3.5 SWS06 - Wavelength Range Scan

SWS06 was used to make observations over a wider wavelength range, as opposed to observing individual lines. Up to 64 wavelength ranges could be chosen anywhere in the SWS grating range, i.e., between 2.38 and 45.2m, under the restriction that each range falls within one of the bands (see Table 2.1).

SWS06 was executed by pointing the telescope to the target position, and then for each wavelength range (or pair of merged ranges) a dark-current measurement was done. If the spectral scan was long enough to need it, a measurement on the reference wavelength was performed. The wavelength range specified in the AOT was observed. Each range was scanned back and forth to provide redundancy. The reference measurement (if any) was repeated after which again a dark current was done. Very long observations were interrupted by wavelength switching to the reference wavelength, as an additional photometric check. If a single up and down scan did not provide enough sensitivity, the measurement sequence was repeated as often as necessary. Photometric checks using the internal calibrator were done at regular intervals, at least once per AOT.

Table 3.3 shows the reset interval, dwell times, stepsize and number of up-down scans for SWS06. The dwell time stops at 4 secs, and for higher sensitivity the scans were repeated. This was not always the case. Earlier versions of the logic (prior to May 1996, see Appendix C) had the dwell time increasing.

Table 3.3: SWS06 parameters

Detector	reset	dwell	stepsize	Number of
band	interval	time	LVDT	up-down
	[s]	[s]		scans
1	1, 2, 4	reset	1, 2, 4	n
2, 3	1, 2	reset	1, 2, 4	n
4	1, 2	reset	1, 2, 3, 6	n

Example of an SWS06 observation:

As an example, an SWS06 observation around 10, 12, 33 and 40m had the following timeline:

1. Acquisition - point aperture 1 to target
2. Dark current measurement (16 sec)
3. Scan up around 12m (24 sec, fast)
4. Scan down around 12m (24 sec, fast)
5. Dark current measurement (16 sec)
6. Switch to aperture 3 (10 sec)
7. Dark current measurement (16 sec)
8. Reference scans near 10 and 33m (16 sec)
9. Scans around 10 and 33m, up (360 sec)
10. Reference scans near 10 and 33m (16 sec)
11. Scans around 10 and 33m, up, continued (360 sec)
12. Reference scans near 10 and 33m (16 sec)
13. Scans around 10 and 33m, down (360 sec)
14. Reference scans near 10 and 33m (16 sec)
15. Scans around 10 and 33m, down, continued (360 sec)
16. Reference scans near 10 and 33m (16 sec)
17. Dark current measurement (16 sec)
18. Dark current measurement (16 sec)
19. Reference scans near 10 and 40m (16 sec)
20. Scans around 10 and 40m, up (96 sec)
21. Scans around 10 and 40m, down (96 sec)
22. Reference scans near 10 and 40m (16 sec)
23. Dark current measurement (16 sec)
24. Internal photometric calibration (40 sec)

Figure 3.9 shows the wavelength layout and grating positions in both grating sections. Figure 3.10 shows the SPD signals, status data, grating position and wavelength for detectors 1, 13, 25 and 37 during an SWS06 observation. The SPD data is in V/s. The periods when apertures 1, 2 and 3 are used, times of dark current measurements and photometric checks are indicated.

Figure 3.11 shows the AAR, flux (in Jy) against wavelength, for the same observation (upper panel). What can be achieved after further processing of such data in either ISAP or OSIA is shown in the lower panel.

Figure 3.9: Example of the wavelength layout during an SWS06 observation. See Figure 3.2 for further explanations.

Figure 3.10: SPD data (in V/s) for the first detector of each grating band for an AOT SWS06. Note the symmetric patterns introduced by the up-down scans. Aperture colour codes are the same as in Figure 3.2.

Figure 3.11: In the upper panel the AAR data from all detectors for an SWS06 observation is shown. In the lower panel it is shown what can be achieved with these SWS06 OLP data after -clipping, flat-fielding, defringing in all bands except band 4 and rebinning.