All Figures

	Figure 1: Observational geometry in the test case of Io, where a thin slit is aligned with the equator of the planet. The relative motion of the body induces a velocity offset V0. Measuring differential Doppler shifts allows the retrieval of the rotational velocity v only.
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In the text

	Figure 2: Each order is a spectral image built from a sequence of monochromatic slit images. The curvature of both the spectral and slit images needs to be corrected.
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In the text

	Figure 3: Spatial profile of an order. The latter is close to a Gaussian function in the case of Io and Titan. By convention, the region corresponding to the centroid of the intensity distribution is r=0.
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In the text

	Figure 4: Breaks due to the CCD grid periodically appear in the extracted spectra when no interpolation is used.
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In the text

	Figure 5: Segment of a two-dimensional spectrum. Each pair corresponds to the eastern and western regions of the target, intensity decreasing with distance from the planet's center.
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In the text

	Figure 6: The slit image curvature is computed by using the Connes algorithm. The curvature is almost the same for each order, as shown by the dotted lines.
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In the text

	Figure 7: The FWHM variation of the spatial profile mainly depends on the seeing and possible drifts of the target during the exposure.
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In the text

	Figure 8: Relative position of the orders. The centering variation along the slit length is larger than the pixel scale for two consecutive observations.
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In the text

	Figure 9: A set of Doppler shifts is computed for each order (dotted lines). We sum independently the orders from the blue part (triangles) and red part (squares) of the spectral range. The slope in the central part corresponds to the direct rotation of Io, as expected.
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In the text

	Figure 10: The measured Doppler shift ideally tends towards a maximal velocity value at infinity, smaller than the target's rotational velocity. An artificial decrease may occur due to normalization error.
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In the text

	Figure 11: Velocity retrieval scheme. The extracted velocity is the maximal Doppler shift value measured in the range where the shifts are symmetrical in relation to the estimated target's center.
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In the text

	Figure 12: Summary of the results for Io. Velocities extracted from the blue and red spectral ranges are plotted with triangles and squares. The spurious variation results from systematic errors, such as the uncertainty on the pointed latitude, drifts, or the degradation inherent to the Earth's atmosphere.
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In the text