Matt Menge writes:

Anyways, it is clear from the stuff after the elipses that these two
devices ultimately perform different functions. But how are they
really different? I mean how much difference can there be between two
devices that separate ultraviolet light into its component
wavelengths. Is the new one simply more sophisticated?


From the STIS instrument handbook, [1]

: The primary design goal of COS is to improve the sensitivity to point
: sources in the far-UV (from about 1100 to 1800 AA). In this wavelength
: range the throughput of the COS FUV channel exceeds that of the STIS
: FUV-MAMA by factors of 10 to 30, and the combination of the
: spectroscopic resolving power (~ 20,000) and wavelength coverage (300
: to 370 AA per setting) of the medium resolution COS FUV modes results
: in a discovery space (throughput times wavelength coverage) for
: observations of faint FUV point sources that is at least 10 times
: larger for most targets than that of STIS modes with comparable
: resolution, and is as much as 70 times greater for faint
: background-limited point sources.

Another obvious difference is that STIS was designed for spatially
resolved spectroscopy, whereas COS is designed for point sources.

Currently, HST has zero UV spectrographs, so one might also consider
STIS and COS backups for each other.

Craig


References
[1] http://www.stsci.edu/hst/stis/docume...entcycle5.html


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Craig B. Markwardt, Ph.D. EMAIL:
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