Astron. Astrophys. 362, 1072-1076 (2000)

2. Observations and data reduction

We observed Mars using an imaging spectrometer in the 0.4 1.05 µm spectral range on the 1.5 m telescope at Sierra Nevada Observatory, Granada (Spain), from 19 March through 24 1997. The planet was at a geocentric distance = 0.659 AU with an angular dimension of 14.2". The telescope was configured at f/8, yielding a scale of 0.4 arcsec per pixel, equivalent to a footprint of 186 Km. However, during the period of our observations, seeing rarely reached the 1", while most of the period seeing was 2". The spectrometer used a CCD detector, 384x288 pixels of 2323 µm dimensions and cooled by liquid nitrogen at -40^o C. The instrument simultaneously acquire contiguous high-resolution spatial and spectral information, thus producing a so-called image cube. The images were obtained scanning the telescope across the planet and acquiring each CCD frame. The frames were then packed into a tridimentional data set (e.g. image cube) composed of 121 monochromatic (bandpass = 50 Å) images. The summary of the observations is reported in Table 1. We report the results of analysis of only one image cube because it presents the best signal to noise ratio. It was acquired on March 20 at 0.20 UT. Dark and flat field frames were acquired and the cubes corrected following standard procedures. The raw data were corrected for various instrumental effects following the procedures described in Bellucci et al. (1998). In order to correct the spectra for instrumental response and telluric absorption features, relative reflectance spectra were created by ratioing the raw spectra to the Mare Serenitatis II (MS2 18^o40'N, 21^o25'E) area on the Moon, which was observed at same Mars air mass and time. A residual telluric feature due to O₂ is however still present in the reduced spectra (see Fig. 2 below). Absolute reflectance spectra were then produced by using the following relation:

where , are the Mars and MS2 raw spectra, respectively, is the reflectivity of MS2 (McCord et al. 1972) and C a scaling factor. C was chosen to scale the average spectrum of Arabia to 0.4 at 1.040 µm (Bell et al., 1999).

Table 1.

In Fig. 1 a reduced data set is shown in the form of six monochromatic images.

Fig. 1. Multispectral images of Mars acquired on March 20. The wavelength in micron is indicated.

The corresponding wavelength (in micron) of each image is also shown. It was summer in the northern hemisphere of Mars at time of our observations ( = 92) and Syrtis Major, Hellas cloud and north pole were visible.

© European Southern Observatory (ESO) 2000

Online publication: October 30, 2000
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