5.2 Operation of the ISOCAM Instrument

For a better understanding of the products we describe here some of the aspects of how the detectors were operated (see Chapter 2 for a more general description.). Either the LW or the SW channel, was in use at any one time. At the beginning of the selected integration time, the pixels were reset to a constant charge and read out, after which the detector was exposed to incoming radiation for the integration time and read again. The quantum of measurement consists, therefore, of a pair of RESET and end-of-integration (EOI) frames and both are usually present in the TDFs. The physically significant quantity proportional to flux is (EOIRESET) which is stored along with RESET values in SPD. The exceptions are for the shortest integration time, T=0.28 s, and when ISOCAM was operating in parallel mode, when telemetry limitations made the use of on-board integration in the accumulated readout mode necessary (Section 2.4.1) for which RESET values are not available. CAM usually operated continuously from switch-on during activation at the beginning of a revolution until de-activation at the end. When CAM was designated prime instrument, data were downloaded via the 24 prime frames of the 32-frame 2 s telemetry block; otherwise a single frame was reserved for parallel mode.

An individual (EOIRESET) EXPOSURE is the lowest in a hierarchy of structures that reflect the instrument's operation and move up through STATE, OP-MODE, CONFIGURATION and AOT.

STATEs

are series of one or more EXPOSUREs at fixed instrumental settings.

OP-MODEs

are series of STATEs of at most one of the following types:

OBS: sky exposure

DRK: dark current measurement

CAL: internal flux calibration

CLN: detector reset flash

IDLE: ISOCAM off

CONFIGURATIONs

are series of OP-MODEs at fixed instrumental settings.

AOTs

are series of one or more CONFIGURATIONs performed on a given target.

The Astronomical Observation Template, or AOT, is identified with an observation as a whole and is of one of the types discussed in detail in Chapter 3. A CONFIGURATION has between one and five OP-MODEs of different types, in the order defined by the original observer, including one OBS OP-MODE only. According to the AOT, a CONFIGURATION's central OBS OP-MODE will encompass one of the following:

• an entire staring mode observation;
• one observation of each point in a raster;
• one observation of each position in a beam-switch;
• one observation of each wavelength in a CVF scan;
• one observation of each polariser in a polarisation measurement.

• detector, DEID or CHANNEL (0=SW1=LW)
• entrance wheel position, EWHL (or ENTW)
• Fabry-mirror beam selection wheel position, SWHL (or SELW)
• pixel size lens wheel position, PFOV (or LNSW)
• wavelength and bandwidth defined by the filter wheel position (if applicable), FCVF (or FLTW)
• wavelength interval and scanning step for a CVF scan (if applicable), FCVF
• integration time, TINT
• gain, GAIN

where FCVF means a constant set of multiple FCVF values. The parameters EWHL, SWHL, PFOV and FCVF refer to the position of the selection wheel in question and may be translated into quantities of physical significance, such as wavelength or angular resolution, using the CWHEELS calibration file. It is mostly on the basis of these values that data should be divided up and combined for the production of images. There are other especially important real-time quantities which the user should take into account:

ITK:

each record of raw data is labelled with an `Instrument Time Key' in CIER[1].GPSCTKEY(*) or CISP[1].GPSCTKEY(*), for example, to allow synchronisation with the UTK and UTC keys that apply to other types of data.

RPID:

the `Raster-Point ID' is the index of the instrument's pointing direction and is the other quantity in the General Science Prefix in CIER[1].GPSCRPID(*,2), CISP[1].GPSCRPID(*,2). It identifies the relevant parts of the prime or parallel pointing files by comparison with IRPH[1].RPID(*,2) or CIPH[1].RPID(*,2), for example. For pointing operations which did not involve rasters of several different directions, RPID=(1,1). Occasionally, RPID=(0,0) shows that there were pointing problems.

OTF:

the `On-Target Flag' is ON=1 or OFF=0 to show if the instrument is pointing to the target in a direction closer than the angular threshold, usually 10 , given in CIER[0].ATTOTFTH, IRPH[1].OTFT(*) or CRPH[1].OTFT(*). It serves to show when the manoeuvre to the target is almost complete or the move away has begun, as well as warning observers of rare failures when, for example, it proved impossible to acquire the target at all. Although more detailed pointing information is available if required, the OTF provides a convenient mechanism for rejecting data. The flag is maintained by the on-board pointing system and is available at its fully sampled rate of 2 Hz in IIPH[1].OTF(*) or CIPH[1].OTF(*). The latest value when an F2 block was assembled is given in CIER[1].F2OTFLAG(*) while CIER[1].F2OTFSUM(*) contains the logical AND of all the OTFs sampled during the F2 accumulation time.

QLF:

the `Quick-Look Flag' CIER[1].F2QLA switches from OFF=0 to ON=1 to show that the preselected number of stabilisation EXPOSUREs, N, has been completed. This is supposed to ensure that the instrument had settled down after the move to the observation's target. Despite these good intentions, the instrument's transient behaviour proved to be one of its more challenging aspects of which N provides only a zero-order approximation. A much better stabilisation model, which takes into account the evolution of individual pixel signals, has been implemented in the pipeline as described in Section 4.4.2.'