Observations of Be 18 were collected during three observing runs at the Kitt Peak National Observatory. Table 1 (click here) gives essential information about these runs.
Table 1: Summary of observations of Be 18
Preliminary processing of the data was done with the standard routines in the
IRAF-CCDPROC package
.
The flat-field frames were prepared by
combining ``dome flats'' and exposures of the twilight sky.
Such procedure was used while reducing data from all 3 runs.
Five overlapping sub-fields covering the central part of the cluster
were observed
during run #1. We obtained four exposures for each sub-field:
120 s and 600 s exposures in the V-band, 240 s and 900 s
exposures in the B-band. The sky was dark and clear during these
observations. The seeing ranged from 1.1 to 1.3 arcsec.
The instrumental photometry was extracted using DAOPHOT/ALLSTAR package
(Stetson 1987, 1991).
A point spread function varying linearly with
X and Y coordinates was adopted.
12 standard stars from the Landolt (1983) were observed on the
night of October 3, 1991. In addition we observed twice
5 standards from the field around Ru 149 (Landolt 1992)
and several standards from the so called ``North consortium field''
located in M92 (Stetson & Harris 1988). The transformation
equations adopted for the night of October 3, 1991 are:
where X is the airmass and the lower case letters refer to the
instrumental magnitudes normalized to 1 s exposures. The standard
deviations about magnitudes and colors for standard stars
are 0.016 in V and 0.019 in (B-V).
During our run the ST1K camera suffered from a charge transfer
problem. The problem led to the formation of ``shadow-like''
traces on one side of very bright stars present in the frame, and
affected photometry of some faint stars located in vicinity of
the bright objects.
The color-magnitude diagram (CMD) of Be 18 based on observations
collected during run #1 is shown in Fig. 1 (click here).
Figure 1: V vs. B-V CMD for stars from the Be 18 region based
on photometry obtained with the KPNO 2.1-m telescope + ST1K camera
in October 1991
One field centered on the cluster center was observed during run #2. During this run, the 0.9-m telescope produced images with strong positional variability of the PSF. Stellar images were particularly poor for the first 500 columns of the chip. This section of frames was not used for photometry. About half of the frames were obtained with the telescope pointing offset by 500 pixels along rows. Hence, the effectively observed field had a size equivalent to the full field of view of the T2KA camera. A total of six images were obtained for each of BVI filters. One short exposure (180 s, 120 s and 70 s for B, V and I filters, respectively) and a pair of long exposures (900 s, 600 s and 420 s, for B, V and I filters, respectively) were obtained for each filter and for each pointing. The seeing ranged from 1.65 arcsec to 2.45 arcsec. Moreover, on the night of October 19 the sky was covered with thin cirrus clouds. The the point spread function varying quadratically with X and Y coordinates was used to extract the profile photometry. The night of October 20 was photometric and the data obtained on that night were used to establish a transformation from the instrumental to the standard system. See Kaluzny & Rucinski (1995) for details about the calibration procedure. The derived CMD's are presented in Fig. 2 (click here).
One field covering the central part of Be 18 was observed during run #3. This run was plagued by bad weather. Some usefull data could be collected only during the last few hours of the last night of the run. Therefore, only a few exposures of the cluster were obtained. The log of observations is given in Table 2 (click here). Note that only one, relatively short exposure, was obtained in the B-band. The reduction procedure and details about transformation from the instrumental to the standard system were described in Kaluzny (1995). The derived CMD's are presented in Fig. 3 (click here).
Figure 2: V vs. B-V and V vs. V-I CMD's for stars from the
Be 18 region based on photometry obtained with the KPNO 0.9-m
telescope
Figure 3: V vs. B-V and V vs. V-I CMD's for stars from the
Be 18 region based on photometry obtained with the KPNO 2.1-m
telescope + T1KA camera in October 1992
The three data sets described above were calibrated independently of each other. The zero points of the BVI photometry derived during runs #2 and #3 are in a good agreement. The largest systematic differences do not exceed 0.015 mag. The B-V colors based on the data from run #1 are systematicaly bluer by about 0.035 in comparison with colors based on runs #2 and #3.
Table 2: Log of Be 18 observations collected during run #3
Figure 4:
The observed luminosity function for the field of Be 18
observed with the KPNO 0.9-m telescope. LF is the number of stars
in intervals 0.25 mag wide
Figure 5:
Star density as a function of radial distance from the center
of Be 18 for objects with 18<V<21.0