2 Observations

Optical positions for the CERS were obtained by means of a hybrid multi-step procedure involving CCD and photographic observations, which responds both to the need for attaining usefully small rms errors at very faint magnitudes, and the need to refer the positions to a standard reference system.

On account of their faintness, most of the objects in our program cannot be adequately observed by means of direct photography with wide-field astrographs, so the use of CCD detectors is necessary. However, due to the limited size of the existing CCD chips, and the low density of present fundamental catalogues, CCD astrometry by itself produces only relative positions. Both CCD and wide-field photographic observations are therefore needed; the first for very precise astrometry in the local field of the targets, and the second to link the results to a standard system.

2.1 Photographic observations

These observations were carried out with the flat-field 70/100/210 cm Maksutov Astrograph (field: $5\hbox{$^\circ$}\times 5\hbox{$^\circ$}$,scale: $99.4 \hbox{$^{\prime\prime}$}/{\rm mm} )$ at the EACR.

Two forming gas hypersensitized Kodak IIIaJ plates, one long exposure (50 min) and one short exposure (2 min), were taken centered on each CERS; both on the same night, and near culmination. A Schott GG385 filter was used throughout. Although not ideal, the use of the blue bandpass was dictated by emulsion availability.

As a first step we identify in the short exposure plate all stars from the Hipparcos Catalogue (which was adopted as primary reference frame) present in the $5\hbox{$^\circ$}\times 5\hbox{$^\circ$}$ field around the CERS. Excluding bright stars, close pairs and stars near the edges of the plates, roughly 60 homogeneously distributed measurable catalogue stars were identified on average in the fields of the sources. We then select about 30 well distributed stars with a magnitude $B \sim 12-14$ in the central $\sim 1.5\hbox{$^\circ$}\times 1.5\hbox{$^\circ$}$ field around the source. The positions of these latter stars are then determined with respect to the stars of the Hipparcos Catalogue, which leads to the establishment of a secondary reference system to be used for position determinations on the long exposure plate. Because of their magnitude, these secondary stars are adequately measurable both in the short and in the long exposure plates (this is not the case of the much brighter primary stars from the Hipparcos Catalogue, which are well overexposed in the deep plates).

In the second step of the procedure we identify all non saturated stars in the $\sim 8.5\hbox{$^\prime$}\times 8.5\hbox{$^\prime$}$ field of the CCD image, which can at the same time be adequately measured on the long exposure plate. They turn out to be stars roughly in the magnitude range $B\sim 15 - 18$. On average, 20 stars satisfying the above conditions could be identified in the fields of the CCD images. The positions of these stars are then measured with respect to the previously established secondary frame and this leads to the definition of a tertiary reference system which can be used for position determinations on the CCD image.

It should be noted that, since the Hipparcos Catalogue was constructed to coincide with the ICRS, our ad-hoc secondary and tertiary reference frames are tied to the ICRS. The positions of the CERS determined with respect to the tertiary stars in the field of the CCD image will therefore be obtained in the system of the ICRS.

2.2 CCD observations

The CCD observations were carried out with the 1.5 m telescope at Cerro Tololo Interamerican Observatory (CTIO) and with the 2.5 m telescope at Las Campanas Observatory (LCO). Both telescopes provide a very similar set-up in terms of scale and field: $0.24 \hbox{$^{\prime\prime}$}$/pixel, 8.19$\hbox{$^\prime$}\times$ 8.19$\hbox{$^\prime$}$, CTIO and $0.26 \hbox{$^{\prime\prime}$}$/pixel, 8.87$\hbox{$^\prime$}\times$ 8.87$\hbox{$^\prime$}$,LCO. In both cases the CCD detectors used were Tektronix 2048 $\times$ 2048 chips with 24$\mu$ pixels. The small scales not only allow an optimum definition of the centroids of the Point Spread Functions (PSF), but also make it possible in a few cases to investigate the optical morphology of the CERS.

Four frames were typically obtained of each target, which were registered and then combined to produce a single work image (see next section). Extensive testing showed that 600 s. exposures (CTIO; 300 s. LCO) were necessary to obtain a good signal to noise ratio for the sources, without saturating the tertiary reference stars. A Johnson B filter was employed throughout for consistency with the photographic observations. All observations were made during culmination to minimize refraction effects, and only with good seeing ($FWHM\leq1.5\hbox{$^{\prime\prime}$}$). In optimum seeing conditions ($FWHM\leq1.0\hbox{$^{\prime\prime}$}$) we estimate that the limit of detection of our survey is $B\sim 24$.

  

Table 1: Identification of the sources

1 indicates a newly identified optical counterpart,
LC stands for Las Campanas,
CT stands for Cerro Tololo.
References to finding charts
[1] This paper
[2] Bolton J.G., Shimmins A.J., Wall J.V., 1975, A.J.P. Astrophys. Supp. 34, 1
[3] Drinkwater M.J., Webster E.L., Francis P.J., et al., 1997, MNRAS 284, 85
[4] Jauncey D.L., Savage A., Morabito D.D., et al., 1989, AJ 98, 54
[5] Jauncey D.L., Batty M.J., Gulkis S., et al., 1982, AJ 87, 763
[6] Torres C., Wroblewski H., 1987, A&AS 69, 23

In all but one case the CCD and the photographic observations were secured at reasonably similar epochs, ensuring a negligible error contribution from the unknown proper motions of the intermediate reference stars. Table 1 gives the epochs of the observations together with information related to the identification of the sources. CT indicates that the CCD observations were carried out at CTIO; LC that they were carried out at LCO. Finding charts for the newly identified optical counterparts (Figs. 1 to 7) were produced from their corresponding combined CCD images. The approximate B magnitudes given for them in Table 1 (and those given for sources that were detected to have varied considerably) were estimated from their signal to noise ratio on the combined CCD frames. An asterisk in the Remarks column indicates that additional comments are made in Sect. 6 (Notes on individual objects).

 

Figure 1:  Figures 1--7: Finding charts for the newly identified optical counterparts, as indicated in Table~1. Charts are 4.1 arcmin on a side. See Sect.~6 for details