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Astron. Astrophys. 353, 457-464 (2000)

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2. Observations and data reduction

A spectrum of the quasar Q1205-30 can be found in Lanzetta et al. (1991), but its celestial coordinates were never made publicly available. To obtain an image we therefore had to "rediscover" Q1205-30 on a copy of the prism plate UJ9085P kindly made available to us from the UK Schmidt Telescope Objective Prism Survey (for details of the procedure see Fynbo et al. 1999b). Celestial coordinates of Q1205-30 are RA 12 05 35.72, Dec -30 14 25.8 (1950).

Deep imaging of the field was subsequently carried out in NTT service mode (Silva & Quinn 1997, Silva 1998, Woudt & Silva 1999). The field was imaged through standard B and Bessel I filters, as well as through a special 20Å (fwhm) narrow band filter manufactured by Custom Scientific. The narrow band filter (CS 4906/20) is centred at 4906Å, which is the wavelength of redshifted Ly[FORMULA] at [FORMULA]. The service mode data were obtained with the ESO Multi-Mode Instrument (EMMI) on the NTT during several nights of January, February and March, 1998. The CCD used in the red arm of EMMI was a SITe TK2048 with a pixel scale of 0.27 arcsec. The blue arm CCD was a SITe TK1024 with a pixel scale of 0.35 arcsec. The total integration times in each filter and the seeing in each of the combined frames are given in Table 1.


Table 1. Journal of observations, NTT, January through March 1998.

Landolt (1992) photometric standards and three spectrophotometric standard stars Eggr99, Feige 56 and L970 were used for the photometric calibrations.

I-band imaging was obtained with the red arm of EMMI. The CCD in the red arm of EMMI can be read in either single port (D only) or dual port (A + D) read-out mode. Unfortunately it showed up that for our programme a mix of single and dual port read-out had been employed, and we had to follow a somewhat complex reduction procedure to make up for this.

The disadvantage of the single port read-out is the longer read time required (which is particularly a problem while obtaining twilight flats). The disadvantage of the dual port read-out is that the bias level, the gain (and drift of the gain), and the readout noise are different for the two image sections. Also the locations of traps (bad columns) depend on the direction of charge transfer along the columns. In particular the section read through port A has more bad columns than the same section read through port D. The I-band science exposures of Q1205-30 were read out using dual port read-out, whereas the exposures of standard stars were read out using single port mode only. This mixed mode operation requires 2 sets of biases, as well as two sets of flat fields, and two different ways of reducing the images. Twilight flats were only obtained for single port mode (appropriate for the standard star exposures), while dome flat were taken with dual port read-out (appropriate for the science frames). The dual read exposures were treated as if the A and D subsections were two individual frames. Finally, bias frames, flat field exposures, and science exposures were pasted together to form 2048x2046 frames. The subsequent flat-fielding was done in the usual way.

The CCD in the blue arm of EMMI can be read in single port mode only. All science frames obtained with the blue arm of EMMI (all B-band and narrow band data) were bias-subtracted and flat-fielded using standard routines.

Following the basic reduction the images in each of the three filters were combined employing the code described in Moller & Warren (1993a). This code optimizes the signal-to-noise for faint objects in the field, for which the noise is well understood via propagation of read-out-noise and photon shot noise.

All magnitudes quoted in this paper are on the AB system. The narrow band data were calibrated directly onto the AB system using the spectrophotometric standard stars, and these magnitudes are denoted [FORMULA]. We found that the colour terms for both the I and B filters were consistent with zero, hence we use the equations [FORMULA] and [FORMULA] (Fukugita et al. 1995) to put the broad band magnitudes onto the AB system. Details of the sky brightness and sky noise in the combined images are provided in Table 2.


Table 2. Measured level and rms of sky surface brightness.

We reach limiting magnitudes (5[FORMULA]) of n(AB)=25.0, B(AB)=25.8 and I(AB)=25.1. A narrow band AB magnitude of 25.0 corresponds to a Ly[FORMULA] flux of [FORMULA] erg s-1 cm-2.

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© European Southern Observatory (ESO) 2000

Online publication: December 17, 1999