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4.7 Spacecraft Pointing Jitter

The ISO pointing system is described in detail in the ISO Handbook Volume I, [40]. It has been shown to yield the following performance:

random motion of the satellite is 0.5 $^{\prime \prime }$ (2 $\sigma$ ),
absolute pointing drift is smaller than 0.1 $^{\prime \prime }$ /hour,
absolute pointing error (blind pointing accuracy): around 2 $^{\prime \prime }$ (half cone).

This pointing stability was much better than the pre-launch specifications of the spacecraft. However, the random spacecraft jitter, although limited to about 0.5 $^{\prime \prime }$ , can cause a slight displacement of a source from image to image, which is most noticeable when a point source is observed. This pointing jitter has some impact on the data analysis and in principle a correction for this systematic effect would be desirable. So far, despite numerous attempts, none of the proposed algorithms to correct for this effect was very successful. Fortunately, even when using the 1.5 $^{\prime \prime }$ pfov lens, the influence of the pointing jitter is not dramatic (Figure 4.19). More details on the jitter correction currently available in CIA can be found in the ISOCAM Interactive Analysis User's Manual, [28].

**Figure 4.19:** Distribution of pointing jitter offsets as computed with a Gaussian fitting method as available in CIA. Data are taken from observations of the calibration standard star HIC 73005. In this particular case the pointing jitter standard deviation is measured to be 0.065 $^{\prime \prime }$ , a good demonstration of the pointing stability of the ISO telescope.
$\resizebox {12cm}{9cm}{\includegraphics{jitter_stats.eps}}$

Next: 4.8 The Spectral Response Up: 4. Calibration and Performance Previous: 4.6 Point Spread Function

ISO Handbook Volume II (CAM), Version 2.0, SAI/1999-057/Dc