2.4 ORIENT constraints for Extended Sources

Detailed instructions on performing COS spectroscopy on extended sources (i.e., sources with FWHM > 0.6") are provided in Section 5.9. Here we remind users that observing extended sources at different position angles can result in changes in the cross-dispersion and dispersion profiles on the COS detector. Such changes can lead to differences in spectral resolution and flux that may prove problematic when combining spectra across visits or performing science on separate spectra taken at different position angles. This can be mitigated by the user by implementing an ORIENT constraint across visits during within the APT file. ORIENT constraints are considered 'Special Requirements' and must be specified by the user within the Phase I proposal document.