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2 Observations and data reduction

J (1.2 $\mu$m) and K (2.2 $\mu$m) photometry of King 5 was obtained at the 1.5 m Gornergrat Infrared Telescope (TIRGO) equipped with Arcetri Near Infrared Camera (ARNICA) in October 1997. ARNICA is based on a NICMOS3 256 $\times$ 256 pixels array (gain = 20 e-/ADU, read-out noise = 50 e-, angular scale = 1 $\hbox{$^{\prime\prime}$ }/$pixel, and $4 \times 4 \hbox{$^\prime$ }^{2}$ field of view). More details about the observational equipment and infrared camera, and the reduction procedure can be found in Carraro et al. (1999a). Through each filter 4 partially overlapping images of each field were obtained, covering a total field of view of about 8 $\times$ 8 $\hbox{$^\prime$ }^{2}$, in short exposures to avoid sky saturation. One field is located close to the center of King 5. In addition, a sample of the background population of King 5 was obtained observing a field of $2^{\prime} \times 2^{\prime}$ at about $15^{\prime }$ South-East from the cluster center and a second one of the same dimensions located at about $5^{\prime }$ Northward.

The log-book of the observations is presented in Table 1 where the centers of the observed fields and the total exposure times are given. The nights were photometric with a seeing of $1\hbox{$^{\prime\prime}$ }- 1.5\hbox{$^{\prime\prime}$ }$. Figure 1 presents the final mosaic of the 4 frames for King 5 in K passband.


  \begin{figure}{\psfig{file=ds9007f3.eps,width=8.8cm} }
\end{figure} Figure 3: The CMD of two comparison fields taken $15^{\prime }$ eastwards (left panel) and $5^{\prime }$ northwards (right panel) of King 5

The conversion of the instrumental magnitude j and k to the standard J, K was made using stellar fields of standard stars taken from Hunt et al. (1998) list. About 10 standard stars per night have been used. In addition 2 stars (thereafter K5A and K5B) inside the field have been measured with a photometer and used for the calibration. Results are summarized in Table 2.


 

 
Table 2: Photometry of K5A and K5B
Star K J
K5A 9.20 $\pm$ 0.04 10.41 $\pm$ 0.05
K5B 8.74 $\pm$ 0.04 10.10 $\pm$ 0.05


The relations in usage per 1 s exposure time are:

J = j+19.51 (1)


K = k+18.94 (2)

with standard deviation of the zero points of 0.03 mag for the J and 0.04 for the K magnitude. This error is only due to the linear interpolation of the standard stars. The calibration uncertainty is dominated by the error due to the correction from aperture photometry to PSF fitting magnitude. The standard stars used for the calibration do not cover the entire colour range of the data, because of the lack of stars redder than $(J-K) \sim 0.8$. From our data, no colour term is found for K mag, whereas we cannot exclude it for the J magnitude. Taking all into account, we estimate that the total error on the calibration is about 0.1 mag both in J and K pass-bands.


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