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4.5 AOT 7

The purpose of AOT 7 was to make Fabry-Pérot observations. If requested, Short Wave grating observations were made in parallel.

For the FP the stepsize is always two FP units, about 1/4 of an FP wavelength resolution. Reset intervals are 1 or 2 seconds, with the dwell time equal to the reset interval. The number of scans is greater than, or equal to, three, and only ``up'' grating scans are commanded.

Any requested SW grating observations were carried out in an identical manner to those of an SWS06 measurement. Users should read section 4.4 for a description of that.

Due to a slight internal misalignment of the Long Wave FP there are problems merging FP observations with grating observations when observing through aperture 3. The `effective' aperture of the FP was displaced from the center of the grating aperture by approximately 10 arcseconds, causing an approximately 40% drop in throughput of the FP when pointing is determined by the grating. The solution decided on was to define a fourth `effective' aperture, centered on the FP aperture, and carry out all observations through this. This meant the SW grating lost approximately 40% of its throughput. Therefore LW FP observations (26 - 44 $\mu m$ ) and SW grating observations (7.0 - 13.16 $\mu m$ ) through aperture 3 could not be merged. Any requested aperture 3 grating observations were performed after the FP observations. This change was considered acceptable as the main purpose of AOT 7 was FP observations. Merging was still performed on observations using apertures 1 or 2 .

However, some early AOT 7 observations were carried out before this change was implemented. Observers may therefore receive FP data merged with SW grating observations in the range 7 to 13.16 $\mu m$ .

Note that band 2C ends at 13.16 $\mu m$ for AOT SWS07 - see table 3.2.

4.5.1 Example AOT 7 timeline

The AOT SWS07 was 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.

AOT 7 is achieved by using combinations of ICS's 9 and 4. After the instrument has been set up ICS 9 is used to make a dark-current measurement. Another ICS 9 is then used to start the FP. While the FP is operating ICS 4 is used to make SW grating scans, if any are requested. More than one of these may be uplinked while the FP scan is continuing. To observe more lines with the FP more ICS 9's are uplinked. Again ICS 4's could be uplinked to observe with the SW grating section. After all observations have concluded a final ICS 9 is used to make a dark measurement.

4.5.2 Example AOT 7 data

Figure 4.19 shows ERD, SPD and aperture data for part of an AOT 7. The ERD is shown as signal in bit values against time (ITK), and the SPD as signal (in $\mu V/sec$ ) against time (seconds). While the line is hard to see in the ERD, it can easily be seen in the SPD.

Figure 4.20 shows the LW grating position, the FP position and the resulting wavelength of light falling on detector during the part of an AOT 7.

Figure 4.21 shows SPD signal (in $\mu V/sec$ ) and AAR flux (in Jy), both against wavelength, for the same period of the example AOT 7. Note the wavelength shift in the location of the peak between the SPD and AAR. This is because the SPD wavelength scale is `ISOcentric', whereas the AAR is corrected to be heliocentric.

**Figure 4.19:** ERD, SPD and aperture data for part of an AOT 7
$\begin{figure} \begin{tabular}{c} \centerline{\epsfig{file={sws07_a.eps},width=15.0cm}}\\ \end{tabular}\end{figure}$

**Figure 4.20:** LW grating position, FP position and resulting wavelength for part of an AOT 7

**Figure 4.21:** SPD and AAR data for AOT 7
$\begin{figure} \begin{tabular}{c} \centerline{\epsfig{file={sws07_b.eps},width=15.0cm}}\\ \end{tabular}\end{figure}$

Next: 4.6 AOT behaviour Up: 4. AOT overview Previous: 4.4 AOT 6

SWS Instrument & Data Manual, Issue 1.0, SAI/98-095/Dc