3.4 SWS02 - Line Scan

SWS02 was optimised for sensitive observations of single lines. It could observe up to 64 individual small wavelength intervals per spectrum (each one covering approximately 10 resolution elements, i.e., 1500-3000 km/s) using the SWS grating mode. Each range was scanned back and forth to provide redundancy with minimal scan time of 100 seconds. Dark current measurements were performed before and after the spectral scan. And a photometric check was performed at the end, using the internal calibrators. In cases of very long observations ( 3600 sec) there were extra photometric checks.

Table 3.2 shows the reset intervals, dwell times, stepsize and number of up-down scans for SWS02. The dwell time may be higher for some merged scans.

Table 3.2: SWS02 parameters

Detector	Reset	Dwell	Stepsize	Number of
band	interval	time	LVDT	up-down
	[s]	[s]		scans
1	1, 2, 4	= reset	1	n
2,3	1, 2	= reset	1	n
4	1, 2	= reset	2	n

Example of an SWS02 observation:

As an example, to observe lines at 10, 12, 35 and 40m the SWS02 carried out the following sequence:

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

Figure 3.6 shows the wavelength coverage in time of an SWS02. The particular SWS02 is quite a short one, performing only 5 scans. As the individual scans are also short, the up-down character of each of them can hardly be distinguished. Figure 3.7 displays the timeline (SPD) of the first detector in each of the 4 grating bands; not much seems to be happening. Figure 3.8 shows only part of the AAR in the overlap region between band 1D and 2A (upper panel). A faint absorption line is visible in band 1D while it is mostly noise in band 2A. In the lower panel it is shown what can be achieved by post-AAR processing.

Figure 3.6: The wavelength layout of an SWS02. A few short scans were planned by the observer. See Figure 3.2 for further explanations.

Figure 3.7: SPD data (in V/s) for an SWS02 as a function of time. See Figure 3.2 for aperture colour codes.

Figure 3.8: AAR data from overlapping scans of bands 1 (black) and 2 (red) for an SWS02 as a function of wavelength. In the upper panel the original AAR data is displayed while in the lower panel it is shown what can be achieved by post-AAR processing. In this case -clipping, flat-fielding and rebinning. No defringing was done in the bands displayed.