2 Observations and reductions

2.1 Observations

Optical images were obtained with the 2.56 m Nordic Optical Telescope (NOT) at La Palma during three observing runs in March, May and December 1994. Table 2 summarises the instrumentation used. In addition, we have some supplementary observations from other observing runs. I-band observations of RC 1510+0438 were made with "Stockholm'' CCD in July 1994 and RC 2013+0508 was observed with Brocam1 in September 1994. The complete log of observations is given in Table 3. For each observed object it contains the filter used, number of separate images, total integration time, seeing, and date. Calibration stars from Landolt (1992) were observed several times each night at a range of air masses.

 

Table 2: Instruments

 

Table 3: Journal of observations

In this paper, we shall restrict the discussion to observations made under excellent or good seeing (1) conditions, totaling 22 objects. We present only R-band images except for a few cases where the morphology has a strong wavelength dependence and the S/N-ratio in other passbands is high enough. All observations presented were made under photometric conditions.

"Blooming'' of the CCD was a serious problem with the IAC CCD. Some objects were close to bright stars which limited the longest possible exposure time, or a bright star had to be moved outside the CCD. RC 1735+0454 lies close to the galactic plane, hence the field is crowded with bright stars. We could not obtain long exposures of this faint object and had to move it close to the edge of the CCD. The bright star northeast from the centre of gravity of RC 1219+0446 hampered the observations and the northern part of the radio source was not observed.

2.2 Reductions

The reductions were carried out using standard IRAF routines (bias subtraction, trimming, flat fielding). The average bias frame was constructed for each night. The flatfielding was made by twilight flats obtained each evening and morning. All the scientific frames were flattened at better than a 1%-level. The exposures of each object were registered in position using several stars in the field and then averaged. The number of reference stars varied from three up to a dozen.

The astrometric calibration was carried out using the APM Catalogue (Irwin et al. 1994) whenever possible. For the objects near the galactic plane the Guide Star Catalog (GSC) (Lasker et al. 1990) was used. Due to the small field of view of the CCDs there were typically only a few reference stars in the frame. The number of stars and hence the accuracy of the astrometry strongly depends on galactic latitude. We estimate the accuracy of the astrometric calibration to be typically better than 1 s of arc. This is enough for the current study, because the typical resolution of the radio map is about 1$\hbox{$.\!\!^{\prime\prime}$}$5 and most of the radio sources are so compact that the optical identification is straightforward.

As a check of our photometry in the March and May 1994 run we measured comparison stars of OJ 287 (Fiorucci & Tosti 1996). The derived brightnesses were consistent with each other within 0.1 magnitudes.

 

Table 4: NOT imaging data. The diameter of the aperture is indicated in arcseconds. The magnitudes are without correction of galactic extinction. Ellipticity and position angle of resolved sources is measured with the same aperture as the magnitudes. The radio position angles are measured from Kopylov et al. (1995a)