for preparing residuals and displaying progress; it does not prescribe the optimizer.
Start with the fitting tutorial for a complete script.
This reference explains the purpose and use of each helper. Their
docstrings give exact arguments and defaults: try
help(ba.valid_pixel_residual) or help(ba.FitMonitor) after importing
bornagain as ba.