2 Observations and reductions

The observations reported on here were acquired on two nights, June 13/14 and 15/16, 1994, using the 100'' telescope of the Las Campanas Observatory. The detector was a TEKTRONIC 2 CCD ($1024\times1024$ pixels) with a readout noise of $8{\rm e}^-$ rms; the pixel size was 21 $\mu$m. The center of the measured field is approximately in 2' to the east and 1' to the north of the cluster center; a $4\times4$ arcmin field is covered by the frames. We used a total of 10 frames in U (exposure time from $50^{\rm s}$to $900^{\rm s}$), 12 frames in B ($12^{\rm s}$ to $500^{\rm s}$), 11 frames in V ($10^{\rm s}$ to $360^{\rm s}$), 12 frames in R ($20^{\rm s}$ to $420^{\rm s}$), and 12 frames in I ($8^{\rm s}$ to $360^{\rm s}$). The average seeing estimated during reductions was 1.5''.

The reductions of the photometry were performed at the Institute of Astronomy (Moscow) using the software described by us earlier (Samus et al. 1995a) and based upon the DAOPHOT II ALLSTAR application (Stetson 1991). PSF stars, 15 to 20 per frame, bright but far from saturation, were selected interactively among those having no neighbors or defects within the PSF radius.

Our experience shows that, among standard PSFs provided by DAOPHOT, the PENNY2 function enables us to handle aberrations specific to individual frames most effectively. The photometric errors showed no significant dependence on the position within the frame, so we did not introduce any variations of the PSF.

In the reductions, we used photoelectric standards from Alvarado et al. (1994). As usual for CCD studies of the Isaac Newton Institute, we did not observe any comparison fields, but applied a photoelectric sequence established in the program field itself. The CCD magnitudes and colors of the eight used standards, which cover a rather wide range of color indices, are listed in Table1, along with the deviations (in the sense CCD minus photoelectric) from the photoelectric values.

  

Table 1: Observations of photoelectric standards in NGC 6723

The formulae used in this study to derive magnitudes and color indices in the standard system from DAOPHOT ALLSTAR instrumental magnitudes for NGC 6723 are:

$$\begin{eqnarray} \nonumber V&=&v-0.015(\pm\,0.040)(b-v)-0.017(\pm\,0.041),\\ \no... ...nonumber V-I&=&0.922(\pm\,0.025)(v-i)+0.048(\pm\,0.025). \nonumber\end{eqnarray}$$

In each photometric band, we retained only stars with at least two measurements. Consequently, we obtained magnitudes for 3620 stars in V and R, 3565 stars in B, 2307 stars in I, and 2222 stars in U. The limiting V magnitude was about 22. The results of our photometry are available from N. Samus upon request.

For V<15, the random rms errors are, on average, $0\hbox{$.\!\!^{\rm m}$}021$ in U; $0\hbox{$.\!\!^{\rm m}$}014$ in B; $0\hbox{$.\!\!^{\rm m}$}013$ in V; $0\hbox{$.\!\!^{\rm m}$}010$ in R; and $0\hbox{$.\!\!^{\rm m}$}010$ in I. For 15<V<18, the errors are $0\hbox{$.\!\!^{\rm m}$}030$ in U; $0\hbox{$.\!\!^{\rm m}$}021$ in B; $0\hbox{$.\!\!^{\rm m}$}016$ in V; $0\hbox{$.\!\!^{\rm m}$}011$ in R; and $0\hbox{$.\!\!^{\rm m}$}014$ in I. For V>18, they are, on average, $0\hbox{$.\!\!^{\rm m}$}088$ in U; $0\hbox{$.\!\!^{\rm m}$}055$ in B; $0\hbox{$.\!\!^{\rm m}$}040$ in V; $0\hbox{$.\!\!^{\rm m}$}033$ in R; and $0\hbox{$.\!\!^{\rm m}$}046$ in I.