2.5 Choosing Between COS and STIS

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.

2.5.1 Sensitivity and Wavelength Coverage

The primary design goal of COS was to improve the sensitivity of HST to point sources in the far-ultraviolet (FUV; 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 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 Å.

In the NUV (approximately 1700–3200 Å), COS and STIS have complementary capabilities. To accommodate the NUV detector format, the COS NUV spectrum is split into three non-contiguous stripes, each of which covers a relatively small range in wavelength. Obtaining a full NUV spectrum of an object requires several set-ups and exposures (6 or more for the medium-resolution gratings and 4 for G230L). When broad NUV wavelength coverage is needed, there will be circumstances in which obtaining a single STIS spectrum is more efficient than taking separate COS spectra.

Historically, the dark rate of the COS NUV detector was substantially lower than that of STIS, making it superior for faint sources, even when more exposures were required to achieve full wavelength coverage. However, the dark rate of the COS and STIS NUV detectors have recently converged to similar rates. The COS global mean dark rate has leveled off since its launch to a value of about 0.001 counts/s/pixel [link]. Meanwhile, the mean dark rate of the STIS NUV-MAMA reached a minimum near 0.0015 counts/pixel/s around 2016, increased to a maximum of about 0.002 counts/pix/s around 2022, and is currently decreasing with recent values back around 0.0011 counts/pix/s, which is the current value used by in ETC calculations [link]. Observers are advised to perform detailed calculations using both the COS and STIS ETCs to consider carefully the relative instrument overheads to determine which combination of instruments and modes is best for their science.

2.5.2 Spatial and Spectral Resolution

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).

COS was optimized for point-source observations. While COS has relatively large entrance apertures (2.5 arcseconds diameter), flux from regions more than 0.5 arcseconds from the aperture center is significantly vignetted. These large apertures also mean that objects extended in the dispersion direction will yield spectra with lower spectral resolution. In addition, the optical design of the FUV channel limits the achievable spatial resolution, showing FWHM in the spatial direction that vary between ~0.25 arcseconds and ~1.5 arcseconds depending on grating and wavelength. The COS NUV channel uses a different optical design and has a spatial resolution comparable to that of the STIS first-order NUV modes (~0.05 arcseconds), with somewhat better sampling; however, for sources extending more than 1 arcsecond in the spatial direction, the various NUV spectral segments will begin to overlap.

The line-spread functions (LSFs) of both instruments exhibit non-Gaussian wings due to mid-frequency zonal (polishing) errors in the Optical Telescope Assembly (OTA). Using STIS, one can minimize their effects through the use of narrow apertures. On COS, narrow apertures are not available. The broad wings of the COS LSF, especially in the short wavelengths of FUV band, can affect the detectability of faint, narrow features and blend closely spaced lines. Studies that require accurate knowledge of the line profile will require full consideration of the COS LSF. The non-Gaussian wings of the COS LSF should have only modest impact on science programs targeting broad lines and continuum sources.

The STIS high-dispersion echelle modes E140H and E230H have resolving powers of ~114,000 (or even R ~ 200,000 with the 0.1 × 0.03 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 Å, while the maximum wavelength observable by COS is about 3200 Å.

2.5.3 Brightness Limits and TIME-TAG Mode

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.

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.

2.5.4 Other Possible Considerations

Observers should also note that many COS NUV modes are only sparsely used, and the corresponding STIS NUV modes are likely to have more extensive calibration.

Another consideration for COS FUV usage is lifetime impact. For the COS FUV detectors, approximately 27,000 counts can fall on a pixel before the reduced gain at that location results in significant flux loss. In some cases, STIS may be the preferred instrument for relatively bright sources simply to preserve the lifetime of the COS detectors. The COS FUV also suffers from gain sag, requiring periodic changes in where on the detector spectra are placed (the Lifetime Positions). Users should consult with the COS Documentation for the latest policies and up-to-date information on the calibration of the various Lifetime Positions.