The PHT detectors are coupled with integrating amplifiers and the non-destructive read-out technique is used for the determination of the signal; i.e. when integrating, the voltage on the amplifiers is read-out repeatedly without disturbing the integration process. The change of the voltage with time is proportional to the incoming flux (see Fig. 4). After a certain time period the integration is stopped and the integrator is reset to a null voltage; this process is called a destructive read-out. The time period between two destructive read-outs is called the reset interval.
After a destructive read-out the next integration is started. The read-out settings, the length of the reset interval, and the number of non-destructive read-outs (NDRs) during one reset interval depend on the expected flux level. For brighter sources the integrator stage must be read out more frequently in order not to exceed the valid voltage range and saturate the amplifier. The maximum possible number of non-destructive read-outs is used, but the time interval between two adjacent non-destructive read-outs cannot become arbitrarily short because of limitations in the telemetry rate of the detector data. Then the number of reset intervals has to be reduced.
In general, the number of the non-destructive read-outs decreases when the length of the reset interval decreases. If a minimum of 3 NDRs during a reset cycle of 1/8 second is reached, no further reduction is possible. This means the brightest objects of the IR sky will not be observable with ISOPHOT in all possible filter and aperture combinations.
If the actual flux from the sky deviates significantly from the estimated flux used to determine the read-out setting, the measured signal will not be well matched to the dynamic range of the amplifier, and the integration ramps will either run into saturation (over-exposure) or will not allow for a good determination of the slope (under-exposure). Saturation does not mean that there will be any harm to the detector and it will operate perfectly afterwards. Signals can be still determined from partially saturated ramps, if a sufficient number of non-saturated NDRs are left.
Figure 4 illustrates the read-out intervals for three different flux levels which span about one order of magnitude in flux. The destructive read-outs, the non-destructive read-outs, and the ramps are also illustrated.
The impact of the relative source and background signals on the dynamic range is described in Section 6.2.
