4.4 AOT 6

   

AOT 6 is used to make observations over wavelength ranges, as opposed to observing individual lines. Only ICS  5's are used (after the instrument has been set up). The ICS starts with a dark current measurement  (at the reference wavelength), then makes a reference scan . Then the grating(s) is commanded up to observe a wavelength region. Another reference scan occurs before the down scan of the up-down pair . A final reference scan and dark current measurement are then made. This operation is repeated until all wavelength regions have been scanned.

For short scans the reference scans are dropped, as they take too much time. 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.

Table 4.8 shows the reset  times, dwell times , stepsize and number of up-down scans  for AOT 6. The dwell time stops at 4 secs, and for higher sensitivity the scans repeat. This was not always the case. Earlier versions of the logic (prior to May 1996) had the dwell time increasing. The current scanning strategy to reach the desired S/N ratios is as follows:

1

use 1 sec reset with the max step size.

2

if the S/N ratio is insufficient use a reset interval 2 sec.

3

while the S/N ratio is insufficient and the stepsize is greater than 2 halve the stepsize.

4

if the S/N ratio is still insufficient and it is band 1  then use a reset interval 4 sec.

5

if the S/N ratio is still insufficient then increase the number of scans.

 

Detector reset dwell stepsize Number of

band time time LVDT up-down

sec sec scans

1 1, 2, 4 1, 2, 4 n

2, 3 1, 2 1, 2, 4 n

4 1, 2 1, 2, 3, 6 n

 

The wavelength coverage of AOT 6 is given in table 4.9. It is slightly different from that in V2.0 of the Observers Manual. The main change has been the introduction of band 3E , put in because band 3D  suffered from a light leak at the long wavelength end, rendering observations around 28-29 problematical. Note that there is a slight overlap between bands.

 

Detector Detector Aperture Grating Band Band Band

Array material order start End

1 InSb 1 4 1A 2.38 2.61

1 InSb 1 3 1B 2.60 3.03

1 InSb 2 3 1D 3.02 3.53

1 InSb 2 2 1E 3.52

2 Si:Ga 2 2 2A 4.05 5.31

2 Si:Ga 2 1 2B 5.30 7.01

2 Si:Ga 3 1 2C 7.00 12.1

3 Si:As 1 2 3A 12.0 16.6

3 Si:As 2 2 3C 16.5 19.6

3 Si:As 2 1 3D 19.5 27.6

3 Si:As 3 1 3E 27.5 29.0

4 Ge:Be 3 1 4 28.9 45.2

 

1. The band 1E end wavelength was changed to 4.08 from 4.05 from May 1996 onwards.

Figures 4.10 and 4.11 show ERD data from all bands taken during an AOT 6. Data for detectors 1, 13, 25 and 37 (bands 1, 2, 3 & 4) is shown in bit values against time (ITK ). Only five samples are shown out of the 24 per second for clarity. The periods when apertures 1, 2 & 3  are used, times of dark current measurements  and photometric checks  are indicated.

Figures 4.12 and 4.13 show the wavelength as seen by the middle detector of each band, along with the SW and LW scanner position (LVDT ) for the same period. Finally, figures 4.14 and 4.15 show the SPD, in against time, and AAR, flux (in Jy) against wavelength, for the same observation.

  
Figure 4.10: ERD data for detectors 1 and 13 during an AOT 6

  
Figure 4.11: ERD data for detectors 25 and 37 during an AOT 6

  
Figure 4.12: Scanner position and wavelength seen by detector 6 for bands 1 & 2 during an AOT 6

  
Figure 4.13: Scanner position and wavelength seen by detector 6 for bands 3 & 4 during an AOT 6

  
Figure 4.14: SPD data, in , for the first detector of each band during an AOT 6

  
Figure 4.15: AAR data from all detectors for an AOT 6