5.2 Instrument characterisation procedure

5.2.1 Detector characteristics

Debiasing parameters

These are measured with the internal illuminators, using different compensation voltages at the start of the ramps. The compensation levels induce different effective bias levels for the ramps. From the second order polynomial fits to the data the debiasing parameter a (see section 4.2) can be determined.

Detector Dark Current

The dark currents can only be measured together with any current induced by background or stray radiation. To minimize this background radiation the measurement is done by placing a Fabry-Perot - deliberately made non-parallel - in the beam. Tests have shown that this gives the lowest background on the detectors, the FP acting as a highly efficient blocking filter. ISO is then pointed to a relatively clear part of the sky to get minimal straylight from off-axis sources. The observation is done with several bias levels. The grating is scanned to look for spectral signatures in the dark current, and the illuminators are used to check for responsivity changes during the dark current measurement.

5.2.2 Electronics characteristics

No need for in-orbit characterisation could be identified. All characteristics are determined in the laboratory and while building the instrument. The important characteristics are:

• The time constants of the high pass electronic filters
• The gains of the amplifiers

5.2.3 Fabry Perot characteristics

The optimum offsets for the Fabry-Perot etalons for FPS and FPL are determined by observing an unresolved line at several offset values around the expected optimum values (offset mapping). This gives a set of line spectra. For each of these spectra the FWHM of the line and the peak heights are determined. From the contour maps of the FWHM of the line versus the two FP offset values (for the coils of one FP) the position of the minimum FWHM is determined. The offset values corresponding to this minimum are optimum for parallelism.

5.2.4 Optical characteristics

The beam pattern of the instrument is determined by taking full grating spectra over a raster map of a strong source. The result is a map of the beam-pattern of the instrument as a function of wavelength.