Ratio object spectra to comp stars, combined appropriately to match object airmasses.
For a single comp star, obviously you just get a simple ratio - no real airmass correction. For two or more comp stars, an extinction is derived, and used to correct each comp star spectrum to the object airmass, prior to averaging the comp stars and performing the ratio. Array of object mid-point exposure times. Array of comp star mid-point exposure times. Array of badflags for the object spectra.