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Astron. Astrophys. 353, 465-472 (2000)

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2. Observations and data reduction

The observing run took place from August 13 to 19, 1996, under excellent seeing and transparency conditions (Table 1).


Table 1. Summary of the data sets

The AO correction was performed on the brightest spot of NGC 1068 in the visible continuum light (Lynds et al. 1991). The wavefront sensor (EBCCD after a red dichroic splitter) has a pixel size of 0.7" and takes into account the gravity center of the light within a 6" diameter circular entrance. Due to the pixel size of the wavefront sensor, the contribution of the continuum and the lines from the central 50 pc around the Lynds et al. (1991) peak both fall in one pixel: so the contrast is maximum.

The detector was the COMIC camera (Lacombe et al. 1997), at the f/45 Cassegrain focus, which provides an image scale of 0.1"/pixel, resulting in a field of view of 12.8" [FORMULA] 12.8".

NGC 1068 was observed in an imaging mode through the standard spectral L (3.48µm), L' (3.81µm), and M (4.83µm) bands and through a circular variable filter for the PAH (line 3.3µm rest wavelength, and continuum). Through the L, L' and M bands, we observed in a chopping mode, alternating object and sky images by the use of a field selection mirror. We chose an offset of 10" to the N and 10" to the W.

During the six nights, the visible seeing was measured by the ESO differential image motion monitor. It was excellent during four nights, ranging from 0.4" to 0.7". Therefore, the efficiency of the AO correction was optimal and the images obtained with COMIC were diffraction-limited. However, the gain of the intensifier was not optimized at that time, which resulted in Strehl ratios lower than expected.

In order to minimize position offsets between the calibration star and the AGN, we selected a reference star within 2 degrees of the target. For both the galaxy and the point spread function (PSF) reference star, the air-mass was at most of 1.3, ensuring differential refraction effects to be negligible (less than one pixel).

Individual exposure times were chosen so as to observe under conditions of background limiting performances (BLIP). In this way, the readout-noise is dominated by the background photon noise, and we just take an average of the images to improve the signal-to-noise ratio. We observed several photometric standard stars to obtain a precise flux calibration and another star to determine the PSF for later deconvolution.

Standard infrared data reduction procedures were applied to each individual frame, for both the galaxy and the reference stars: dead pixel removal, sky subtraction, flat fielding from sky images at each wavelength. As the AO system compensates for image shifting, no additional shift correction was applied.

Thanks to the length of our observing run, we obtained a largely redundant data set. We discovered in particular that we had experienced a problem of astigmatism during 2 nights, due to an inappropriate tuning of the visible wavefront sensor leading to a very low signal to noise ratio on the Shack-Hartmann analysor (Alloin & Marco 1997). Being aware of this flaw, we decided to review in depth the AO optimization parameters for the entire data set and to remove all suspicious blocks. In addition, we applied a selection procedure of 32-images data cubes, based upon the seeing value and the Strehl ratio. The corresponding equivalent integration times are reported in Table 1.

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© European Southern Observatory (ESO) 2000

Online publication: December 17, 1999