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2 Measurement procedure and the accuracy of measurements

All measurements were made using the Digitized Palomar observatory Sky Survey (DSS) which is constructed from 6.5 degree square plates which have been digitized with a modified PDS microdensitometer. The resultant pixel size is nominally 25 microns, corresponding to about $1\hbox{$.\!\!^{\prime\prime}$ }7$ arcsec. While the primary motivation for the STScI/DSS was to support Hubble Space Telescope operations, the entire astronomical community has benefited from this service. Earlier the DSS has already been used to derive accurate coordinates for several hundred QSOs (Schneider et al. 1992; Kirhakos et al. 1994), and AGNs (Véron-Cetty & Véron 1996).

Two groups of researchers made an independent measurement of positions of 2978 SBS objects. One group used the facilities of the Special Astrophysical Observatory (SAO), Russia. The second group used the resources available at the Space Telescope Science Institute (STScI) in Baltimore, Maryland, U.S.A.

The measurements were made prior to the near-universal accessibility of the DSS over the Internet (e.g., http://archive.stsci.edu/dss/). Investigators at the SAO used the FITSview (Flexible Image Transport System) softwave (NRAO; Bill Cotton, http://www.cv.nrao.edu/bcotton/fitsview.html). It allows the user to select and display images, to mark the position of objects and to measure the coordinates and pixel brightness of the image. FITSview software was used to display the extracted images from the SAO set of DSS CDroms and, after marking of the positions of identified objects, to measure the accurate coordinate of objects for J2000.0 epoch. The coordinates were converted to B1950.0 epoch by the use of the conversion program described in Lieske (1979). Finding charts of the SBS objects were used to confirm selected objects to be measured.

Independent measurements at the STScI were made by using the DSS, after performing additional extractions of smaller spatial extent, with data resampled to a pixel size of $\sim0\hbox{$.\!\!^{\prime\prime}$ }4 $arcsec. In order to facilitate convenient electronic access to the STScI/DSS, all image data were compressed using a lossy technique. The data were typically compressed by a factor of 10, a level that does not affect the fidelity of astrometry measurements. In all cases the astrometry relied on stars listed in the AGK3 (Heckman et al. 1975). The difference of positions between the two sets of measurement has an rms of $\sim1\hbox{$.\!\!^{\prime\prime}$ }5$ arcsec in both coordinates implying that the accuracy of each set of measurement is $\sim1\hbox{$.\!\!^{\prime\prime}$ }1$ arcsec.

The accurate optical coordinates of the SBS objects (mean of the two sets of measurements) are listed in Tables 1 and 2. Column 1 gives the SBS designation of the objects. Columns 2 and 3 list the B1950.0 right ascension and declination, while the corresponding J2000.0 coordinates are listed in Cols. 4 and 5. An optical magnitude is listed in Col. 6. If the precision is specified in multiples of 0.01 mag, the listed value is a photometric measurement with an accuracy $< 0\hbox{$.\!\!^{\rm m}$ }05$ (Chavushyan et al. 1995); otherwise the value represents an eye estimation of the photographic magnitude with an uncertainty of $0\hbox{$.\!\!^{\rm m}$ }5$ (Stepanian 1994). Finally, alternative names for the SBS objects are listed in Col. 7.

Earlier, in various astrometric programs, accurate optical positions were measured for more than 1000 out of 2978 SBS objects (Green et al. 1986; Sanduleak & Pesch 1987; Nelson et al. 1988; Seal et al. 1990; Simpson 1990; Stepanian 1994; Bowen et al. 1994; Véron-Cetty & Véron 1996).

The comparison of the position measured by us with the mentioned ones is presented in Table 3. Plots of the differences of positions are shown in Figs. 1-8.


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