Cosmic Origins Spectrograph Instrument Handbook for Cycle 17

2.6 Should I Use STIS instead of COS?

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Current plans for SM4 call for the astronauts to carry out a replacement of STIS' failed power supply so that STIS' capabilities can be restored. Proposers for Cycle 17 should assume that both COS and STIS will be operational. In that environment both STIS and COS can be used for spectroscopic observations in the ultraviolet, so which is to be preferred? Here are some key differences to help you decide.

Point sources versus extended or structured objects

COS was designed and built to excel at obtaining ultraviolet spectra of point sources. COS is especially efficient at doing so in the far-UV, below about 2000  Å. Because of its design, COS is not at all suited to obtaining spectra of extended objects or when more than one point source is in the aperture. It is still possible to obtain data under such circumstances, of course, but the analysis of those data will be complex and not within the capabilities of the pipeline software.

In general, COS may be used effectively for single point sources, but other types of targets should use STIS.

Bright objects

Having photon-counting detectors, COS cannot observe objects that are too bright. 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.

The count rate limits for safety reasons are both local and global. In other words, the total (global) count rate over the entire detector cannot exceed a set limit without causing COS to shut down. At the same time, the count rate within any pixel may not exceed the local rate limit. These limits are discussed further below (see Section 8.2.1, "Limiting magnitudes and bright object limits," on page 104) and are included in the COS Exposure Time Calculator (ETC). Note that the same count rate limits apply to all uses of COS, either for imaging and acquisitions, or for spectroscopy.

In general, we strongly recommend use of TIME-TAG mode with COS whenever possible because you will then obtain observations of consistently high quality for several reasons explained below. However, in some cases targets may be safe to observe with COS but will produce counts at a rate that exceeds TIME-TAG capabilities. In such cases you will need to use ACCUM mode, and again the relevant parameters are encoded in the COS ETC.

In some cases, COS can be used to observe bright targets if they are placed in the Bright Object Aperture. The BOA includes a neutral density filter that attenuates by a factor of approximately 200, but the BOA also degrades the image, thereby reducing spectroscopic resolution by a factor of 3 to 5. We anticipate that in almost all cases in which an object is too bright for COS that an observer will prefer to use STIS. Eschew the BOA.

Far-UV spectroscopy

In the far-ultraviolet (from about 1100 to 2000 Å), COS is more sensitive than STIS by factors of 10 to 30. However, COS offers only two spectroscopic resolving powers, 2,500 and 20,000, while STIS also offers a high resolution (R = 100,000) echelle mode and some other options as well.

Near-ultraviolet spectroscopy

In the near-UV, COS' gain over STIS is more modest (factors of 2 to 3) and there are compromises to deal with. In particular, COS' near-UV capability was added after the first design of the instrument. To accommodate the NUV channel and its optics, the NUV spectrum of COS is split into three non-contiguous sub-spectra. Obtaining a full spectrum of an object in the near-UV then requires several separate set-ups and exposures, and it is just coincidental if a single set-up contains all the spectroscopic features of astrophysical interest to the observer.

In other words, at any one near-UV wavelength, COS is more sensitive than STIS, but STIS is probably to be preferred for obtaining a broad-band near-UV spectrum. On the other hand, COS' background count rate is substantially lower than for STIS so that COS is superior for extremely faint sources.

As with the far-UV, in the near-UV STIS offers more capabilities than does COS, including a high resolution (R = 100,000) echelle mode.