AOT 1 scans the entire SWS wavelength range at a reduced resolution . After resetting the instrument, the AOT is composed of three SS0005 ICSs, one for each aperture. The sequence is aperture 1, 2 then 3. Each ICS 5 stars with a dark, then makes the measurement. An ICS 6 (dark) is used to make another dark current measurement after each ICS 5. A photometric check is made between the aperture 1 ICS 6 and aperture 2 ICS 5. From May 1996 an extra photometric check was added at the end of the AOT.
Table 4.3 shows the reset times, dwell times , stepsize and number of up-down scans for AOT 1. The reset time is the time between detector resets, the dwell time is the time between the grating moving and the stepsize, in LVDT , is the amount by which the grating moves every dwell time. An LVDT is an internal unit of measurement of the grating position. A movement of 1 LVDT corresponds to approximately 1/8 of a resolution element. The pipeline resolution degradation is discussed in section 4.2.3.
For an AOT 01 measurement, where the grating moves during a reset interval, the grating position in the SWSPGPOS field of the SPD is set to the start position of the grating during the reset interval. This contrasts with the wavelength in the SWSPWAVE field of the SPD which is the effective wavelength, i.e. calcuated from the grating position taken from the middle of the reset interval.
Figures 4.1 and 4.2 show ERD data for detectors 1, 13, 25 and 37 (bands 1, 2, 3 & 4) during an example AOT 1 observation. The signal is shown in bit values against time (ITK) and only four 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 4.3 and 4.4 show the wavelengths seen by the middle detector of each band during the AOT 1 observation along with the scanner position (LVDT ). The nature of up-down scans can be seen in the plots, ref section 8.3.6.
Figure 4.5 shows SPD data, in , for detector 1 of each band against time. Figure 4.6 shows the AAR for the same observation, now as flux (Jy) against wavelength. The spread of flux between different detectors in the same band can be seen in the band 4 data.
Figure 4.1: ERD band 1 & 2 data for AOT 1
Figure 4.2: ERD band 3 & 4 data for AOT 1
Figure 4.3: Wavelengths seen by bands 1 & 2 during an AOT 1
Figure 4.4: Wavelengths seen by bands 3 & 4 during an AOT 1
Figure 4.5: SPD data from the first detector of each band during an AOT 1. The signals have been arbitrarily scaled.
Figure 4.6: AAR data from all detectors against wavelength
The pipeline resolution degradation, given in table 4.3 is the amount by which the resolution of the spectrometer is degraded after the data has been processed by the pipeline. The pipeline processes data for blocks of length of the reset interval (1 or 2 seconds), during which the grating will have moved several times. The pipeline effectively smears out the resolution. Both theory and observations can be used to characterise the amount of degredation.
Note that section 3.4.3, `AOT 1', of the SWS IDUM V3.0 discusses the resolution expected from the pipeline products of an AOT 1 observation.
The degradation is given by the equation:
For a source filling the slit, the number of steps per resolution element is about 4.5, except for bands 3E and 4 , where it is about 7.0.
Hence the effective resolution of AOT 1 data processed by the pipeline is between a factor of 2 and 7 worse than that expected from an AOT 2 pipeline product.
Preliminary measurements of the resolution of AOT 01 scans at all four speeds were carried out on observation of the PNe NGC 6543 and a speed 4 observations was carried out on the PNe NGC 7027. The assumption was made that the lines from these objects would not be resolved by the SWS01 scans. NGC 7027 contained by far the most lines of the two objects.
The resolution quoted in tables 4.4 and 4.5 is the usual
|Wavelength||Speed 4||Speed 3||Speed 2||Speed 1|
The measurements on NGC 7027 give more reliable results than those on NGC 6543. No systematic difference between the speed 4 resolutions of the two objects could be found.
The speed 4 resolutions vary between approximately 800 and 1700, increasing with wavelength per order. This is approximately half of the AOT 2 resolution, matching expectations. The FWHM remains nearly constant.
We can compare the resolutions of the different speeds using the NGC 6543 observations. The results, which match expectations, are:
R(3) / R(4) = 0.6
R(2) / R(4) = 0.3
R(1) / R(4) 0.3 - 0.4