Prime Observing Modes

There are two main observing modes  with the SWS.

1. Grating-Only Observations (Medium spectral resolution ). The spectral region of interest is directed to an array of 12 detectors. The spectral range instantaneously covered by the array is about 8 grating resolution elements, with gaps between the individual detectors   (except for Band 3). For an observation, the grating is scanned  in small steps to fully sample the desired wavelength range and provide a certain redundancy in case of detector failures or detector memory effects . Different scan schemes are used to obtain high fidelity line profiles, scans of wavelength ranges, and quick low-resolution  full scans of the SWS wavelength range.
2. Fabry-Pérot / Grating Combination Observations (High spectral resolution ). The wavelength is selected by setting the Fabry-Pérot unit to a gap where one of the transmitting orders matches the correct wavelength. The F-P unit is then scanned  in steps of about 1/4 F-P resolution element. The grating is used as an order sorter, with the maximum of the grating transmission tuned to the desired wavelength so that unwanted F-P orders are suppressed. Because the two detectors of a F-P detector   pair look at slightly different grating resolution elements, this tuning can normally be done only for one detector. The other detector will not simultaneously deliver a meaningful signal. Use of this redundant detector would require commanding of a slightly different grating position.

A number of factors have to be considered in the selection of the main observing mode . The grating mode offers the only possibility to cover large wavelength ranges, and offers much better sensitivity for continuum sources and broad, well-resolved features. Natural advantages of the high spectral resolution  F-P mode are the abilities to resolve close line groups and to study line profiles not resolved by the grating. A more detailed tradeoff has to be made for the detection of faint lines. For broad lines, and for any line not affected by line crowding or strong underlying continuum, the grating is superior because of its higher transmission. In contrast, the F-P is superior for the detection of narrow emission and absorption lines in the presence of a strong continuum. Here, the increase in line-to-continuum ratio due to the higher spectral resolution   more than compensates for the reduced sensitivity caused by additional elements in the optical path.