Astron. Astrophys. 336, 823-828 (1998)

2. Observations and data reduction

We observed the central region of NGC 7469 at 3.48, 3.81 and 4.83 µm with ADONIS, the adaptive optics system installed at the ESO 3.60-meter telescope (La Silla, Chile). The adaptive optics system is fully described in Beuzit et al. (1994). The ADONIS system was equipped with the COMIC camera, working from 3 to 5µm . The wavefront was directly sensed on the bright visible nucleus of NGC 7469 and the adaptive optics correction on the nucleus was performed in the infrared.

The observations were made on August 17, 1996, under excellent seeing and transparency conditions (seeing 0.6"), as indicated in Table 1.

Table 1. Summary of the data sets

We used the COMIC camera (Bonaccini et al., 1996), at the f/45 Cassegrain focus, which provides an image scale of 0.1"/pixel, resulting in a field of view of 12.8" 12.8". The COMIC detector is a 128128 HgCdTe photovoltaic focal plane array. At 77 K, the average dark current is 2000 e^-/sec and the total capacity is about 6 10⁶ electrons. The readout noise is about 1000 e^- rms allowing to observe under conditions of background limiting performances (BLIP) for individual exposure times larger than 500, 260 and 150 ms for the L, L' and M bands respectively (Lacombe et al., 1997). In this way, the readout-noise is dominated by the background photon noise, and we just take a mean of a series of individual images to improve the signal-to-noise ratio.

For 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 north and 10" to the west.

We observed several photometric standard stars to obtain a good calibration in flux, and another star to determine the point spread function (PSF) for 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. We payed careful attention to flatfielding, spending several hours at the start or at the end of the nights, to record sky images and to derive an excellent flatfield for each wavelength. The adaptive optics system compensates for image shifting. We cross-correlated individual images to track residual shifts: the centroid coincidence between individual frames was found to be always better than 1 pixel (i.e. 0.05"). So no additional shift correction had to be applied.

© European Southern Observatory (ESO) 1998

Online publication: July 27, 1998
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