The orders can be straightened by performing a 2D geometric correction to the frame using CDIST. If the repeated exposures are in the same place on the slit, exposures of a single object and spectral region can be co-added prior to application of CDIST. However, beware of atmospheric dispersion which can move the bluer orders by different amounts than the redder orders.
Having corrected the distortions, ECHSELECT can be run to identify, for each order, the rows to be used for the object and for sky. The result is a `collapsed echellogram' where X is wavelength and Y is order number. The object image gives rise to both an object and a sky echellogram.
For quick--look reductions, a mask extraction can be done instead. This process is described first: