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Astron. Astrophys. 346, 892-896 (1999)

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2. Observations and data reduction

We observed ZCMa at the CFHT, on January 23rd 1998 during a commissioning night of the integral field spectrograph OASIS coupled to the adaptive optics bonette PUE'O.

In the TIGER mode (Bacon et al., 1995) of the OASIS instrument the field of view is sampled by an array of micro-lenses. The image produced by each micro-lens is then dispersed with a grism and filtered with a large bandpass filter to avoid the overlapping of spectra on the CCD. An OASIS configuration is defined by the sky projected micro-lens separation, the filter, and grism used. Each configuration must be calibrated with additional frames taken at zenith: undispersed micro-pupils, halogen lamp and associated arc, flux standard and associated arc. Because the configuration calibrations are, in general, done at different zenithal angles from the science target, each science exposure must be preceded/followed by an arc exposure to correct for instrument flexure effects in the wavelength solution.

The OASIS configuration used covered the 6200-6532 Å range with a spectral resolution of 3200; the sky sampling was 0.11"/microlens and the field of view [FORMULA]. We took a 300 s exposure of Z CMa followed, at the same telescope position, by a neon arc calibration frame. These exposures were complemented by the standard configuration calibration frames.

The data were processed according to the standard OASIS procedures within the XOasis software. The raw CCD images were offset and bias subtracted. The micro-lenses' positions on the CCD were computed using the micro-pupil frame taken without the grism and filter wheel, the optics distortion corrected, then the spectra crests were found in the halogen frame. Using the previous information and the arc frame associated with the halogen exposure, a model for the spectrograph optics was fitted and the extraction mask created.

The 1100 Z CMa spectra were then extracted from the CCD frame and wavelength calibrated, using the mask and Z CMa associated neon arc, thus creating a raw TIGER data cube. Cosmic rays were removed from the raw cube. These procedures were repeated for the halogen and photometric standard, which were needed to calibrate the Z CMa raw cube for the spatial and spectral dependence of the instrument transmission.

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© European Southern Observatory (ESO) 1999

Online publication: June 17, 1999