3.2 SWS

3.2.1 The Instrument

 

In brief, SWS is composed of two grating  systems, one covering the short wavelength (SW) range, 2.38 to 12.0 (or 13.16 when used with the Fabry-Pérot), the other the long wavelength (LW) range, 12 to 45.2 . These operate independently with the proviso that they must both observe through the same aperture . The option exists to redirect light from the LW grating into a Fabry-Pérot (FP) system to increase the spectral resolution , in which case light falls on one of two FP subsystems.

The wavelength coverage of the SWS is broken down into 12 grating bands and 5 FP bands. For historical reasons they are named Astronomical Observation Templates (AOT) bands , and are listed in table 3.1. The AOT bands are combinations of detector array, aperture and grating orders such that the relevant detector  array sees an unique order. Other detector arrays may see only one order or may see a mixture of orders. The wavelengths of the SWS data products will not be on an equidistant wavelength grid and there may be gaps over certain wavelength ranges. There may also be flux jumps between spectra taken in adjacent AOT bands.

        

band order aperture detector detector wavelength

filter type number

1A 4 1 14-20 InSb 1 - 12 2.38 - 2.60 1870 - 2110 756

SW-gr 1B 3 1 t 14-20 InSb 1 - 12 2.60 - 3.02 1470 - 1750 1043

SW-gr 1D 3 2 t LiF 14-20 InSb 1 - 12 3.02 - 3.52 1750 - 2150 1282

SW-gr 2 2 t LiF 14-20 InSb 1 - 12 3.52 - 4.08 1290 - 1540 867

SW-gr 2 2 t LiF 14-20 Si:Ga 13 - 24 4.08 - 5.30 1540 - 2130 2115

SW-gr 2B 1 2 t LiF 14-20 Si:Ga 13 - 24 5.30 - 7.00 930 - 1250 1377

SW-gr 1 3 t 14-20 Si:Ga 13 - 24 7.00 - 12.0 1250 - 2450 4276

3A 2 1 14-27 Si:As 25 - 36 12.0 - 16.5 1250 - 1760 2047

LW-gr 3C 2 2 r LiF 14-27 Si:As 25 - 36 16.5 - 19.5 1760 - 2380 1879

LW-gr 3D 1 2 r LiF 14-27 Si:As 25 - 36 19.5 - 27.5 980 - 1270 2524

LW-gr 3E 1 3 r 20-27 Si:As 25 - 36 27.5 - 29.0 1300 500

LW-gr 4 1 3 r 20-33 Ge:Be 37 - 48 29.0 - 45.2 1020 - 1630 4324

F-P1 5A 3 1 r 10-39 Si:Sb 11.4 - 12.2 20600 - 24000

F-P1 5B 2 1 r 10-39 Si:Sb 12.2 - 16.0 24000 - 32000

F-P1 5C 2 2 r LiF 10-39 Si:Sb 16.0 - 19.0 32000 - 34500

F-P1 5D 1 2 r LiF 10-39 Si:Sb 19.0 - 26.0 34500 - 35500

F-P2 6 1 r 17-40 Ge:Be 26.0 - 44.5 29000 - 31000

 

Notes
1. SW = short-wavelength region
2. LW = long-wavelength region
3. t = transmission
4. r = reflection
5. `Aperture area' refers to the dimensions of the SWS entrance apertures projected on the sky
6. The resolution given is that obtained when observing an extended source  
7. = total number of scan steps in AOT band
8. AOTs SWS06 and SWS07 (SW grating) use finite (but small) band overlaps
9. Band 2C ends at 13.16 for AOT SWS07.
10. Detector 49 is used for FP observations in band 5 .
11. Detector 51 is used for FP observations in band 6 .
12. These observations are made through the virtual aperture 4  (see section 4.5) but are flagged in the data as being through aperture 3.
13. The band 1E/2A limit changed from 4.05 to 4.08 after the end of PV.

The SW and LW gratings are associated with 4 different detector arrays of 12 elements each. The 2 SWS Fabry-Pérots are associated with 2 double detectors. All detectors are operated simultaneously. The correspondence between detector number, bands and detector material is shown in table 3.1. At any given time, the astronomical source of interest to the observer is centered into one and only one of the three SWS apertures , feeding light on many, or all, of the 52 detector elements via two rotatable scanning mirrors, one for each grating system. While it is possible to find scanner settings such that all of the detector arrays will contain valid data, the instrument will normally be operated such that only one or two of the detector arrays will receive one order of the grating spectrometer, the other four or five receiving a mixture of orders. The observer will, however, receive data from all 52 detectors.

As photons fall onto the detectors the current generated charges up a capacitor. The voltage across this capacitor is read-out and digitised into a bit value 24 times a second, i.e. the SWS detectors are read-out every 1/24 second. At a set interval the capacitor is discharged (basically short-circuited). The time interval between two detector resets  is automatically determined from the user's input data (flux and SNR, or execution speed) and is either 1, 2 or 4 seconds. To observe faint sources, integrations over several reset intervals are used. Raw data is saved in an Edited Raw Data (ERD ) file, with an ERD file for one second consisting of an array of 52 ramps of 24 read-out's each. The Standard Processed Data (SPD ) and Auto-Analysis Results (AAR ) files contain for each detector a single flux estimate for each reset interval. As mentioned previously, normally only a subset of the array will contain clearly interpretable data.

Examples of ERD, SPD and AAR for each AOT are shown in sections 4.2 to 4.5.

3.2.2 Entrance apertures and spacecraft axis

     

The three entrance apertures are aligned along in z-y plane of the spacecraft.

z-axis 

(sometimes called n), parallel to the spacecraft z-axis, is the dispersion direction.

y-axis 

(sometimes called m), parallel to the spacecraft y-axis, is the non-dispersion, or cross-dispersion, direction.

The size of the aperture in the y-axis is either 20, 27 or 33 arcseconds, and in the z-axis it is either 14 or 20 arcseconds.

To derive the position angle of an aperture information on the spacecrafts position on the sky must be used. This can be found in the header keywords INSTRA , INSTDEC , INSTROLL  found, for example, in the AAR. Specifically, INSTROLL is the angle, measured anticlockwise, between north and the spacecraft z-axis (ref. section 3.5 ISO DUM  V2.2). See also section 5.7.