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6.3 Response to Off-axis and Extended Sources: Spectrum
Fracturing

For point sources observed off-axis, and to a lesser extent extended sources, the stitching between adjacent detectors breaks down when the source lies in a particular part of the beam (see Figure 6.3). The reason for this is unknown but the part of the beam affected is the edge that passes close to the cutout in the contour field mirror, between Mirror 2 and the collimator. At shorter wavelengths the flux levels are generally higher in this region compared to similar radii in other parts of the beam, and at longer wavelengths the effect of fringing is reduced. The spectrum takes on a broken or fractured appearance with large discontinuities between detectors. The effect is worse at shorter wavelengths and for the SW detectors it introduces a strong gradient in the flux with wavelength. The variation of the mean flux in the fractured region is the main cause of the asymmetry in the beam, and of the difficulty in modelling it. Two routines in LIA, CORRECT_SPECTRUM for point sources and EXTENDED_FLUX for extended sources, can be applied to reduce this effect in the spectra.

Figure 6.3: Spectra of Mars observed at various positions off-axis corresponding to those in Figure 5.16. The fracturing of the spectra can be seen on one side of the field, and the effects of fringing at longer wavelengths.
\resizebox {13cm}{!}{\includegraphics{MarsLambdaRatio_1.eps}}


next up previous contents index
Next: 6.4 Dark Current Subtraction Up: 6. Caveats and Unexpected Previous: 6.2 Response to Off-axis
ISO Handbook Volume III (LWS), Version 2.1, SAI/1999-057/Dc