The grism chain searches for those files in the current ODF whose FILTER keyword is set to Grism1 or Grism2. If such files (grism images) are found, omgchain calls a number of OM-specific SAS tasks in order to correct these images for distortion, align them with respect to the pixel grid (for spectral extraction optimisation), and identify and extract spectra of sources for which both the zero- and first orders lie within the OSW. Where no grism images exist or when there are no usable spectra in the OSW, omgchain finishes smoothly, generating a warning message. Thus, running omgchain, the user does not have to specify input or output files in the task parameter list, although a number of parameters may be set by the user (omgchain has no mandatory parameters). Omgchain will attempt to extract a spectrum from a default region specifically related to the location of the target object (based on the specified target coordinates in the ODF files), even when no corresponding spectrum is identified by the automated detection.
