2 Observations

2.1 The target galaxy

Figure 1: Contour plot of ESO 342-G017 showing levels from 20.0 to 27.0 R mag/sq arcsec in 0.5 mag/sq arcsec steps. The image is a central subsection of our total ESO 342-G017 mosaic. The inability to trace smooth contours at the lowest light levels and the noisier background on the western end of the source is due to fewer frames making up the mosaic on this side of ESO 342-G017.

The target, ESO 342-G017, is a nearby, edge-on galaxy, selected on the basis of its right ascension and declination, very high disk inclination, absence of a prominent bulge, low extinction correction, and optimal angular size. The latter is important in order to adequately resolve the disk scale height while maintaining sensitivity to faint surface brightness in the halo. Our deep R-band image obtained with the VLT-UT1 test camera is shown in Fig. 1 and the basic properties of the source are given in Table 1.

   
2.2 Observing strategy and resulting data

 

 

Table 2: Summary of observations.

Field	Filter	Dates	Total integration	Median seeing
		August 1998	(s)	(arcsec)
ESO 342-G017	Bessel-V	22	3300	1 $.\!\!^{\prime\prime}$1
	Bessel-R	18, 22, 25	10320	0 $.\!\!^{\prime\prime}$9
Flat-field frames:
HDF-Sa	Bessel-V	18, 22, 23, 26, 27	16200	0 $.\!\!^{\prime\prime}$9
	Bessel-R	18, 22, 23, 25, 26	15300	1 $.\!\!^{\prime\prime}$0
EIS0046-2930	Bessel-V	17	2700	0 $.\!\!^{\prime\prime}$8
	Bessel-R	17	2700	0 $.\!\!^{\prime\prime}$8
EIS0046-2951	Bessel-V	22	2700	0 $.\!\!^{\prime\prime}$9

Bessel V and R observations of ESO 342-G017 were made on the nights of 18, 22 and 25 August 1998 as part of the ESO VLT-UT1 Science Verification (SV) program. A complete description of the VLT SV program telescope and instrument set-up can be found in Leibundgut et al. (1998) and Giacconi et al. (1999). We give only a summary of the issues important for our observations of ESO 342-G017.

The VLT Test Camera, an engineering grade Tektronix 2048² CCD, was rebinned $2\times2$ to improve its surface brightness sensitivity, resulting in a binned scale of 0.091 arcsecpixel^-1and a field of view of 93 arcsec on a side. The camera was rotated approximately 60 degrees in order to position the galaxy major axis along the x-axis of the detector. For economy of prose throughout the paper, we will refer to the northeast and southwest sides of ESO 342-G017 as the "northern'' and "southern'' sides, respectively.

A challenge to our data reduction was the fact that the Test Camera CCD is not a science-grade device. As such, it displays more than the customary number of cosmetic flaws, most noticeably, a large region ($\sim$ $120\times130$ pixels) near the center of the chip with a lower sensitivity than its surroundings. Although this "stain'' has a strong colour dependence (it is more prominent in the blue), we found it to be temporally stable and therefore easily corrected with our science frame flat-fields (see Sect. 3.2). Furthermore, ESO 342-G017 was always positioned well away from this feature.

Fortuitously, long total integrations were made of the HDF-south and two EIS cluster candidate fields on the nights of 17, 18, 22, 23, 25, and 26 August, in the same filters as our observations. Using these images to create our deep sky flat-fields obviated the time-intensive strategy of observing off-source fields for ESO 342-G017. Each of the images used to make our superflats, as well as the observations of ESO 342-G017 itself, were dithered on average by more than 10 $^{\prime\prime}$ in both $\alpha$ and $\delta$. This allowed for the removal of cosmic rays from our galaxy field, and the removal of stars in the super sky flat (see Sect. 3.2).