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Subsections



3.4 The Scientific Instruments

The scientific payload consisted of four instruments: a camera, ISOCAM; an imaging photo-polarimeter, ISOPHOT; a long wavelength spectrometer, LWS; and a short wavelength spectrometer, SWS. Each instrument was built by an international consortium of scientific institutes and industry, headed by a Principal Investigator, using national funding (see Section 2.3). Although developed separately, the four instruments were designed to form a complete, complementary and versatile common-user package. Table 3.5 summarizes the main characteristics of the instruments.

Table 3.5: Main characteristics of the ISO instruments.

Instrument Wavelength
(Principal Range and Outline Spectral Spatial
Investigator) Main Funct. Description Resolution Resolution

ISOCAM 2.5-17 µm (i) 32 x 32 array (i) 11 filters 2$\leq$R$\leq$20       Choice of
(C. Cesarsky, Camera and     for 2.5-5 µm      circ. var. filt. R$\sim$40 1.5, 3, 6 or
CEA-Saclay, F) polarimeter (ii) 32 x 32 array (ii) 10 filt. 2$\leq$R$\leq$14 12'' per pix.
    for 4.5-17 µm     circ. var. filt. R$\sim$40

ISOPHOT 2.5-240 µm (i) Multi-aperture, (i) 14 filters 2$\leq$R$\leq$15 (i) Choice of
(D. Lemke, Imaging photo-     multi-band     diffraction-
MPI für polarimeter     photo-polarimeter     limited to
Astronomie     (3-120 µm)      3' apertures
Heidelberg, D) (ii) Far-infrared camera   (ii) (ii)
    50-120 µm: 3 x 3 pix.     6 filters 1$\leq$R$\leq$3      43'' per pix.
    90-240 µm: 2 x 2 pix.     5 filters 2$\leq$R$\leq$3      89'' per pix.
(iii) Spectrophotometer (iii) grating, R$\sim$90 (iii) 24'' x 24''
    (2.5-12 µm)     aperture

ISO-SWS 2.38-45.2 µm (i) Two gratings (i) R$\sim$1000-2000 (i) 14'' x 20'', 14'' x 27''
(Th. de Graauw, Short wavel.      2.38-45.2 µm      20'' x 27'', and
Lab. for Space spectrometer (ii) Two Fabry-Pérot (ii) R$\sim$3 x 104      20'' x 33''
Research,     interferometers (ii) 10'' x 39'', and
Groningen, NL)     11.4-44.5 µm      17'' x 40''

ISO-LWS 43-196.7 µm (i) Grating (i) R$\sim$200 1.65'
(P. Clegg, Long wavel. (ii) Two Fabry-Pérot (ii) R$\sim$ 104 diameter
Queen Mary and spectrometer     interferometers aperture
Westfield College,
London, UK)

Only one instrument was operational in prime mode at a time. However, when the camera was not the main instrument, it was used in parallel mode to acquire extra astronomical data (Siebenmorgen et al. 1996, [151]). Whenever possible, the long-wavelength channel of the photometer was used during satellite slews. This serendipity mode (e.g. Stickel et al. 1999, [154]) led to a partial sky survey, covering approximately 15% of the sky, at wavelengths around 200$\mu $m, a spectral region not covered by the IRAS survey. After launch, a parallel/serendipity mode was added for the LWS (e.g. Clegg 1999, [27]), in which narrow-band data were obtained at 10 fixed wavelengths in parallel with the main instrument and also during slews.

With ISO, photometry was possible in broad and narrow spectral bands across its entire wavelength range of 2.5 to around 240$\mu $m. A variety of apertures, ranging from 5 to 180 $^{\prime \prime}$, was selectable out to 120$\mu $m. For spectroscopy, resolving powers ranging from 50 to 30000 were available. ISO was capable of direct imaging in broad and narrow spectral bands across the complete wavelength range at spatial resolutions ranging from 1.5 $^{\prime \prime}$ (at the shortest wavelengths) to 90 $^{\prime \prime}$ (at the longer wavelengths). In addition, mapping could be carried out using sequences of pointings.

Each of the four instruments had a number of possible operating modes. To simplify the definition of an observation and to allow users to specify their observation in terms familiar to them, a set of astronomically-useful operating modes was defined and presented to users as a set of `Astronomical Observation Templates' (AOTs). Each AOT was designed to carry out a specific type of astronomical observation. The observations resulting from the use of these AOTs are the basic building blocks of the ISO Data Archive. Note, however, that the AOTs consist of lower level structure, e.g. source, background, internal calibrator, dark current, etc. measurements. Further information on the individual AOTs, including engineering and calibration AOTs, is given in the following instrument specific subsections, where we give a short, compact high-level summary description of the 4 scientific instruments (CAM, LWS, PHT, SWS) on board ISO. More detailed instrument descriptions are given in volumes II to V of this Handbook.


3.4.1 The ISO Camera: ISOCAM

The ISOCAM instrument (Cesarsky et al. 1996, [16]; Cesarsky 1999, [17]) consisted of two optical channels, used one at a time, each with a $32\times32$ element detector array. These arrays operated, respectively, in the wavelength ranges 2.5-5.5$\,\mu$m and 4-17$\,\mu$m. The short wavelength (SW) array used an InSb detector with a Charge Injection Device (CID) readout and the long wavelength (LW) detector was made of Si:Ga with a direct readout. A selection wheel carried Fabry mirrors which directed the light beam of the ISO telescope towards one or other of the detectors; this wheel also carried an internal calibration source for flat-fielding purposes. Each channel contained two further selection wheels: one carried various filters (10-13 fixed and 1 or 2 Circular Variable Filters (CVF), with a resolution of $\sim\,$45) and the other one carried lenses for choosing a pixel field of view of 1.5, 3, 6, or 12 $^{\prime \prime}$. Polarisers were mounted on an entrance wheel -- common to both channels -- which also had a hole and a shutter. Figure 3.8 shows a schematic representation of ISOCAM.

ISOCAM observations were taken in four main modes. These are designated as CAM01 (General observation), CAM03 (Beam switching), CAM04 (Spectrophotometry) and CAM05 (Polarisation).

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CAM01 was dedicated to photometric imaging in one of the two channels, using one or more pixel field of views at one or more wavelengths. Observations could be made in the bandpass filters as well as at individual CVF positions. Use of the spacecraft's raster pointing capabilities gave the possibilities not only of mapping areas larger than the camera's field of view, but also of improving the flat-field accuracy. Micro-scanning techniques were employed to increase the redundancy and, thereby, improve detection limits and photometric accuracy.

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CAM03 was also dedicated to photometry and used a beam switching mode. An on-source measurement was made followed by a background measurement on a nearby empty reference field. The use of up to 4 different reference fields was possible.

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CAM04 allowed spectral imaging. The spectrum was observed by obtaining a series of spectral points. A complete CVF spectrum took at most 115 steps in the short wavelength channel and 85 steps for each of the two long wavelength CVFs.

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CAM05 allowed polarisation maps to be obtained by taking successive images through the three polarisers. (Operationally, it was implemented in a slightly different manner to the above AOTs).

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CAM60, CAM61, CAM62, CAM63: Engineering Data
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CAM99: Non-standard data, mainly used for special calibration purposes

Figure 3.8: Schematic of the Camera (ISOCAM).
\rotatebox {0}{\resizebox{13.5cm}{10cm}{\includegraphics{CAM__Vol._2_.ps}}}

In orbit, the instrument behaved extremely well. Its sensitivity was as good (LW detector) or better (SW detector) than predicted from ground-based tests. In particular, ISOCAM was able to detect faint point and extended sources through long observations. At 15$\,\mu$m, good detections at the level of a few tens of $\mu $Jy have been made. The broken readout cable for column 24 of the LW array found during pre-launch test, continued to render column 24 unreadable during operations (i.e., the detector was still active but no data were available). In common with the other instruments, ISOCAM experienced transients and glitches (see Sections 5.6.2 and 5.6.1); these have been tackled with increasingly sophisticated methods and ever-growing success. Work on minimising the effects of transients and glitches continues as do efforts relating to removing ghosts from CVF images and for detecting faint sources close to bright ones. The overall absolute calibration is better than 20% in practice with repeatability better than a few per cent.

It is worth noting that a few camera settings have been used much more extensively than the rest, in particular the LW2 (around 7$\,\mu$m) and LW3 (around 15$\,\mu$m) filters, which became the ISOCAM colours, used with the 6 $^{\prime \prime}$ pixel field of view so as to take advantage of the whole field of view offered by the ISO satellite.

More details about ISOCAM are given in The ISO Handbook, Volume II: CAM - The ISO Camera, [11].

3.4.2 The ISO Long Wavelength Spectrometer: LWS

The LWS (Clegg et al. 1996, [26]; Clegg 1999 [27]) covered the wavelength range 43-196.7$\,\mu$m with a spectral resolving power of $\sim\,$200. Using also the Fabry-Pérot (FP) etalons, the resolution could be increased to around 10000.

Figure 3.9: Schematic of the Long Wavelength Spectrometer (LWS).
\rotatebox {0}{\resizebox{13.5cm}{10cm}{\includegraphics{LWS__Vol._3_.ps}}}

The LWS instrument consisted essentially of a reflection diffraction grating, two FP etalons and an array of 10 discrete detectors. The grating was ruled with 7.9 lines per millimetre at a blaze angle of 30$^\circ$ on a rotationally-symmetric Schmidt profile. It was used in second order for the wavelength range 43-94.6$\mu $m and in first order for the wavelength range 94.6-196.9$\mu $m. The FP subsystem, which was situated in the collimated part of the beam, consisted of a wheel carrying two FP interferometers. The wheel could be set in any of four positions: in one of these, the beam passed straight through the subsystem whilst in another, the beam was completely obscured. In the remaining two positions, one or other FP was placed in the beam and modulated it spectrally. The two FPs covered the wavelength ranges 47-70$\mu $m and 70-196.6$\mu $m respectively. The instrument contained ten detectors made of Ge:Be and Ge:Ga (stressed and unstressed) material and read out by integrating amplifiers: five of these detectors covered the short-wavelength range $\sim$43-90$\mu $m in nominally 10$\mu $m-wide channels while the others covered the long-wavelength range $\sim$90-197$\mu $m in nominally 20$\mu $m-wide channels. Five internal illuminators were used to monitor and calibrate the stability of response of the detectors. The single fixed LWS circular field of view was designed to match the diffraction limit of the telescope at 118$\,\mu$m (i.e. 100 $^{\prime \prime}$) and was a compromise for the wavelength range of the instrument. In practice, the beam was somewhat narrower than this. Figure 3.9 gives a schematic of the LWS instrument.

LWS observations were taken in four main modes. Two involved use of the grating only: LWS01 (Grating range scan) and LWS02 (Grating line scan); while the other two also used the FPs: LWS03 (FP wavelength range) and LWS04 (FP line spectrum). Observers had to add background observations separately.

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LWS01 took a spectrum over a user-specified range of wavelengths up to the full LWS range. The spectrum could be sampled at 1, 1/2, 1/4 or 1/8 of a resolution element. The spacecraft raster mode could also be used.
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LWS02 took a spectrum around a number (up to 10) of user-specified wavelengths. The user specified the number of resolution elements around each line. An alternative use of this mode was `narrow-band photometry', where the grating was stationary and spectro-photometry was obtained at 10 fixed wavelengths in the LWS range. The spacecraft raster mode could also be used.
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LWS03 took a high-resolution spectrum over a user-specified range of wavelengths between 47.0$\mu $m and 196.7$\mu $m. The user specified the spectral sampling interval. The spacecraft raster mode could also be used.
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LWS04 took a high-resolution spectrum around a number (up to 10) of user-specified wavelengths between 47.0$\mu $m and 196.7$\mu $m. The user specified the spectral scan width and the sampling interval. The spacecraft raster mode could also be used.
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LWS22, LWS23, LWS70, LWS71, LWS72, LWS76: Engineering Data
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LWS99: Non-standard data, mainly used for special calibration purposes

In orbit, the LWS performed very well. The spectral resolution was as expected. The grating wavelength accuracy was 0.25 $\,\Delta\lambda$ with a long term stability of better than 0.5 $\,\Delta\lambda$. For both FPs, the wavelength accuracy was better than 0.5 $\,\Delta\lambda$. The flux calibration for the grating is better than 20%. The effects of charged particle impacts reduced the overall sensitivity of the instrument. Operationally, the biggest concern was the precautionary suspension of LWS use for a time while characteristics of the interchange wheel were further studied; upon resumption, no restrictions were needed on scientific use of the instrument.

More details about LWS are given in The ISO Handbook, Volume III: LWS - The Long Wavelength Spectrometer, [68].


3.4.3 The ISO Imaging Photo-Polarimeter: ISOPHOT

The ISOPHOT instrument (Lemke et al. 1996, [110]; Lemke & Klaas 1999, [111]) consisted of three subsystems:

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ISOPHOT-C: two photometric far-infrared cameras, used one at a time, for the wavelength range 50-240$\,\mu$m. The `C100' camera contained a $3\times3$ array of Ge:Ga detectors, each with a pixel field of view of 43.5 $^{\prime \prime}$, and 6 filters covering wavelengths up to 105$\,\mu$m. The `C200' camera used a $2\times2$ array of stressed Ge:Ga detectors with a pixel field of view of 89 $^{\prime \prime}$ and had 5 filters covering wavelengths longwards of 100$\,\mu$m.

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ISOPHOT-P: a multi-band, multi-aperture photo-polarimeter for the wavelength range 3-110$\,\mu$m. It contained 13 apertures ranging in size from 5 $^{\prime \prime}$ to 180 $^{\prime \prime}$ and 14 different filters. It had three single detectors, used one at a time, made of Si:Ga, Si:B and Ge:Ga.

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ISOPHOT-S: a dual grating spectrophotometer which provided a resolving power of $\sim\,$90 in two wavelength bands simultaneously (2.5-5$\,\mu$m and 6-12$\,\mu$m). It contained two 64-element Si:Ga detector arrays with a square entrance aperture of 24 $^{\prime\prime}\,\times$24 $^{\prime \prime}$.

A focal plane chopper with a selectable beam throw of up to 3$^{\prime}$ was also included in ISOPHOT. Selection between the different modes of the various sub-systems was achieved with appropriate settings of three ratchet wheels. Two redundant sets of thermal radiation sources (fine calibration sources, FCSs) were located symmetrically about the centre of the ISOPHOT field of view and were used for calibration and to monitor the time evolution of detector responsivity. Figure 3.10 shows a schematic representation of ISOPHOT.

Figure 3.10: Schematic of the Photo-polarimeter (ISOPHOT).
\rotatebox {0}{\resizebox{13.5cm}{10cm}{\includegraphics{PHT__Vol._4_.ps}}}

ISOPHOT had 11 operating modes, grouped into 4 categories, plus non-standard and engineering modes.

  • Photometry: Single pointing and Staring Raster Modes
    -
    PHT03: Standard mode for multi-filter photometry using ISOPHOT-P detectors. An arbitrary number (up to 14) of filters could be selected plus one aperture per filter. Used in stare, raster or chop modes, the production of maps and scans was possible.
    -
    PHT22: Analogue of PHT-03 for long-wavelength photometry using ISOPHOT-C detectors. Up to 11 filters could be selected; no aperture selection was needed as the field of view was defined by the detector arrays. Used in stare, raster or chop modes, the production of maps and scans was possible.
    -
    PHT05: Absolute photometry using the ISOPHOT-P detectors. Absolute flux calibration was achieved by long internal fine calibration source measurements and the possibility to perform zero level measurements at the time of the sky measurement. The user selected one filter and one aperture.
    -
    PHT25: Absolute photometry using the ISOPHOT-C detectors, i.e. a long wavelength analogue of PHT05. No aperture selection was needed as the field of view was defined by the detector arrays.
    -
    PHT04: Multi-aperture photometry using the ISOPHOT-P detectors. Only one filter could be selected and the minimum number of apertures was 2. Also used in chop mode.
  • Photometry: Scanning/Mapping Modes
    -
    PHT32: Multi-filter mapping using the ISOPHOT-C detector arrays. Designed for high spatial resolution by over-sampling.
    -
    PHT17/18/19: Sparse mapping with the ISOPHOT-P detectors. A sequence was measured of up to 30 positions, which could be irregularly distributed on the sky within a field of radius 1.5$^\circ$. The sequence had to start with a PHT17 measurement and end with a PHT19 one; intervening positions used PHT18.
    -
    PHT37/38/39: Sparse mapping with the ISOPHOT-C detectors, i.e. a long wavelength analogue of PHT17/18/19. No aperture selection was needed as the field of view was defined by the detector arrays.
  • Spectrophotometry
    -
    PHT40: Spectrophotometry simultaneously at wavelengths 2.5-5$\,\mu$m and 6-12$\,\mu$m using the ISOPHOT-S detector arrays. Staring or chopped observations could be performed.
  • Polarimetry
    -
    PHT50: Polarimetry using the ISOPHOT-P2 detector at 25$\,\mu$m. The target was observed at 25$\,\mu$m with one aperture (79 $^{\prime \prime}$) through the three different polarisers.
    -
    PHT51: Polarimetry using the ISOPHOT-C200 detector array at 170$\,\mu$m, i.e. a long wavelength analogue of PHT50.
    (Operationally, PHT50/51 were implemented in a different manner to the other modes).
  • PHT77: Data taken by the ISOPHOT Serendipity Survey.
  • PHT80, PHT81, PHT82, PHT83: Engineering Data with instrument activation sequence and responsivity checks of each detector after curing.
  • PHT84: Engineering Data with de-activation sequence, including the PHT-S responsivity check at the end of the science window.
  • PHT99: Non-standard data, mainly used for special calibration purposes

    In orbit, ISOPHOT worked well; however, establishment of its detection limits and detailed calibration proceeded more slowly than was first expected. This was due to the complexity of the instrument with its many operating modes needing a very detailed stepwise approach with many pieces having to be completed and interlinked. Additionally, the complexity of the sky at far infrared wavelengths made it necessary to develope new dedicated observing strategies. Some of the detectors had reduced sensitivity in flight as compared to pre-launch estimates.

    More details are given in The ISO Handbook, Volume IV: PHT - The Imaging Photo-Polarimeter, [107].


    3.4.4 The ISO Short Wavelength Spectrometer: SWS

    The SWS (de Graauw et al. 1996, [62]; de Graauw 1999, [63]) covered the wavelength range 2.38-45.2$\,\mu$m with a spectral resolving power of the order of 1000-2500. Using also the Fabry-Pérot (FP) etalons, the resolution could be increased to more than 25000 for the wavelength range 11.4-44.5$\,\mu$m.

    The SWS instrument consisted of two nearly independent grating spectrometers plus two scanning Fabry-Pérot filters. The short wavelength section (SW) used a 100-lines/mm grating in the first four orders covering the range 2.3-12.0$\,\mu$m. The long wavelength (LW) section had a 30-lines/mm grating in the first two orders covering the range 11-45$\,\mu$m. The two FPs were at the output of the LW section and used the first three orders of the LW grating. The SWS had three apertures and a shutter system. This allowed use of one of them while keeping the other two closed. For astronomical observations, the spacecraft pointing had to be adjusted to have the target imaged onto the selected aperture. Each aperture was used for two wavelength ranges, one of the SW section and one of the LW section. This was achieved by using `reststrahlen' crystal filters as wavelength-selective beam splitters behind the apertures. The transmitted beams entered the SW section; the reflected beams entered the LW section. The actual spectrometer slits were located behind the beam splitting crystals. Interference filters or crystal filters took care of further order sorting. Depending on wavelength, the aperture sizes for the grating sections ranged from 14'' x 20'' to 20'' x 33''. Each grating had its own scanner, allowing the use of both grating sections (SW and LW) at the same time, although the observed wavelength ranges were linked. The output of each of the two grating sections was re-imaged onto two small (1$\,\times\,$12) detector arrays, located in-line. The materials used for the grating detectors were InSb, Si:Ga, Si:As and Ge:Be, while the FPs used 1$\,\times\,$2 elements of Si:Sb and Ge:Be. Figure 3.11 gives a schematic of the SWS instrument.

    Figure 3.11: Schematic of the Short Wavelength Spectrometer (SWS).
    \rotatebox {0}{\resizebox{13.5cm}{10cm}{\includegraphics{SWS__Vol._5_.ps}}}

    SWS observations were taken in four main modes. Three involved use of the gratings only: SWS01 (Full grating scan), SWS02 (Grating line profile scan), SWS06 (Grating wavelength range scan); while the fourth used also the FPs: SWS07 (FP line scan). These modes did not include use of the spacecraft raster mode; maps had to be made by concatenating individual pointings (see Section 4.5).

    -
    SWS01 provided a low-resolution full-wavelength grating scan. Different scan speeds (1, 2, 3, 4) could be selected by the user corresponding to resolving powers of the order of 400, 400, 800 and 1600, respectively.
    -
    SWS02 had a scanning scheme optimised to obtain grating scans of individual spectral lines. The user could specify up to 64 different lines in a single observation.
    -
    SWS06 was designed to observe arbitrary wavelength intervals at full resolution. The user could specify up to 64 different ranges in a single observation.
    -
    SWS07 was used for the FP observations. The LW grating section was used as the order sorter and was kept in tune with the FP wavelength to minimise leakage from the adjacent FP orders.
    -
    SWS90, SWS91, SWS92, SWS93, SWS94, SWS95, SWS96, SWS97, SWS98: Engineering Data
    -
    SWS99: Non-standard data, mainly used for special calibration purposes

    In orbit, the instrument behaved extremely well. The performance was in all aspects as expected except for the detector sensitivity where the noise was dominated by effects from particle radiation -- initial estimates of the loss in sensitivity were up to a factor of 5. Further analysis of the instrument behaviour is now allowing recovery of some of the loss. The pre-launch goal of a 30% absolute flux calibration accuracy was achieved with stability better than $\sim\,$5% at the shorter wavelengths and $\sim\,$15% at the longest. The wavelength calibration (goal: 1/10 of a resolution element) was 1/8 (long wavelengths) and 1/16 (short wavelengths).

    More details about SWS are given in The ISO Handbook, Volume V: SWS - The Short Wavelength Spectrometer, [108].


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    Next: 4. ISO Operations Up: 3. The ISO Spacecraft Previous: 3.3 Spacecraft subsystems
    ISO Handbook Volume I (GEN), Version 2.0, SAI/2000-035/Dc