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Astron. Astrophys. 346, 243-259 (1999)

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2. Observations, data reduction, and calibration

2.1. Adaptive optics NIR imaging

Near-infrared imaging was performed in August 1995 using ESO's AO system ADONIS (Beuzit et al. 1994) on the 3.6 m telescope at La Silla/Chile. High-resolution images were obtained in H and [FORMULA]. In each band, a mosaic of three frames was obtained resulting in a total integration time of 400 to 600 s, depending on the location in the image. During the observations the seeing was 1". The high-order AO correction improved the FWHM of the PSF to 0:004 in [FORMULA] and 0:005 in H. All frames were subject to standard bad-pixel removal, flat fielding, and dark-frame subtraction processes before being combined in the resulting images. For calibration purposes, images were taken of the UKIRT standard Y4338.

2.2. NIR narrow-band imaging

Narrow-band images were taken at ESO's 2.2 m telescope on La Silla/Chile in June 1998. IRAC2b served as camera. Four filters were used with central wavelengths 2.111 µm (BP4), 2.126 µm (BP5, used as H2(1-0)S1 filter), 2.154 µm (BP7), and 2.170 µm (BP8, used as Br[FORMULA] filter). The central wavelengths of the filters were computed from the filter transmission curves provided by ESO. They are not the same as stated in the IRAC2b manual. The use of lens C resulted in a pixel scale of 0:00507 per pixel. A mosaic of five frames was taken with the object once in the centre of the detector and once in the centre of each quadrant. The total integration time on source added up to 20 minutes. The same procedure was done for the standard star HD169588 with 10 minutes of total integration time.

The rather complex procedure of calibrating the narrow-band images and doing the continuum subtraction is described in Appendix A.

2.3. MIR imaging

2.3.1. L-band imaging

The L band image was obtained in May 1996 using the COMIC camera in combination with ADONIS. 432 frames of 3 s integration time each were combined in the resulting image. As the original goal was to do polarimetry, these frames were obtained at four positions of the polariser. Because the signal-to-noise ratio proved to be too low for polarimetry, all frames were combined in the resulting image. The mean Strehl ratio given by the ADONIS software was 0.075, the PSF FWHM in the image is 0:0052 while the seeing monitor reported a seeing of 1:002. No photometric calibration was done for this image.

2.3.2. 11.7 µm observations

The 11.7 µm image was obtained using SpectroCam-10 (Hayward et al. 1993) at the 200-inch Hale Telescope of the Palomar Observatory 1. The effective wavelength of the filter was 11.7 µm with FWHM = 1 µm. A 5 frame mosaic was combined into the image. The average on-source integration time at each pixel is 20 s. The star [FORMULA]Lyr served as a standard for flux calibration. This image will be referred to as the 12 µm image in the discussion.

2.3.3. Q-band observations

The Q-band data were taken in October 1996 at ESO's 2.2 m telescope on La Silla/Chile. The mid-infrared camera MANIAC (Böker et al. 1997) was used to obtain images using its [FORMULA] µm (Q) filter. The total integration time on the object summed up to 210 s including combination of the two chopping beams. Photometric calibration was obtained by observing the standard [FORMULA] Sgr.

2.3.4. MIR narrow-band observations

Narrow-band observations in the mid-infrared were carried out at ESO's 3.6 m telescope with the TIMMI camera (Käufl et al. 1992). The filters [SIV ] ([FORMULA] µm , FWHM = 0.4 µm) and [NeII ] ([FORMULA] µm , FWHM = 0.4 µm) were used. Chopping and nodding was done to obtain 6[FORMULA]20 images of 37.74 ms integration time in each of the two filters. For photometric calibration, the standard star [FORMULA] Sgr was observed. To derive its brightness at the desired wavelengths, we used its IRAS LRS spectrum.

2.4. 1.3 mm continuum measurements

The continuum radiation of G 5.89-0.39 was mapped in March 1996 using the 15 m SEST telescope at La Silla, Chile, together with the 3He-cooled single-channel bolometer system (Kreysa 1990). The equivalent bandwidth of the bolometer is [FORMULA] 50 GHz centred on a frequency of [FORMULA]GHz ([FORMULA]). The effective beam size at this wavelength is [FORMULA]".

The source was mapped four times with the "double beam" technique described first by Emerson et al. (1979). To generate the dual beams, a focal plane chopper with a chopping frequency of 6 Hz was used. Chopping was done in azimuth. The chopper throw was 67". The map rows were generated by moving the telescope continuously along the direction of the beam separation (i.e. in azimuth) with a scanning velocity of 8"/sec and an elevation separation between adjacent scans of 8".

Calibration maps of the planet Uranus (adopted brightness temperature 96 K; Griffin & Orton 1993) were obtained with the same technique and parameters as used for G 5.89. The atmospheric transmission was measured by sky dips. Telescope pointing was found to be repeatable within [FORMULA]5".

Data reduction was performed with the SEST standard software and with the software package MOPSI (written by R. Zylka) which use the NOD2 and GAG libraries.

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© European Southern Observatory (ESO) 1999

Online publication: May 6, 1999