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Next: 3.3 Entrance Apertures Up: 3. Instrument overview Previous: 3.1 Introduction


3.2 The Instrument

Fig. 3.1 gives an impression of the lay-out of the SWS, an optical block diagram is shown in Fig. 3.2, and an optical schematic is shown in Fig. 3.3.

Light from the telescope is reflected into the SWS by the ISO pyramidal mirror. The SWS had three entrance apertures, each with its own dichroic beamsplitter feeding the Short Wavelength section and the Long Wavelength section. The appropriate entrance aperture was selected by specific pointing of the ISO satellite. A four-position selection mechanism permitted opening of any one of these apertures or blocking of all three - see section 3.3.1 for further details.

Figure 3.1: Lay-out of SWS
\begin{figure} \begin{tabular}{cc} \epsfig{file={inst_laya.eps},width={9.5cm}}& \epsfig{file={inst_layb.eps},width={5.5cm}}\end{tabular}\end{figure}

Each grating had its own scanning mechanism, enabling the use of both parts of the spectrograph at the same time, albeit through the same aperture. Wavelength scanning was achieved by rotating a flat mirror close to each grating in discrete scan steps.

After reflection from the gratings, the light almost retraced its path and, by means of small-diameter re-imaging relay optics, the high resolution spectral image of each wavelength band was re-imaged onto the detector block. These relay optics have various functions:
- they relayed the high resolution image to a more easily-accessible location on the detector block,
- they changed the relative opening of the beam to a value that was optimised to the detector dimensions,
- they enabled efficient straylight rejection.

To use the F-P's, the radiation returning from the LW grating was collimated again, transmitted through a tunable F-P interference filter and imaged onto separate detectors in the detector block. This second light path is also shown in Figure 3.2. The two F-P's were mounted on a single pair of parallel plates. Their separation and parallelism could be varied by changing the currents in three pull coils.


Table 3.1: Detector definition for the SWS AOT bands
  band order aperture area5 detector detector wavelength8
  name number15   filter ('' x '') type number ($\mu m$)
SW1 1A 1 4 1 t3 Al2O3 14-20 InSb 1 - 12 2.38 - 2.60
SW 1B 2 3 1 t Al2O3 14-20 InSb 1 - 12 2.60 - 3.02
SW 1D 3 3 2 t LiF 14-20 InSb 1 - 12 3.02 - 3.52
SW 1E13 4 2 2 t LiF 14-20 InSb 1 - 12 3.52 - 4.08
SW 2A13 5 2 2 t LiF 14-20 Si:Ga 13 - 24 4.08 - 5.30
SW 2B 6 1 2 t LiF 14-20 Si:Ga 13 - 24 5.30 - 7.00
SW 2C9 7 1 3 t SrF2 14-20 Si:Ga 13 - 24 7.00 - 12.0
LW2 3A 9 2 1 r4 Al2O3 14-27 Si:As14 25 - 36 12.0 - 16.5
LW 3C 10 2 2 r LiF 14-27 Si:As14 25 - 36 16.5 - 19.5
LW 3D 11 1 2 r LiF 14-27 Si:As14 25 - 36 19.5 - 27.5
LW 3E 12 1 3 r SrF2 20-27 Si:As14 25 - 36 27.5 - 29.0
LW 4 13 1 3 r SrF2 20-33 Ge:Be 37 - 48 29.0 - 45.2
F-P1 5A 15 3 1 r Al2O3 10-39 Si:Sb 49 - 5010 11.4 - 12.2
F-P1 5B 16 2 1 r Al2O3 10-39 Si:Sb 49 - 5010 12.2 - 16.0
F-P1 5C 17 2 2 r LiF 10-39 Si:Sb 49 - 5010 16.0 - 19.0
F-P1 5D 18 1 2 r LiF 10-39 Si:Sb 49 - 5010 19.0 - 26.0
F-P2 6 19 1 312 r SrF2 17-40 Ge:Be 51 - 5211 26.0 - 44.5

Notes:
1.
SW = short-wavelength grating region
2.
LW = long-wavelength grating region
3.
t = transmission
4.
r = reflection
5.
`Aperture area' refers to the dimensions of the SWS entrance apertures projected on the sky - see section 3.3
8.
AOTs SWS06 and SWS07 (SW grating) use finite (but small) band overlaps
9.
Band 2C ends at 13.16$\mu m$ 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$\mu m$ after the end of PV.
14.
These are Back-illuminated Blocked impurity Band (BIBIB) detectors.
15.
Band numbers are stored in the AAR and are explained in section 9.4.2, ``SWAALINE and SWAASCNT''.


Table 3.2: Wavelength range definition of the SWS AOT bands
  band detector wavelength8 resolution6 LAOT7
  name number15 number ($\mu m$) ( $R = \lambda/\Delta\lambda$)  
SW1 1A 1 1 - 12 2.38 - 2.60 1870 - 2110 756
SW 1B 2 1 - 12 2.60 - 3.02 1470 - 1750 1043
SW 1D 3 1 - 12 3.02 - 3.52 1750 - 2150 1282
SW 1E13 4 1 - 12 3.52 - 4.08 1290 - 1540 867
SW 2A13 5 13 - 24 4.08 - 5.30 1540 - 2130 2115
SW 2B 6 13 - 24 5.30 - 7.00 930 - 1250 1377
SW 2C9 7 13 - 24 7.00 - 12.0 1250 - 2450 4276
LW2 3A 9 25 - 36 12.0 - 16.5 1250 - 1760 2047
LW 3C 10 25 - 36 16.5 - 19.5 1760 - 2380 1879
LW 3D 11 25 - 36 19.5 - 27.5 980 - 1270 2524
LW 3E 12 25 - 36 27.5 - 29.0 1300 500
LW 4 13 37 - 48 29.0 - 45.2 1020 - 1630 4324
F-P1 5A 15 49 - 5010 11.4 - 12.2 20600 - 24000  
F-P1 5B 16 49 - 5010 12.2 - 16.0 24000 - 32000  
F-P1 5C 17 49 - 5010 16.0 - 19.0 32000 - 34500  
F-P1 5D 18 49 - 5010 19.0 - 26.0 34500 - 35500  
F-P2 6 19 51 - 5211 26.0 - 44.5 29000 - 31000  

Notes:
1.
SW = short-wavelength grating region
2.
LW = long-wavelength grating region
6.
The resolution given is that obtained when observing an extended source
7.
LAOT = 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$\mu m$ 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.
13.
The band 1E/2A limit changed from 4.05 to 4.08$\mu m$ after the end of PV.
15.
Band numbers are stored in the AAR and are explained in section 9.4.2, ``SWAALINE and SWAASCNT''.

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 tables 3.1 and 3.2. 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.

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 4 x 12 + 2 x 2 = 52 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 was 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 was possible to find scanner settings such that all of the detector arrays will contain valid data, the instrument was normally operated such that only one or two of the detector arrays received 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.

Figure 3.2: Block diagram of the SWS. The diagram shows the optical functions of the spectrometer, excluding its internal calibration sources, but including the shutter, the collimation and the imaging optics. It excludes band 3E.

Figure 3.3: Optical schematic of the SWS. The diagram indicates the location of the six separate entrance slits behind the dichroics after the three apertures. It shows all the spectral order-separation filters and the internal wavelength calibrators. The shutters, collimation and imaging optics and band 3E have been left out
\begin{figure} \begin{tabular}{c} \centerline{\epsfig{file={swsschematic.eps},width=15.0cm}}\\ \end{tabular}\end{figure}

next up previous contents index
Next: 3.3 Entrance Apertures Up: 3. Instrument overview Previous: 3.1 Introduction
SWS Instrument & Data Manual, Issue 1.0, SAI/98-095/Dc