The reduction session consists of the following steps:
Flat-field correction is optional as well as response curve correction, but instrumental effects show up as large variations on varying scales. There are two possibilities: either you have a standard star (STD) or you don't. If you did not take a STD exposure, because you were not interested in an absolute or relative flux calibration, you will now have serious problems in producing an acceptable spectrum. There are correction methods for the instrumental response with no standard star but they do not always provide a satisfactory solution and are time consuming.
A summary of the different operations performed by commands RESPONSE/ECHELLE and REDUCE/ECHELLE is provided in D.13.