2 Observations and reductions

The observations of NGC 6789 were made in 1996 and 1997 at the prime focus of the 6 m telescope, at the Cassegrain focus of the 1 m telescope of SAO (Russia) with standard Johnson B, V and Cousins $R_{\rm c}$, $I_{\rm c}$ filters, and also at the Nordic 2.5-meter telescope (NOT) with the HiRAC instrument in the V, $I_{\rm c}$, and H$\alpha$ bands. At the 6 m telescope we used a ISD017A CCD detector with $1050\times1170$ pixels, each $16\times16~\mu$m ( $0\hbox{$.\!\!^{\prime\prime}$ }137\times0\hbox{$.\!\!^{\prime\prime}$ }137$), at the 1 m telescope we had a K983 CCD detector with 530 $\times$ 580 pixels, each $18\times24~\mu$m, and at the NOT the observations were carried out with Loral Lesser-thinned CCD chip having 2048 by 2048 pixels of 15 microns, that provided a $3\hbox{$.\mkern-4mu^\prime$ }7\times3\hbox{$.\mkern-4mu^\prime$ }7$ view field with a resolution of $0\hbox{$.\!\!^{\prime\prime}$ }11$ per pixel. The journal of observations is given in Table 1. We used only the frames obtained under good photometric conditions with a seeing of $FMHM =0\hbox{$.\!\!^{\prime\prime}$ }7-1\hbox{$.\!\!^{\prime\prime}$ }4$.

 

 

Table 1: Journal of observations

Telescope	Filter	Exposure	Seeing
Date		(s)	(arcsec)
BTA 6 m	I	$2\times600$	1.1
Apr. 23, 1996	R	600	1.4
BTA 6 m	I	600	0.9
Jul. 14, 1996	V	600	0.9
	B	600	0.9
Zeiss 1 m	B	$2\times600$	1.1
Aug. 17, 1996	V	600	1.1
	R	600	1.1
	I	600	1.1
BTA 6 m	R	600	1.3
Jun. 14, 1997
NOT 2.5 m	V	$3\times1000$	0.7
Jul. 26, 1997	I	$3\times900$	0.7
	H$\alpha$	$2\times1200$
	H $\alpha_{\rm Cont}$	$2\times1200$

After dark subtraction, flat-fielding and cleaning for cosmic events the photometric processing of the frames was performed with DAOPHOT and ALLSTAR packages (Stetson [1987]) running within MIDAS. Equatorial photometric standards from Landolt ([1992]) were observed in all observing runs to transform instrumental magnitudes into the standard B, V, $R_{\rm c}$ & $I_{\rm c}$ system. The zero-point error does not exceed $0\hbox{$.\!\!^{\rm m}$ }05$ for each spectral band.

Figure 1 shows the I image of NGC 6789 obtained with the NOT. For the analysis of stellar content, the observed field has been divided into several parts as indicated in the figure caption.