Astron. Astrophys. 336, 648-653 (1998)

2. Observations and data reduction

The observations presented here were carried out at the 3.6m CFHT (Canada-France-Hawaii Telescope) on 19 and 20 April 1995 in good seeing conditions (between 0.3 and 0.6 arcsec FWHM in the visible) with the INSU-OPM (Institut National des Sciences de l'Univers, Observatoire de Paris-Meudon) infrared camera CIRCUS. The CIRCUS camera included a InSb array detector which was optimized for the thermal infrared bands ( 3 µm). The camera was used in speckle mode data acquisition i.e. single exposures were kept relatively short. Measurements were made in the K(=2.22 µm, =0.36 µm), L'(=3.87 µm, =0.61 µm), M(=4.75 µm, =0.43 µm) bands and in the UIR emission band (=3.3 µm, =0.16 µm).

The speckle data include a set of short exposure images of the source, an unresolved reference star and their adjoining sky backgrounds. The nearby star SAO 133091 was used as a point source to determine the point spread function (PSF). We switched every few minutes between the object and the reference star in order to sample the temporal seeing variations so as to minimize seeing effects during the reduction process. The reference star was chosen to have brightness similar to that of the source. Proximity in the sky and similarity in brightness of the reference was chosen in order to prevent non-linear effects due to the detector defects or to the variation of the optical transfer function.

During the observations, we used a sub-array of the detector selected for its best performances. The magnification was 0.109 arcsec per pixel whitch gave a field of view of 6.4 6.4 arcsec². The integration time was selected to achieve the best resolution with a sufficiently large signal-to-noise ratio. We made images with exposure time of 147 ms in K and L', 183.9 ms in the M band and 294.6 ms at 3.28µm. In order to estimate the contribution of the detector and of the sky we used the same integration time for the sky close to the source (typically 40 arcsec away) and reference frames for each bandwidth. The journal of observations is given in Table 1.

Table 1. Journal of observations

Data were analyzed using the package developed by Tessier (1993) within IRAF by the following procedure. Each image first had the background subtracted and was interpolated over bad pixels. The best images of the source were then co-added using the shift-and-add (SAA) technique. The SAA images were therefore deconvolved using PSF's derived from similar data for the reference star. We used the IRAF Lucy algorithm to deconvolve images. The angular resolution after image reconstruction is of 0.2 arcsec at K, L' and 3.3 µm, and 0.3 arcsec at M.

The spatial calibration was determined from the binary star Ser giving an absolute spatial scale better than 1 and a position angle (PA) less than 1 degree. No photometric calibrations have been done.

Our 3.3 µm UIR image is obtained through a so-called PAH filter centered at 3.28 µm with a spectral width of 0.16 µm. The filter contains not only the UIR feature but also a continuum which was not subtracted. From Geballe et al. (1989), we estimate the contribution from the continuum in this filter at approximatively 60 of the PAH emission.

© European Southern Observatory (ESO) 1998

Online publication: July 20, 1998
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