Open Access

Fig. 5


Example of data preprocessing on an image taken during the 2008 campaign on May 5. Color scale is inverted, i.e., dark is bright and white is faint. The image is centered on the Fe doublet at 557 nm. Left panel: raw image, which is the result of a 1 min exposure on Venus during daytime. The dark area corresponds to the region occupied by Venus spectrum, while the Fraunhofer lines scattered by Earth’s sky are clearly visible out of it. The sky lines are tilted because of the geometric distortions mentioned in the text. The areas delimited by dotted lines and indicated by two vertical arrows are used to compute the average sky spectrum, which is then subtracted from the whole image. Right panel: “cleaned” image after dark field removal, rectification of geometric distortion, flat-field division, and mean sky lines subtraction.

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