4 Presentation of relative astrometry data

Figure 1: Internal errors on separation vs. angular separation for two classes of $\Delta m$.

Table 3 lists the relative astrometry and differential photometry for 239 Hipparcos double stars, one apparently double (ADS 6223), one triple system (SAO 244567) and 12 astrometric standard stars. In the first column we list the identifier: generally a running Hipparcos number or, in a single case, a double star identifier. If no Hipparcos number exists, they are identified by their HD or Durchmusterung designation. Astrometric standard stars with an Hipparcos identifier are filed among the programme stars but can easily be recognized by the suffix "a'' attached to the identifier. In the following columns, from left to right, we give the number of different observations, $N_{\rm obs}$, the component identification (Cp), the total number of frames, N, the mean separation (mean of two filters) with its standard deviation, the mean position angle (mean of two filters) with its standard deviation, the instrumental magnitude differences in V and i (with standard deviations), the Bessel epoch and a two letter code for the observer. Hereafter, both angular separation and position angle will refer to their mean taken in both filters V and i. N_V and N_I are the number of frames taken in V and i respectively. Mean values are listed in case of repeated observations of the same double star on a different night. In addition, an observation of ADS 6223 and SAO 244567, which appeared as a "wide'' triple system, had been requested. Such data are listed at the end of each table. Negative detections (no components found) are not listed: this concerns HIP 1876 (on 16/08/92 and 18/08/94), HIP 21765 (on 23/12/93) and HIP 31711 (on 25/12/93). The last column gives the difference between the CCD and the Hipparcos relative position in arcsec. This "distance'' or difference between the two measurements in position of the secondary with respect to the primary component, $d_{\rm H}$, may be computed as

$$d_{\rm H} = \sqrt{\rho_{\rm ccd}^2 + \rho_{\rm H}^2 - 2 \rho_{\rm ccd} \rho_{\rm H} \cos(\theta_{\rm ccd} - \theta_{\rm H})}.$$ (1)

In one case a different component than the one measured by Hipparcos was evidently observed (HIP 21577). The agreement is generally excellent as shown in a previous analysis based on a set of some 400 observed visual double stars (Oblak et al. 1997). In total, some 50 campaigns were performed in various observatories. This paper is the second one of the series grouping the data of five missions alltogether. It seems therefore preferable to us to postpone the comparison between the CCD and the Hipparcos relative positions until after all the network results are available. Nonetheless, we hereby list $d_{\rm H}$ to illustrate the overall quality of our data.