Subsections

• 4.3.1 Example AOT 2 timeline
• 4.3.2 Example AOT 2 data

4.3 AOT 2

AOT 2 is used to observe up to 64 individual lines in an objects spectrum using the SWS grating mode. The wavelength interval covered when observing a line is approximately 10 resolution elements, i.e., 3,000-1,500 km/s.

Table 4.4 shows the reset intervals, dwell times, stepsize and number of up-down scans for AOT 2. The dwell time may be higher for some merged scans.

The band start wavelengths for AOT 2 are given in table 4.8.

Table 4.8: AOT 2 wavelength ranges

Detector	Detector	Aperture	Grating	Band
Array	material		order	start
1	InSb	1	4	2.38
1	InSb	1	3	2.60
1	InSb	2	3	3.02
1	InSb	2	2	3.52
2	Si:Ga	2	2	4.081
2	Si:Ga	2	1	5.30
2	Si:Ga	3	1	7.00
3	Si:As	1	2	12.0
3	Si:As	2	2	16.5
3	Si:As	2	1	19.5
3	Si:As	3	1	27.5
4	Ge:Be	3	1	29.0

1.

The band 2A start wavelength was changed from 4.05 to 4.08 in May 1996, and hence the band 1A end wavelength was moved accordingly.

4.3.1 Example AOT 2 timeline

AOT 2 is executed the following way:

• The telescope is pointed to the target position.
• A range of 10 resolution elements around each wavelength specified in the AOT is observed. Each range is scanned back and forth to provide redundancy. The minimal scan time is 100 seconds.
• Dark current measurements are only performed before and after the spectral scan.
• A photometric check is performed, using the internal stimulators.

As an example, to observe lines at 10, 12, 35 & 40 AOT 2 would carry out the following sequence:

1. Acquisition - point aperture 1 to target
2. Dark current measurement (20 sec)
3. Scan line at 12, up (50 sec)
4. Scan line at 12, down (50 sec)
5. Dark current measurement (20 sec)
6. Switch to aperture 3 (10 sec)
7. Dark current measurement (20 sec)
8. Simultaneously scan lines at 10 and 40, up (100 sec)
9. Simultaneously scan lines at 10 and 40, down (100 sec)
10. Dark current measurement (20 sec)
11. Simultaneously scan lines at 10 and 35, up (100 sec)
12. Simultaneously scan lines at 10 and 35, down (100 sec)
13. Dark current measurement (20 sec)
14. Internal photometric calibration (40 sec)

This timeline is carried out using chains of ICS 5. Each ICS 5 starts with a dark, then observes a line. ICS 6 is used to make a dark measurement when: the gain changes; the reset interval changes; or the aperture changes. Every approximately 3600 s there is a photometric check. If the entire observation takes less than 3600 s a photometric check is done at the end.

The length of the up and down scans in an AOT 2 may occasionally differ by small amounts. This is due to the runflag going down too early, signifying an early end to the observation.

4.3.2 Example AOT 2 data

Figure 4.9 shows the ERD and SPD from detector 25 as SWS scans across one particular line with an AOT 2. Figure 4.10 shows LW scanner positions, aperture information and wavelength seen by detector 25 during the same period, while 4.11 shows the SPD and AAR data from detector 25 during the same period.

Figure 4.9: ERD and SPD data for part of an AOT 2

Figure 4.10: Scanner position, apertures and wavelength seen by detector 25 during an AOT 2

Figure 4.11: SPD and AAR data from band 3 for AOT 2