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8.6 Pointing Effects

8.6.1 Introduction

The SWS was designed for a pointing performance consistent with the specification of the telescope optics, but exceeding the specifications of the ISO satellite. Fortunately the satellite pointing was significantly better than specified. Nevertheless, for SWS the data quality was affected by the ISO pointing performance and further improvements on the pointing were triggered by these findings. Even then, pointing problems have remained a limitation for many SWS observations.

The accuracy of the target input coordinates, and of the pointing, have a large impact on the instrumental throughput and the point source flux accuracy, but only a minor impact on the wavelength accuracy of SWS for point sources. Several documents were written covering the pointing accuracy. Leech & Heras 1997, [26], gives examples of what can go wrong when incorrect target coordinates are used. In summary, a star offset from the centre of the aperture by 6 $ ^{\prime\prime}$ in the cross-dispersion direction (y-axis) loses approximately 40% in throughput (at 17 $ \mu $m). A similar offset in the dispersion direction (z-axis) does not result in as large a flux decrease, due to the beam profile (see Section 8.4). An offset in the dispersion direction does, however, affect the wavelength calibration, with a 4 $ ^{\prime\prime}$ mispointing resulting in a 1 LVDT (1/8 grating resolution) wavelength offset. As the maximum possible offset is about 2 LVDT, or about $ 1/4$ of a grating spectral resolution element, and within the required wavelength accuracy, errors introduced into the wavelength scale can usually be ignored. Still, random pointing errors are the largest contributors of uncertainty in the wavelength scale. They also impact on the exact fringe patterns, see fringes in Sections 8.6.2 and 9.7.

Heras 1998, [14] extended this study by comparing the ratio of on- and off-source observations of a star with the pointing jitter. It was found that the sudden flux jumps are associated with pointing fluctuations. She noted that even for nominally on-source observations (defined to be those where the pointing error is less than about 4 $ ^{\prime\prime}$), there is a correlation between noise in an observation and pointing jitter.

During the ISO mission several stars were observed every few weeks for purposes of wavelength and flux calibration. The flux of one of these targets, $ \gamma $ Dra, was seen to have a modulation that is suspected to be due to pointing errors. The report by Feuchtgruber 1998c, [8], can be read for further information on this.

Information on how the spacecraft pointing can affect observations of extended objects can be read in Feuchtgruber 1998b, [7]. In this case the observed line fluxes varied by 10% over the course of several months. This was attributed to the spatial extent of NGC 6543, the SWS slit size projected on the sky, see Section 8.2.2, and the change in roll angle between the observations.

To derive the position angle of an aperture, information on the spacecraft's position on the sky must be used. This can be found in the header keywords CINSTRA, CINSTDEC, CINSTROLL in ERD, SPD, or AAR products. (see the ISO Handbook Volume I, [17] for a description of these keywords).

8.6.2 Pointing effects on fringes

The effect of pointing offsets on the position and intensity of the instrumental fringes, most prominent in band 3A, was characterised with dedicated spectral scans performed at various positions with respect to NML Cyg as illustrated in Figure 8.7. The dependence of the fringe pattern on the pointing also implies that for extended sources or point-like sources off-axis, a perfect correction for the instrumental response function is not possible.

Figure 8.7: Pointing offset dependence of the fringes seen in detector 26, band 3A, in cross-dispersion (upper panel) and dispersion (lower panel) directions. Each fringe pattern corresponds to a grating scan performed at positions on the sky separated by 2 $ ^{\prime\prime}$.
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8.6.3 Pointing jitter induced correlated noise

The ISO pointing jitter is seen by the SWS detectors as the target moves across the beam profile, and results in correlated noise in the band. This correlated noise is discussed in Heras 1998, [14], where the correlation between ISO's pointing jitter and jumps in the flux of SWS observations were first studied. The source DO 24107 was observed off source because of an error in the input coordinates ($ +$4.7 $ ^{\prime\prime}$ and $ -$8.6 $ ^{\prime\prime}$ off in dispersion and cross-dispersion directions respectively). Such a large discrepancy implies that the observation is carried out at a position where the beam profile is very steep, and therefore small oscillations of the pointing around the average position induce large variations of the measured flux.

8.6.4 Signal modulation on $ \gamma $ Dra

The SWS flux calibration included reproducibility checks on the standard star $ \gamma $ Dra. A total of 120 observations were carried out on this star from revolution 24 to revolution 867. These were systematically processed at the end of the mission and the resulting signals were shown to present flux modulations over the mission (see Feuchtgruber 1998c, [8], for details). Figure 8.8 gives all the derived fluxes within the passbands of the related band versus revolution. The three $ \gamma $-Dra raster exercises in revolutions 126, 368 and 369 are also included in this sample, clearly to be seen for example in band 1D.

Flux modulations can clearly be seen at the key wavelengths in bands 2A, 2B, 2C, 2C$ ^{\prime}$ (at 11 $ \mu $m), 3A, 3C, 3D, 3E, 4A (offband data) and 4D (offband data). No related modulation of comparable amplitude can be seen in band 1. Fluxes are heavily affected by pointing inaccuracies, as can be seen in the period before revolution 369, when significant pointing errors happened. Also the use of different guide stars with different accuracies are suspected of introducing additional noise in the flux trends. The best explanation for the flux modulation, still unproven however, is a systematic pointing error towards $ \gamma $ Dra. Band 1 has basically 0 FPG offset, while all the other bands are offset by 2 $ ^{\prime\prime}$$ -$3 $ ^{\prime\prime}$. This makes bands 2, 3 and 4 sensitive to even small ( 1 $ ^{\prime\prime}$) pointing errors, as they are operated close to the edge of the flat top of the SWS beam profile. Begin centred, band 1 remains rather robust against pointing errors.

Figure 8.8: $ \gamma $-Dra fluxes of all passbands vs. revolution number divided by their median. The flux modulations can clearly be seen in the bands 2A, 2B, 2C, 2C $ ^{\prime}$, 3A, 3C, 3D, 3E, 4A, 4D. Band 4 and 4$ ^{\prime}$ have too low flux to provide a statement there. No related modulation of comparable amplitude can be seen in band 1.

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Next: 9. Caveats Up: 8. Beam Profiles, Pointing Previous: 8.5 Straylight
ISO Handbook Volume V (SWS), Version 2.0.1, SAI/2000-008/Dc