2 Observations

In May 1996 we used the Danish 1.54 m telescope at La Silla, ESO, to obtain Strömgren photometry of the two open clusters NGC 6134 and NGC 3680 over a period of five nights. We also made observations of 19 bright Strömgren standard stars from the catalogue by Olsen et al. (1994) in order to transform the instrumental magnitudes to standard Strömgren indices.

The combination of the focal reducer instrument Danish Faint Object Spectrograph and Camera (DFOSC) and a Loral/Lesser CCD (W11-4) permits a large area of the clusters to be observed in one frame, i.e. 13.6 by 13.6 arcmin (2048 by 2048 pixels). Due to a failed UV-flooding, the CCD was insensitive in the u band, making the Strömgren photometry incomplete. A large decrease in sensitivity towards the edges was also seen in the remaining filters.

The observations of NGC 6134 were made with several different shifts of the telescope (a few arc minutes) and rotations of the CCD camera ( $\pm90^\circ$). This was done in order to detect any dependency of the derived magnitudes on the position of the CCD due to large scale flat field errors. As noted by Andersen et al. (1995) focal reducer instruments, like the DFOSC mounted on the Danish 1.54 m telescope, may suffer from scattered light problems due to insufficient baffling and/or internal reflection in the optics of the instrument. These effects will result in erroneous flat fields. In Table 1 we note the number of frames obtained in each of the Strömgren filters.

For the observations of the Strömgren standard stars we defocused the telescope in order to get a higher duty cycle. The degree of defocusing was different for each target, i.e. depending on the brightness of each star (V in the range 9.7 to 7.2). We also read out only a small 750 by 400 window on the CCD for these observations.

   

Table 1: Number of frames obtained of the open clusters NGC 6134 and NGC 3680 in each of the Strömgren filters. The two $\beta$ filters are labeled W (wide) and N (narrow)

Cluster	u	v	b	y	$\beta_{\rm W}$	$\beta_{\rm N}$	Total
NGC 6134	1	19	18	24	10	14	86
NGC 3680	0	11	17	9	7	8	52