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Next: 3. Derive SPD level Up: ISOPHOT Error Budgets: Derive_SPD Previous: 1. Introduction


2. Definition of terms

statistical uncertainty:
comes in whenever the mean or median value is derived of a sample of signals. ISOPHOT's measurement principle is based on collecting a large number of redundant data points: several non-destructive read-outs to establish the slope (=signal in V/s) of an integration ramp and several integration ramps to make up the final signal. The statistical uncertainty is determined by the width of the signal distribution and includes photon noise and detector noise.

systematic uncertainty:
comes in (1) whenever the sample of signals contains non-random outliers or (2) whenever a data point is subject to a arithmetic operation (scaling, addition, or subtraction) which requires parameters obtained from independent calibration observations. The uncertainty of the calibration parameters can be purely statistical. An important fraction of the systematic uncertainty comes from the FCS measurement. An error in the detector responsivity derived from the FCS signal causes the same (systematic) scaling error in the flux densities of the sky measurements.

photometric bias:
is the photometric uncertainty in ISOPHOT-data due to uncertainties in the predicted fluxes of the celestial standards upon which the ISOPHOT flux or inband power calibration tables are based. The input fluxes of the photometric calibration standards are based on model predictions [1]. The photometric accuracy of celestial standards can vary from object to object (e.g. due to type of asteroid, spectral type of star, particular planet, etc) which may cause a different photometric bias for different flux regimes at a given wavelength. The photometric bias becomes important when comparing ISOPHOT data with results from other instruments. This term is important for the final absolute accuracy.

relative accuracy:
is the filter to filter or aperture to aperture accuracy for a given detector assuming a single value for the detector responsivity. The relative accuracy is of importance in a multi-filter and/or multi-aperture observation where a single FCS measurement is used to calibrate several sky measurements with different filters and/or apertures. The relative accuracy limits the accuracy in the determination of colours in case of multi-filter photometry and of extended emission components in case of multi-aperture photometry.

detection limit:
is the minimum point-source flux that can be detected within a given measurement time. The detection limit is determined by the source-background separation which depends on the observing strategy like minimaps, chopped observations, on- and off-position AOTs, sparse maps, nodding, etc. Cirrus and galaxy confusion are important external limitations depending on wavelength, background flux level, and spatial resolution. The photometric bias has no significant impact on the detection limit.

flux reproducibility:
determines the width of the flux distribution of a large sample of identical observations (same AOT with identical settings on the same target) obtained at different times during the ISO mission. The reproducibility gives a global assessment of the best accuracy that can be achieved for a given observing mode. It lumps together several uncertainties: the statistical uncertainty in the sky measurement, the systematic uncertainty due to the FCS calibration measurement and the uncertainties due to external noise sources. It can also be regarded as a measure of the stability of the instrument. This term becomes important in the analysis of variable sources.


next up previous contents
Next: 3. Derive SPD level Up: ISOPHOT Error Budgets: Derive_SPD Previous: 1. Introduction
ISOPHOT Error Budgets: Derive_SPD Processing Steps, Version 1.0, SAI/98-091/Dc