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Astron. Astrophys. 322, 554-564 (1997)

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2. Observations and data reduction

2.1. Sub-arcsecond imaging

HD 5980 was observed on 1991 September 26 using the ESO New Technology Telescope (NTT) equipped with the SUperb Seeing Imager (SUSI) which functions with an active optics system (see ESO Web site for more information). The detector was a Tektronix CCD (#25) with 10242 pixels of 24 µm, each corresponding to 0 [FORMULA].13 on the sky. The filters were R, V, and B, and the seeing was around 0 [FORMULA].7 during the observations. Also, bias and flat-field frames necessary for image calibrations were secured during the run.

We observed HD 5980 also using the ESO VLT adaptive optics prototype installed at the Cassegrain focus of the 3.6 m telescope at La Silla (Rigaut et al. 1991 , Hubin et al. 1993 ). On 1993 November 8 the COME-ON-PLUS system was used and on 1996 January 1st the more advanced system ADONIS. In both systems the incoming light from a reference source is analyzed by a wavefront sensor. Using this information, the surface of a deformable mirror is modified in real time by a servo-control to compensate the wavefront distortions induced by the atmospheric turbulence. This technique yields resolution close to the diffraction limit of the telescope, at least longward of 1 µm wavelength. The visible magnitude of HD 5980 (brighter than 12) being within the sensitivity range of the wavefront sensor detector, HD 5980 was used as its own reference for the wavefront measurement. The corrected images were recorded on a 256 [FORMULA] 256 infrared camera (Sharp II) with a 0 [FORMULA].05 pixel scale on the sky. Images were obtained in standard photometric bands J (1.25 µm), H (1.68 µm), and K (2.25 µm) with an elementary integration time of 10 s. As a typical example, a total integration time of 200 s in J and 400 s in H and K were achieved on the source in the 1993 run. An internal chopping mirror allowed to move the source on two different areas of the detector and thus to simultaneously record both the source and the sky background for later subtraction. Exposures were also recorded in the same conditions on the reference stars SAO 248288, 255763, and 255729 for later deconvolution of the images.

We applied the standard infrared reduction procedure (sky subtraction, dead pixel removal, and flat fielding) using the IRAF/NOAO image reduction package. Some periodic features due to the detector electronics were minimized by a treatment in the Fourier plane. The different cleaned frames were eventually centered and averaged. In order to improve the resolution, we used two deconvolution methods in the IRAF/STSDAS package, the Lucy-Richardson and maximum entropy algorithms. The results were reconvolved by a gaussian to avoid an unrealistic resolution. The FWHM of the final images are 0 [FORMULA].28 in J, 0 [FORMULA].24 in H and 0 [FORMULA].17 in K.

2.2. CASPEC observations

HD 5980 was observed with the CASPEC spectrograph attached to the 3.6 m telescope on 1989 September 14. The 31.6 lines mm-1 grating was used with a 300 lines mm-1 cross dispersion grating and an f/1.5 camera. The detector was CCD #8, a high resolution chip of type RCA SID 006 EX with 1024 [FORMULA] 640 pixels and a pixel size of 15 µm. The central wavelength was [FORMULA] 4250 Å and the useful wavelength range 3975 to 4820Å corresponding to orders 118 to 142 of the Thorium-Argon calibration arc. The resulting FWHM resolution as measured on the calibration lines is [FORMULA] 0.2 Å.

2.3. EMMI echelle spectroscopy

The New Technology Telescope (NTT) coupled with the ESO Multi-Mode Instrument (EMMI) was used during two runs, 1993 September 21-23 and 1994 September 10-13 in order to get high dispersion spectra of HD 5980. The instrument mode was the red arm REMD. Grating # 9 defined the dispersion and grism #3 served as a cross-disperser. The resulting dispersion was 18.3 Å mm-1, corresponding to a resolving power of 6900 for a 1 [FORMULA] [FORMULA] [FORMULA] decker. The detector was a CCD chip (Loral # 34) with 20482 pixels of 15 µm corresponding to 0 [FORMULA].35 on the sky. This covered (in 1994) orders 22 to 41 corresponding to [FORMULA] [FORMULA] 3920-7610 Å.

The echelle spectra were reduced using the echelle context implemented in the MIDAS package. Flat-field exposures were used to define the order positions. The sky spectrum was extracted over 6 pixels aside of the object spectrum and the wavelength calibration was performed using exposures of a Thorium-Argon lamp. The resolution as measured from the calibration lines is [FORMULA] 0.8 Å at 5000 Å.

Due to the low level of the flat-field exposures in the blue region we had to use the stellar spectrum itself to define the positions of the highest orders. For the same reason we were not able to perform an appropriate background definition in the blue region of the flat-field and no flat-field correction was applied between [FORMULA] 4130 and [FORMULA] 5200 for the 1993 spectra and [FORMULA] 3920 and [FORMULA] 4750 for the 1994 data. The wavelength calibration was done separately for each of the blue orders leading to residuals of 0.01 Å. Individual orders were corrected for the blaze function using properly chosen continuum windows. The quality of the normalization was checked by comparing overlapping regions of adjacent orders.

2.4. EMMI slit spectroscopy

Several moderate long-slit spectra were taken of HD 5980 using NTT+EMMI with grating # 12 on 1991 December 27, 1993 September 21-23, and 1994 September 10-13. The CCD detector in 1991 was Tektronix #28, while in 1993 and 1994 we used Tektronix # 31. In both cases the format was 10242 pixels with a pixel size of 24 µm. The range was [FORMULA] [FORMULA] 3810-4740 Å and the dispersion 38 Å mm-1, giving FWHM resolutions of [FORMULA] pixels or [FORMULA] Å for a 1 [FORMULA].0 slit.

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© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998