Table 3: Characteristics of the co-added WFI images. The limiting magnitudes quoted in Col. 4 are defined in the Vega system via $m_{\rm lim}=ZP-2.5\log(\sqrt{N_{\rm pix}}\cdot 3\cdot\sigma)$ , where ZP is the magnitude zeropoint, $N_{\rm pix}$ is the number of image pixels in a circle with radius $2\hbox{$.\!\!^{\prime\prime}$ }0$ and $\sigma$ the sky background noise. The seeing in Col. 5 was measured with the SExtractor FWHM_IMAGE parameter. The groups in Bonn (B) and Innsbruck (I) observed Z3146 with an offset of about $5\hbox {$.\mkern -4mu^\prime $ }0$ In Ra and $8\hbox {$.\mkern -4mu^\prime $ }0$ in Dec. The exact layout is given in Fig. 8. For the R band we created, besides the individual stacks, a deep mosaic in the common area (A). We note that the Innsbruck R band has nearly double the exposure time of the Bonn image but about the same limiting magnitude as it was observed during less favourable moon phases. The quality of our photometric calibration is crosschecked in Fig. 9.
Filter Image region exp. time (s) limiting magnitude ( $3\sigma$ in a $2\hbox{$.\!\!^{\prime\prime}$ }0$ aperture) Seeing (arcsec)

B (B) 1500 24.70 1.21

V (I) 8040 25.02 1.16

R (I) 16077 25.27 1.02

R (B) 8850 25.24 1.11

R (A) 24927 25.71 1.04

**Table 3:** Characteristics of the co-added WFI images. The limiting magnitudes quoted in Col. 4 are defined in the Vega system via $m_{\rm lim}=ZP-2.5\log(\sqrt{N_{\rm pix}}\cdot 3\cdot\sigma)$ , where ZP is the magnitude zeropoint, $N_{\rm pix}$ is the number of image pixels in a circle with radius $2\hbox{$.\!\!^{\prime\prime}$ }0$ and $\sigma$ the sky background noise. The seeing in Col. 5 was measured with the `SExtractor` FWHM_IMAGE parameter. The groups in Bonn (B) and Innsbruck (I) observed Z3146 with an offset of about $5\hbox {$.\mkern -4mu^\prime $ }0$ In Ra and $8\hbox {$.\mkern -4mu^\prime $ }0$ in Dec. The exact layout is given in Fig. 8. For the R band we created, besides the individual stacks, a deep mosaic in the common area (A). We note that the Innsbruck R band has nearly double the exposure time of the Bonn image but about the same limiting magnitude as it was observed during less favourable moon phases. The quality of our photometric calibration is crosschecked in Fig. 9.
Filter	Image region	exp. time (s)	limiting magnitude ( $3\sigma$ in a $2\hbox{$.\!\!^{\prime\prime}$ }0$ aperture)	Seeing (arcsec)
B	(B)	1500	24.70	1.21
V	(I)	8040	25.02	1.16
R	(I)	16077	25.27	1.02
R	(B)	8850	25.24	1.11
R	(A)	24927	25.71	1.04

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