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STIS Instrument Handbook for Cycle 24 |
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With the installation of COS and the repair of STIS, HST has two spectrographs with significant overlap in spectral range and resolving power. Each has unique capabilities, and the decision of which to use will be driven by the 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 of HST to point sources in the far-ultraviolet (FUV) (from about 1100 to 1800 е). 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 е 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 greater 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. COS also has the unique capability to observe down to wavelengths as short as 900 е.For observations of 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 ultraviolet (UV), optical, and NIR. The STIS long slits, when used with the first-order gratings, allow spatially-resolved observations that exploit the intrinsically high resolution of HST over the full width of the detectors (approximately 0.05 arcseconds per 2-pixel spatial resolution element over a length of 25 arcseconds with the NUV and FUV MAMAs, and ~ 0.1 arcseconds per 2-pixel spatial resolution element over a length of 52 arcseconds with the CCD).Both COS detectors and the STIS MAMA detectors are prohibited from observing objects that exceed specific brightness levels (see Sections 7.7, 13.8, and 14.8 in this handbook, and Chapter 10 in the COS IHB). 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 that attenuates by a factor of about 200 but 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 COS with its neutral-density aperture. 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 (or even R ~ 200,000 with the 0.1x0.03 aperture and specialized data reduction; see Section 12.6), significantly higher than the best COS resolution. Also, STIS can obtain spectra in the optical and NIR at wavelengths up to 10,200 е, while the maximum wavelength observable by COS is about 3200 е.Both STIS and COS can perform observations in TIME-TAG mode, whereby 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 require such a high sampling rate. Due to its lower sensitivity, STIS may be able to observe a target in TIME-TAG mode that is too bright for TIME-TAG observations with COS. On the other hand, TIME-TAG data acquired with the COS FUV detector includes information on the pulse-height distribution, while TIME-TAG data obtained with the STIS MAMAs and the COS NUV-MAMA do not. Pulse-height information can be valuable in identifying and rejecting background counts in the spectra of faint sources.