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.
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| Instrument | Wavelength | |||
| (Principal | Range and | Outline | Spectral | Spatial |
| Investigator) | Main Funct. | Description | Resolution | Resolution |
| |
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| ISOCAM | 2.5-17 µm | (i) 32 x 32 array | (i) 11 filters 2 R20 |
Choice of |
| (C. Cesarsky, | Camera and | for 2.5-5 µm | circ. var. filt. R 40 |
1.5, 3, 6 or |
| CEA-Saclay, F) | polarimeter | (ii) 32 x 32 array | (ii) 10 filt. 2R14 |
12'' per pix. |
| for 4.5-17 µm | circ. var. filt. R40 |
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| ISOPHOT | 2.5-240 µm | (i) Multi-aperture, | (i) 14 filters 2R15 |
(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 1R3 |
43'' per pix. | ||
| 90-240 µm: 2 x 2 pix. | 5 filters 2R3 |
89'' per pix. | ||
| (iii) Spectrophotometer | (iii) grating, R90 |
(iii) 24'' x 24'' | ||
| (2.5-12 µm) | aperture | |||
| |
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| ISO-SWS | 2.38-45.2 µm | (i) Two gratings | (i) R1000-2000 |
(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) R3 x 104 |
20'' x 33'' |
| Research, | interferometers | (ii) 10'' x 39'', and | ||
| Groningen, NL) | 11.4-44.5 µm | 17'' x 40'' | ||
| |
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| ISO-LWS | 43-196.7 µm | (i) Grating | (i) R200 |
1.65' |
| (P. Clegg, | Long wavel. | (ii) Two Fabry-Pérot | (ii) R 104 |
diameter |
| Queen Mary and | spectrometer | interferometers | aperture | |
| Westfield College, | ||||
| London, UK) | ||||
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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
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 240m. A
variety of apertures, ranging from 5 to 180
, was
selectable out to 120m. 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
(at the shortest
wavelengths) to 90
(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.
The ISOCAM instrument (Cesarsky et al. 1996, [16];
Cesarsky 1999, [17]) consisted of
two optical channels, used one at a time, each with a 
element detector array. These arrays operated, respectively, in the
wavelength ranges 2.5-5.5
m and 4-17m. 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 
45) and the other one carried lenses for choosing
a pixel field of view
of 1.5, 3, 6, or 12
.
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).
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 15m, good
detections at the level of a few tens of 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 7m) and
LW3 (around 15m) filters, which became the ISOCAM colours,
used with the 6
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].
The LWS (Clegg et al. 1996, [26];
Clegg 1999 [27]) covered the wavelength range
43-196.7m with a spectral resolving power of 200.
Using also the Fabry-Pérot (FP) etalons, the resolution could be
increased to around 10000.
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
on a rotationally-symmetric Schmidt profile. It was used in
second order for the wavelength range 43-94.6m and in first
order for the wavelength range 94.6-196.9m. 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 47-70m and
70-196.6m 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 43-90m in nominally
10m-wide channels while the others covered the long-wavelength
range 90-197m in nominally 20m-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
118m (i.e. 100
) 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.
m and 196.7m. The user
specified the spectral sampling interval. The spacecraft raster mode
could also be used.
m and
196.7m. The user specified the spectral scan width and the
sampling interval. The spacecraft raster mode could also be used.
In orbit, the LWS performed very well. The spectral resolution was
as expected. The grating wavelength accuracy was 0.25
with a long term stability of better
than 0.5
. For both FPs, the wavelength accuracy was
better than 0.5
. 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].
The ISOPHOT instrument (Lemke et al. 1996, [110]; Lemke & Klaas 1999, [111]) consisted of three subsystems:
m. The `C100' camera
contained a 
array of Ge:Ga detectors, each with a pixel field
of view of 43.5
, and 6 filters covering
wavelengths up to 105m. The `C200' camera used a 
array of stressed Ge:Ga detectors with a pixel field of view
of 89
and had 5 filters covering wavelengths longwards of 100m.
m. It contained 13 apertures ranging
in size from 5
to 180
and 14 different filters. It had
three single detectors, used one at a time, made of Si:Ga, Si:B and Ge:Ga.
90 in two wavelength bands simultaneously
(2.5-5m and 6-12m). It contained two 64-element Si:Ga
detector arrays with a square entrance aperture of
24
24
.
A focal plane chopper with a selectable beam throw of up to 3
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.
ISOPHOT had 11 operating modes, grouped into 4 categories, plus non-standard and engineering modes.
. The
sequence had to start with a PHT17 measurement and end with a PHT19 one;
intervening positions used PHT18.
m and 6-12m using the ISOPHOT-S detector arrays.
Staring or chopped observations could be performed.
m. The target was observed at 25m with one aperture
(79
) through the three different polarisers.
m, i.e. a long wavelength analogue of PHT50.
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].
The SWS (de Graauw et al. 1996, [62];
de Graauw 1999, [63]) covered the wavelength
range 2.38-45.2m 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 11.4-44.5m.
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 2.3-12.0m. The long wavelength (LW)
section had a 30-lines/mm grating in the first two orders covering
the range 11-45m. 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
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 12 elements of Si:Sb and
Ge:Be. Figure 3.11 gives a schematic of the
SWS instrument.
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).
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 5%
at the shorter wavelengths and 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].
