2. Observations

For the measurements we have used a CCD detector placed at the prime focus of the Schmidt telescope at Campo Imperatore (AQ) Observatory (Natali et al. 1990). The dimensions of the telescope are (60/90/180) cm. The detector was equipped with a Thomson chip TH7882, front illuminated with a metachrome 2 coating to enhance its sensibility to the U and B photometric bands. The linear dimensions of the sensitive area are , organized in pixels ), which are equivalent at the telescope focus, to a field of view of (scalefactor = 2.64 arcsec/pixel). The sample has been selected according to the following criteria:

a)

homogeneity: all objects have morphological types between Sbc and Scd, to avoid any dependence of the measured quantities on morphology;

b)

small inclination angles (, except for NGC 6946, with ), to reduce autoabsorption effects;

c)

angular diameters between 15' and 3.3', for a better use of the great field of view.

The objects are listed in Table 1 (click here); all the parameters listed in the table, except some values of the inclination angle, which are taken from the Uppsala General Catalog (when a number is used for the inclination angle), and the inclination angle of NGC 5194, have been taken from the Nearby Galaxies Catalog. For each observing night, flat-field frames have been taken, but we haven't measured any standard stars, therefore the photometric magnitudes are differential.

  
Table 1: Objects observed