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_c, I_c filters, and also at the Nordic 2.5-meter telescope (NOT) with HiRAC instrument in the V, I_c, H bands. At the 6 m telescope we used a ISD017A CCD detector with 1050x1170 pixels, each 16x16µm (0.137"x0.137"), at the 1 m telescope we had a K983 CCD detector with 530x580 pixels, each 18x24µm, and at the NOT the observations were carried out with Loral Lesser-thinned CCD chip having 2048 by 2048 pixels each of 15 microns size, that provided 3.7'x3.7' view field with a resolution of 0.11" per pixel. The journal of observations is given in Table 1. We used frames of the galaxies, which were obtained under good photometric conditions with a seeing of FMHM =0.7"-1.4".

Table 1: Journal of observations

Telescope	Filter	Exposure	Seeing
Date		(sec)	(arcsec)
BTA 6m Apr. 23, 1996	I R	2x600 600	1.1 1.4
BTA 6m Jul. 14, 1996	I V B	600 600 600	0.9 0.9 0.9
Zeiss 1m Aug. 17, 1996	B V R I	2x600 600 600 600	1.1 1.1 1.1 1.1
BTA 6m Jun. 14, 1997	R	600	1.3
NOT 2.5m Jul. 26, 1997	V I H Hc	3x1000 3x900 2x1200 2x1200	0.7 0.7

 

After dark frame subtracting, flat-fielding and cleaning from 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_c & I_c system. According to our estimate a total error in zero-point does not exceed 0.05 mag for each spectral bands.

Figure 1 shows the I image of NGC 6789 obtained with NOT. For the analysis of stellar content (see § 3.1 and § 3.3), the observed field has been divided into several parts. Region C corresponds to the central part of NGC 6789; region E covers the optical external part of the galaxy and regions F map the foreground field stars. The central part is displayed in Fig. 2, which shows a colour composition of all band images. Dark blue (H-band) indicates HII regions, corresponding to a large concentration of blue stars. Left pictures on the figures 1 & 2 are the original frames, and right ones are the same parts after subtracing the frame median smoothed with a window of 10x(FWHM).