Astron. Astrophys. 356, 347-356 (2000)

3. Data reduction

The details of the data reduction are the same as in Billebaud et al. (1995) who used the same camera and wavelength ranges to observe Jupiter. Because of the background emission of the sky and instruments at these wavelengths, observing techniques specific to thermal infared are compulsory. On December 8, we used a "chopping" mode, which substracts the contribution of the sky, and on December 9, we used a "chopping+nodding" mode, which substracts the contribution of the sky and corrects the images for the inhomogeneities of the primary mirror. The resulting images are then flat-fielded and false pixels are corrected. Integration time for an individual image is very short (about 40 ms). It is also possible to add those images which are not separated by more than about 10 minutes, in order to improve the signal-to-noise ratio without degrading the spatial resolution. The deconvolution procedure uses the calibration star images to determine the Point Spread Function (PSF) of the instrument.

The December 8 images are also corrected of a background flux which is not removed by the standard procedure. It is thought to be due to the technique used to make the observations: on December 8, we made the observations without the "nodding" mode. The inhomogeneities of the primary mirror are thus not eliminated. In order to remove this contribution, we make an estimation of the averaged background flux and remove it from the images. Due to bad atmospheric conditions, the absolute calibration is not reliable for the December 8 images. Nevertheless, these images can be used to study the numerous features visible on Saturn and the relative variations of the flux. For their part, the December 9 images are of very good quality, except for the images in the acetylene band, for which the flat-fielding procedure did not remove all the instrumental line-shaped features: some groups of pixel raws of the mosaic show an additional flux. This flux is clearly visible outside the planet, but should also be present on the planet. As we will see later, these "line-shaped features" certainly affect the latitudinal variations of the flux in the acetylene band, especially at 13.29 and 13.48 m. We use the two star Peg and µCeph to calibrate the images. Finally, this gives us two sets of 6 images, one for each night. We have two images at 10.91 m, two at 11.69 m and two at 12.47 m for December the 8th. And for December the 9th, we have one at each wavelength (10.91, 11.69, 12.47, 13.09, 13.29 and 13.48 m). The signal-to-noise ratio depends on the date of the observations and on the wavelengths. The best signal-to-noise ratio is obtained in the ethane band emission for the December 9 images (100) and it decreases with longer wavelengths ( 40 at 13.09 µm,  10 at 13.29 m and 10 at 13.48 m, for the December 9 images), as well as for the 10.91 m image ( 8). The signal-to-noise ratio is quite low in the acetylene band because these wavelengths are at the edge of the atmospheric window, very sensitive to weather conditions. Regarding the December 8 images, the signal-to-noise ratio is lower in the ethane band ( 20) and, at 10.91 m, has the same order of magnitude as in the December 9 image. Fig. 1 and 2 present the images of Saturn at all wavelengths for December 8 and 9.

Fig. 1a - f. C10µ observations of Saturn on December 8, 1992.

Fig. 2a - f. C10µ observations of Saturn on December 9, 1992.

Geometry

The next step is to retrieve the geometry of Saturn and to associate each pixel to a latitude and longitude. First, we transform the 6464 pixel images to 10241024 pixel images by making an interpolation. This leads to a better sampling and a more precise determination of the coordinates of each pixel. Then we localize the position of the center and adjust a circle on the planet. Next we make a deprojection of each pixel in the circle: first of all, we assign to each pixel a (y,z) coordinate (the plane corresponding to corresponds to the plane of the image). Then, we retrieve the x coordinate: , where R is the Saturnian radius determined when drawing the circle. We also take the oblate shape of Saturn into account. Afterwards we make two rotations of the axes to take the tilt of the Saturnian spin axis into account. We can thus associate each set of coordinate (x,y,z) to a longitude and latitude. From planetary ephemerides, we also determine the position of the central meridian at the time of observations, and we relate each pixel to a system III longitude. Moreover, we determine the position of the rings by using a reverse procedure (from the theoretical position of the rings, we determine the (y,z) coordinates of the ring edges on our images). Fig. 3 presents a map of Saturn at 11.69 m on December 8 with the limits of the A and B rings.

Fig. 3. Saturn at 11.69 m the 9 December 1992. We have drawn the meridians and the parallels every 10o. The black meridian corresponds to the central one. The position of the rings is also indicated (A ring, Cassini division, and B ring).

© European Southern Observatory (ESO) 2000

Online publication: March 28, 2000
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