The observations in UBV Johnson and RI Kron - Cousins pass bands were
carried out between 27 March - 2 April and 22 - 28 June 1989 on the ANU 1.0
metre telescope at Siding Spring Observatory, using a blue coated GEC P8603 CCD
in an Astromed camera system mounted at the f/8 Cassegrain focus.
During the observations typical seeing was 1.5 arcsec. The log of the
observations is given in Table 3 (click here). Each square pixel
of the
element CCD corresponds to 0.56 arcsec on the sky.
The read-out noise for the system was about 8 electrons per pixel and
the number of electrons per ADU is equal to 1. Flat-field exposures
ranging from 1 to 5 s in each filter were made on the twilight sky.
Graham's (1982) E-region standards (see Sagar & Cannon
1995) were observed for calibration purposes. They cover a range in
brightness (
) as well as in colour (
). The regions observed for the construction of the CMDs of
the clusters are shown in Figs. 1 (click here)-4 (click here) for NGC 3228, NGC
4103, NGC 5662 and NGC 6087 respectively. More than one region was imaged
for each cluster, except for NGC 4103. The regions were chosen
in a way to exclude the very brightest cluster members which would
have saturated the CCD, but to include some previously
photoelectrically observed stars and to be as near as possible to the
cluster nucleus, thus maximising the number of measurable cluster
members and minimising the proportion of field stars included in the
CMD. The regions selected in this way are located generally well within
the cluster diameter except in NGC 3228 where they are lying almost at
edge of the cluster diameter and it turned out that we are too far from
the cluster centre to catch any bright members or to see any faint ones
clearly, so we are unable to add anything to previous data regarding its
reddening, distance and age. The locations of the these
regions with respect to each cluster centre are given in Table 4 (click here).
There is a small overlap between the two regions of NGC 3228 and NGC
5662 and between the regions 2 and 3 of NGC 6087.
To cover the widest possible range of magnitudes, generally 2 or
3 graded exposures in each pass band were taken (see Table 3 (click here)). They
range from 40 to 600 s in U, from 10 to 600 s in B and from 1 to
300 s in V, R and I.
Table 3: Log of CCD observations
Table 4: Relative positions of the imaged regions
from the cluster centre