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
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
arcsec in both
coordinates implying that the accuracy of each set of measurement is
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
(Chavushyan et al. 1995); otherwise the
value represents an eye estimation of the photographic magnitude with an uncertainty of
(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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