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Astron. Astrophys. 321, 907-920 (1997)

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2. Observations and data reduction

The observations were made in October 1993 and September 1994 with the Canada-France-Hawaii 3.6-meter telescope (CFHT) at Mauna Kea (Hawaii) using a new instrument called BEAR. This instrument uses a new technique, coupling a near-infrared camera (Redeye) equipped with a 256 x 256 pixel NICMOS 3 array to the Fourier Transform Spectrometer (FTS). A detailed description of this instrument and the first observational results have been given by Maillard & Simons (1993), Simons et al. (1994) and Maillard (1995). BEAR simultaneously produces spectra at each point of an extended source over a circular field of view of [FORMULA] in any spectral range between 1 and [FORMULA], allowing monochromatic images of the field at any wavenumber within the spectral bandpass sampled. It offers a choice of spectral resolution (up to [FORMULA]) over a large spectral range, combined with imaging at seeing-limited spatial resolution with a scale of [FORMULA]. Due to the high spectral resolution achievable, BEAR provides the unique capability of measuring the velocity of strong lines over the full field of view (e.g., the 1-0 S(1) H2 line).

The filters used to record the data were a K'-band filter (actually a little narrower, 4480 to 4925  [FORMULA] FWHM) to limit the thermal background, and a narrow-band filter ([FORMULA] 1.1 [FORMULA]) centered on the 1-0 S(1) H2 line (4712.9 cm [FORMULA]) to reach higher spectral resolutions in a minimum number of frames. The K'-band data were recorded in October 1993 for both AFGL 2688 and NGC 7027 with resolutions (FWHM of BEAR's spectral point spread function) of 2.33 and 1.05  [FORMULA], which corresponds at 4712.9 cm-1 to 148 and 67  [FORMULA], respectively. NGC 7027 (in Oct. 1993) and AFGL 2688 (in Sept. 1994) were also observed using the narrow-band filter resulting in a resolution of 0.59  [FORMULA] (37.6 km s-1) for AFGL 2688 and 0.82  [FORMULA] (52.2  [FORMULA]) for NGC 7027. These values do not represent the ultimate capabilities of the technique which is still in the development stage. The data were processed following the steps listed by Maillard (1995). The present results have been derived from the power spectra obtained after inversion of each interferogram through a FFT routine computed with a number of points equal to the number of recorded frames. The resulting images in the final cubes have sizes of 62 x 57 pixels. The numbers of frames is minimum and has been chosen to sample the free spectral range with a step slightly smaller than the resolution. This step corresponds to a velocity channel in the data cubes.

The data cubes (x, y, [FORMULA]) were analyzed using the GRAPHIC software package developed in Grenoble. The flux calibration and correction for telluric absorption lines were made by dividing all the independent spectra of the data cube by a fit to the observed spectrum of the A0 standard star SAO 53596 (K=6.960 mag), which was first divided by a black-body curve at the assumed effective temperature of the star (9850 K). When fitting the stellar spectrum in the K'-band, the Br [FORMULA] (2.166  [FORMULA]) line seen in absorption in the spectrum of SAO 53596 was removed from the fit. Line fluxes were calculated by fitting Gaussian functions to the observed line profiles.

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998