Subsections

• 3.2.1 Up-Down scans
• 3.2.2 Reference scans

3.2 AOT Overview

There were two main observing modes with the SWS.

1. Grating-Only Observations (Medium spectral resolution) - SWS01, SWS02 and SWS06
   The spectral region of interest was directed to an array of 12 detectors. The spectral range instantaneously covered by the array was about 8 grating resolution elements, with gaps between the individual detectors (except for Band 3). For an observation, the grating was 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 were 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) - SWS07
   The wavelength was selected by setting the Fabry-Pérot (FP) unit to a gap where one of the transmitting orders matched the correct wavelength. The FP unit was then scanned in steps of about 1/4 FP resolution element. The grating was used as an order sorter, with the maximum of the grating transmission tuned to the desired wavelength so that unwanted FP orders were suppressed. Because the two detectors of a FP detector pair look at slightly different grating resolution elements, this tuning could normally be done only for one detector. The other detector would not simultaneously deliver a useful signal as use of this redundant detector would require commanding of a slightly different grating position.

Figure 3.1 gives an overview of the wavelength ranges that could be simultaneously covered by the SW and LW or FP detector systems.

Figure 3.1: Overview of possibilities for simultaneous use of LW grating and SW grating (top) and FP and SW grating (bottom). Only combinations within the hatched area were allowed, and simultaneous observations through aperture 3 when observing with the FP were not allowed. The corresponding band designations and aperture numbers are indicated.

During an AOT observation, the source spectrum was measured by rotating the scanning mirror of the grating or varying the gap of the Fabry-Pérot (FP). Each AOT may have done one or more exposures with an internal calibrator for a photometric check and a number of dark current measurements with the aperture closed. Hence, the data produced by any single AOT was generally of several different types.

Four AOTs, listed below, were defined for SWS. Operating modes changed during the mission, and any such changes are noted in Appendix C.

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SWS01: a single up-down scan for each aperture. It had four possible scan speeds and was primarily intended to quickly scan the entire SWS wavelength range at degraded resolution.

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SWS02: slow short up-down scans, designed to measure line profiles.

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SWS06: medium length grating scans with various options. Programmed as long up-down scan, possibly with reference scan. Used to observe long wavelength regions at full resolution.

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SWS07: LW section devoted to FP observations. The SW section did almost the same as in SWS06. In this AOT priority was given to the FP.

Example outputs from all AOTs are given in Sections 3.3 to 3.6. For all AOTs the measurements of photometric check, dark current and science data are interleaved in the ERD and SPD, while the AAR only contains science detector data for which a unique wavelength is assigned to the light falling on that detector.

3.2.1 Up-Down scans

All grating observations employed what were known as up-down scans. In these, the grating initially scans a wavelength range in one direction, then reverses direction to cover the same wavelength region again. e.g. the scanner may start at 35m, scan down to 30m and then go back up to 35m. Having two scans, in different directions, enables a discrimination to be made between real spectral structure and detector memory effects, as the latter can mimic the former. They are called up-down scans, rather than down-up scans, as it is the scanner step number (aka LVDT) that increases then decreases, and wavelength is inversely correlated with LVDT. Note that `up-down scans' only refer to what the grating was doing. For FP observations both the grating and the FP always increased in position, hence decreased in wavelength, with time. An example of an up-down scan is shown in Figure 3.2, taken from an SWS01 observation.

For observations of bright sources ( 100 Jy) the uncalibrated fluxes seen in bands 2 and 4 in the up-scan can differ by 20% compared to the that seen in the down-scan due to memory effects - see Sections 9.2 and 5.7.2. The other grating bands did not suffer from memory effects.

3.2.2 Reference scans

SWS06 and SWS07 observations planned before July 10, 1997 employed what was known as 'reference scans'. These were supposed to enable a determination of the detector drifts caused by memory effects. The method used, was for the grating to occasionally switch back to a fixed grating position, and hence wavelength. Any changes in the detector response at this wavelength over the time scale of the observation were due to detector memory effects, which could therefore be calibrated out. These reference scans were only present in long AOTs.

However, no proper method based on the reference scans could be devised to correct the memory effects while on the other hand a good method was found for band 2 using the Fouks-Schubert model. Furthermore, it was found that including such reference scans actually harmed observations due to the interplay of reference scans and memory effects, described in Section 9.2.3. The time span between reference scans was therefore increased from observations planned from July 10, 1997 such that most SWS06's did not suffer from them.