COS offers dramatic improvement in sensitivity to faint objects over previous UV spectroscopic instruments flown aboard HST. COS achieves high sensitivity, particularly in the far-UV (FUV), by minimizing the number of reflections, which leads to an inherently simple spectrograph design. Because the unique capabilities of COS derive from a fundamentally different design approach, and not from new technology, we are able to provide an instrument with flight heritage in all of its critical areas, such as optics, detectors, and electronics.
In designing COS, we have assumed that STIS will continue to work well. COS is not intended to duplicate the powerful capabilities of STIS for observing bright or extended sources. COS does not have many ``bells and whistles,'' but the capabilities of COS are unique. The order-of-magnitude gains in UV sensitivity over STIS and previous UV spectrographs will open a huge volume of discovery space. Researchers will be able to obtain moderate resolution spectra of objects down to unprecedented flux levels, drawing on the new target lists compiled by UV imaging telescopes, such as UIT and GALEX.