These showed that broad O VI emission responds immediately to continuum changes, to within a resolution of 1-2 days. The saturated broad absorption lines such as Ly and O VI show little response to continuum changes, presumably because the changes in column density required to produce noticeable changes in equivalent width are larger than those induced by the varying continuum. While it probes a zone in the BLR similar in ionization to He II, it is much brighter than He II 1640. For studies of warm absorbers, the O VI lines and the higher-order Lyman lines offer several advantages, as noted earlier. Observing in this bandpass requires some observational trade-offs, however.
Good throughput requires LiF coatings on the optics and open-window detectors.
Both of these features entail significant costs in development and in contamination management throughout the instrument development and integration process.