Cosmic Origins Spectrograph Instrument Handbook for Cycle 17

2.3 Should I Use COS or STIS?

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Current plans for SM4 include both the installation of COS and the repair of STIS. While the success of these operations and the actual performance of each instrument during Cycle 17 cannot be known in advance, proposers should assume that both instruments will be available, and that HST will thus have two spectrographs with significant overlap in spectral range and resolving power. However, despite these similarities, each instrument has its unique strengths and the decision about which to use will be driven by science goals of the program and the nature of the target to be observed.

The primary design goal of COS is to improve the sensitivity to point sources in the far-UV (from about 1100 to 1800 Å). 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 Å 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.

In the near-UV (approximately 1700 - 3200 Å), COS and STIS have complementary capabilities, and the choice of instrument should be guided by the specific science requirements of an individual program. To accommodate the NUV detector format, the NUV spectrum of COS is split into three non-contiguous sub-spectra, each of which covers a relatively small range in wavelength. Obtaining a full spectrum of an object in the near-UV requires several separate set-ups and exposures (6 or more for the medium-resolution gratings and 4 for G230L). When broad near-UV wavelength coverage is needed, there will be circumstances when obtaining a single STIS spectrum is more efficient than taking separate COS spectra. However, the background count rate for COS/NUV is expected to be substantially lower than for STIS (by a factor of about four) so that COS will often be superior for very faint sources, even when more exposures are required. Observers are advised to perform detailed calculations using both the COS and STIS ETCs and to carefully consider the relative instrument overheads in order to decide which combination of instruments and modes is best for their particular science program.

In deciding which instrument to use to observe extended sources, the spatial resolution offered by STIS must be weighed against the superior sensitivity of COS. One of the primary design goals of STIS was to provide spatially-resolved spectra in the UV, optical, and near-IR. The STIS long slits, when used with the 1st order gratings, allow spatially-resolved observations that exploit the intrinsically high resolution of HST over the full width of the detectors (approximately 0.05 arcsec per 2-pixel spatial resolution element over a length of 25 arcsec with the NUV and FUV MAMAs, and ~ 0.1 arcsec per 2-pixel spatial resolution element over a length of 52 arcsec with the CCD). COS was optimized for point-source observations, and this results in some compromises when observing extended sources. COS has relatively large entrance apertures (2.5 arcsec diameter), which are significantly vignetted for any flux off-center by more than 0.5 arcsec. These large apertures also mean that objects extended in the dispersion direction will result in degraded spectral resolution. In addition, the optical design of the FUV channel provides intrinsic limits to the achievable spatial resolution, making it impossible to separate multiple point sources in the aperture unless they are separated by about 1 arcsec in the cross dispersion direction. The COS NUV channel uses a different optical design, and has spatial resolution comparable to that of STIS first-order NUV modes (~ 0.05 arcsec), with somewhat better sampling; however, for sources more than 1 arcsec in extent in the spatial direction, the different NUV spectral segments will begin to overlap.

Both COS detectors and the STIS MAMA detectors are prohibited from observing objects that exceed specific brightness levels (see Section 11.5 in this handbook and Sections 13.8 and 14.8 of the STIS Instrument Handbook). Some brightness limits have been established for the health and safety of the instrument, while others are practical limits that are set to ensure good data quality. Because STIS is less sensitive than COS the brightness limits for STIS tend to be significantly less stringent. In the NUV range, the STIS G230LB and G230MB gratings can also be used with the STIS CCD, which has no bright object limitations. STIS also has a number of small and neutral density apertures that can be used with the MAMA detectors to attenuate the light of a too-bright object. COS has only a single neutral density filter which attenuates by a factor of about 200, but which also degrades the spectral resolution by a factor of 3 to 5. In most cases, some combination of STIS gratings and apertures will be a better choice for observing a UV-bright object than using COS with its neutral density aperture would be. Users are advised to compare results from the COS and STIS ETCs to decide on an appropriate strategy for their target.

The STIS high dispersion echelle modes E140H and E230H have resolving powers of ~114,000, significantly higher than the best COS resolution. Also, STIS can obtain spectra in the optical and near-IR at wavelengths up to 10,200 Å, while the maximum wavelength observable by COS is about 3,200 Å.

Both STIS and COS can perform observations in TIME-TAG mode, where the time of each photon's arrival is recorded. STIS is capable of a much finer time resolution (125 microseconds vs. 32 milliseconds for COS), although few programs are expected to require such a high sampling rate. Due to its lower sensitivity, STIS may also sometimes be able to observe a target in TIME-TAG mode that is too bright for TIME-TAG observations with COS. Also, the TIME-TAG data acquired with COS includes information on the pulse-height distribution, while STIS NUV MAMA and COS NUV MAMA observations do not. The pulse-height information can be valuable in identifying and rejecting background counts in faint source spectra.