4. Data reduction

4.1. Initial processing

Reduction of the obtained images was done with the MIDAS package developed by ESO. Initial processing of the data contained usual steps such as bias and dark frame subtraction and flat-fielding. Cosmic rays were then removed from each galaxy image with the FILTER/COSMIC procedure. The galaxy frames of the same band were summed, if several exposures were made. Images of the observed galaxies in B band are presented in Fig. 1.

Figure 1: The images of the sample galaxies in B band, which were obtained at the 1.2-m telescope of Observatoire de Haute-Provence (France). The field size of the images is $11.8' \times 11.8'$. For all the images North is top and East is left

Figure 1: continued

Figure 1: continued

Figure 1: continued

Figure 1: continued

Figure 1: continued

Figure 1: continued

4.2. Sky background determination

Background stars were removed from the frames by fitting a second-degree surface in circular pixel-area. The sky background was then approximated by a tilted plane, created from a 2-dimension polynomial, using the least-squares method (FIT/FLAT_SKY). The accuracy of the sky background determination is about 0.3% of the original sky level. A typical value of the sky surface brightness is near 22$^{\rm m}$/sq s in B band. Therefore, the mean error of sky determination must not exceed 0.07 mag.

   

Table 2: Observational log

Ident.	Date	B expos.(sec)	V expos.(sec)	I expos.(sec)
UGC 3755	11.04.1997	2$\times$400	2$\times$400	2$\times$500
NGC 2366	05.04.1997	600	500	600
UGC 3974	11.04.1997	2$\times$500	2$\times$500	2$\times$600
K 52	10.04.1997	2$\times$600	2$\times$600	2$\times$900
UGC 4459	05.04.1997	2$\times$600	2$\times$500	2$\times$600
UGC 4483	09.04.1997	2$\times$400	2$\times$400	2$\times$600
F464-v3	12.04.1997	3$\times$600	3$\times$600	3$\times$900
UGC 5086	11.04.1997	2$\times$400	2$\times$400	2$\times$600
Holmberg I	02.04.1997	600+1200	1000	800
UGC 5288	12.04.1997	2$\times$400	2$\times$400	2$\times$600
BK3N	12.04.1997	3$\times$600	3$\times$600	3$\times$900
Holmberg IX	06.04.1997	2$\times$800	2$\times$800	2$\times$1000
UGC 5423	07.04.1997	2$\times$600	2$\times$600	2$\times$800
UGC 5692	03.04.1997	3$\times$400	2$\times$300	2$\times$300
	09.04.1997	400	60	60
UGC 5918	09.04.1997	3$\times$500	3$\times$500	3$\times$600
K 73	12.04.1997	2$\times$600	2$\times$600	2$\times$900
UGC 6817	02.04.1997	2$\times$600	2$\times$500	2$\times$450
NGC 4068	04.04.1997	600	2$\times$300	2$\times$300
	10.04.1997	100	100	100
NGC 4144	04.04.1997	2$\times$300	300	300
	10.04.1997	100	100	100
NGC 4163	05.04.1997	600	500	700
PGC 38998	09.04.1997	2$\times$300	2$\times$300	2$\times$500
UGC 7298	07.04.1997	3$\times$600	3$\times$600	3$\times$800
UGC 7321	04.04.1997	1000	800	600
	10.04.1997	100	100	100
IC 3308	09.04.1997	2$\times$300	2$\times$300	2$\times$500
UGC 7577	06.04.1997	2$\times$400	2$\times$400	2$\times$500
NGC 4449	02.04.1997	100	100	100
UGC 7698	10.04.1997	600	600	2$\times$900
IC 3687	04.04.1997	2$\times$400	2$\times$400	2$\times$400
	10.04.1997	100	100	100
IC 4182	02.04.1997	600	500	2$\times$300
UGC 8201	10.04.1997	400	400	2$\times$600
UGC 8308	02.04.1997	2$\times$1200	1000	1000+900
UGC 8320	04.04.1997	2$\times$400	2$\times$400	2$\times$500
	10.04.1997	100	100	100
UGC 8331	05.04.1997	2$\times$500	2$\times$500	2$\times$600
UGC 8508	06.04.1997	2$\times$400	2$\times$400	2$\times$500
NGC 5229	07.04.1997	600	600	900
Holmberg IV	08.04.1997	2$\times$600	2$\times$600	3$\times$800
	10.04.1997	100	100	100
NGC 5474	06.04.1997	500	500	600
NGC 6503	02.04.1997	100	100	120
NGC 6946	05.04.1997	3$\times$100	100	100

The center coordinates for about 1/3 of all the observed galaxies were determined by ellipse fitting of outer parts of the galaxy (FIT/ELL3). Most of other galaxy centers were determined interactively, because a significant part of the sample consists of the galaxies of irregular morphology. For some galaxies with a star-like center the coordinates of the center were determined using first moment of the intensity pixel values.

4.3. Photometric calibration

For the calibration, the photometric standard stars from Landolt's (1992) list were observed during the observational run after every two galaxies. Due to extinction present on some nights, a number of the galaxies were exposed once more with short exposures under better photometric conditions (see Table 2). Deeper images were then calibrated using these short-exposure data.

Zero-point errors of the transformation to the standard Johnson-Cousins system are estimated to be about 0.15$^{\rm m}$ in each filter.