Description

This AOT allows the user to observe up to 64 different high resolution line profiles , with SWS in the Fabry-Pérot mode. Because of the high spectral resolution of the Fabry-Pérots, SWS07 is an excellent tool for detection of faint narrow lines in the presence of a strong continuum, resolution of close line groups, and for profile studies of lines not resolved by the grating. In contrast, studies of broad features require flux densities much higher than for the grating AOTs, and are very time consuming because of the need to sample many resolution elements.

AOT SWS07 makes use of the SWS capability for simultaneous observations in the two spectrometer sections . In the LW section, F-P line scans are executed, while the SW section is used to do grating scans similar to AOT SWS06. It is recommended to also study the description for AOT SWS06, since not all details will be repeated here.

The basic idea of AOT SWS07 is to use the long integration times typically needed for deep F-P spectra, to simultaneously get almost free grating data for part of the SW grating wavelength range. The detailed timelining of observations will however in most cases induce an extra overhead compared to pure F-P observations. If the observer is interested only in F-P data and does not want to explore the SW grating range, no grating observations for the SW part should be specified.

The gain in observing time obtained with AOT SWS07 critically depends on the selection of the wavelengths observed with both sections. Simultaneous observations are only possible if both the line observed with the F-P and the wavelength range observed with the SW grating use the same input aperture. This restricts the useful combination to those depicted in Fig. 3.1 (p. ). Other combinations, e.g., F-P lines using only aperture 3 and a grating scan using aperture 2, are accepted by the AOT, they will however not result in a gain in observing time. The `merging' process that combines observations in the two spectrometer sections is discussed below in section 6.8.4.

For the F-P part, AOT SWS07 allows the user to observe up to 64 different high resolution  line profiles. The central wavelength(s) may be chosen anywhere in the SWS F-P range (i.e. the nominal range 15-35 m, or, under the restrictions discussed in section 5.2, the extended ranges 11.4-15 m and 35-44.5 m). The scan length is defined by giving radial velocity ranges centered on each of the given wavelengths. The scan-length is limited to larger than 50 km/s and less than 1000 km/s. The spectrum is sampled at intervals of approximately 1/4 resolution element.

Wavelengths are specified by giving the source's heliocentric radial velocity and the vacuum rest wavelength of the line. At the observatory, before execution, a correction of the radial velocity for the projected component of the earth's orbital motion is introduced. This is done to avoid shifts of up to three resolution elements caused by the Earth's motion.

For the grating part, AOT SWS07 allows the user to observe up to 64 different wavelength ranges at medium resolution , similar to AOT SWS06. The wavelength range(s) may however be chosen only in the range covered by the SW grating, i.e. from 2.38 m to 13.16 m. Each range must fall within one of the AOT bands.

The AOT SWS07 is executed the following way:

• The telescope is pointed to the target position.
• The F-P lines and grating wavelength ranges specified in the AOT are observed. Each FP line is scanned at least three times and each SW wavelength range is scanned at least twice to provide redundancy.
• Dark current checks are performed at regular intervals, at least before and after a set of FP scans on the same line and at least once before the reference wavelength of each SW range.
• F-P scans may have to be broken into segments because the end of the mechanical F-P range is reached and the scan has to be continued in a different F-P order.
• With all F-P scans the LW grating is scanned slowly in coordination with the F-P scan to keep the pre-dispersion tuned to the actual wavelength.
• The phasing of the measurements is essentially independent for the two spectrometer sections, at the cost of increased overheads but giving flexibility in combination.
• If the line list contains wavelengths corresponding to different SWS apertures, the aperture is switched accordingly and the satellite re-pointed to bring the source to the center of the new aperture.
• At regular intervals, between observations, photometric checks are performed, at least once per AOT.