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Astron. Astrophys. 339, L13-L16 (1998)

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2. Observations and image analysis

We observed HE 2149-2745 on August 8th, 1997, with SUSI on the 3.5m ESO NTT. SUSI uses a TEK CCD with 10242 24 µm pixels corresponding to 0:0013 in a [FORMULA] sky field of view. Despite a rather variable seeing several exposures could be taken under [FORMULA] seeing conditions. This was lucky given the bad weather conditions just days before the observations. The night was not photometric.

We obtained science exposures in the three bands B (5 exposures à 600 s), V (5 à 400 s), and R (9 à 300 s). We also obtained short exposures of the photometric standard star field PG2213-006 (Landolt 1992) to check for possible spatial variations of the point-spread function (PSF) and to normalize our magnitudes to the standard system. After bias-subtraction the images were corrected for detector pixel-to-pixel variations using combined twilight sky flat-fields. The effective width of a point source in the science frames, as measured for the only well-exposed star in the SUSI field (hereafter "star 3"; cf. Fig. 1 in Wisotzki et al. 1996), range from 1:003 to 1:005 in the B-band frames, from 1:003 to 1:004 (V), and from 1:000 to 1:004 (R). Due to this quality dispersion we decided to analyse each image separately .

All BVR exposures clearly resolve the QSO components (hereafter A and B, respectively). We used the DAOPHOT II package to derive the PSF and to obtain astrometric and photometric parameters in the frames containing the QSOs. The PSF was modeled with a [FORMULA] Moffat function whose parameters resulted from the fit to star 3, allowing for empirical departures from the analytical form. A similar one-star fit and subsequent subtraction of the scaled PSF in the more crowded standard-star field demonstrated the spatial stability of the PSF shape, thus validating the use of just one star in its calculation.

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© European Southern Observatory (ESO) 1998

Online publication: September 30, 1998