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Improvement of selection criteria

  

Figure: Histograms of redshift distributions for the same BCG samples, as for the previous figure

Having quite low efficiency at this pilot stage it is natural to try to improve the selection criteria. The analysis of all accumulated data discussed here, shows that in order to improve the selection of BCGs the following additional criteria and selection procedures should be applied to the preselected lists of candidates:

1.

Second priority candidates have very low detection rates for BCGs (< 20 %) and pick up mainly ELGs of other types which are not in the focus of our project. So, these objects should not be observed further except as backup sample. This will result in an underestimate of the BCG surface density of $\la$ 20 %

2.

It appears that the majority of bright (m_B < $18\fm0$) stellar-like objects are either blue or M-dwarf stars. Thus, additional examination of candidates on direct images in order to discriminate between stellar and fuzzy images will allow to remove most of the obvious brighter star-type interlopers.

3.

The results of follow-up spectroscopy have shown that even part of the prominent first priority candidates turned out non-emission objects. Subsequent check of the plate material have shown that some defects or strong noise peaks looked like strong emission features on objective prism spectra. Thus, we concluded that it is necessary to make a careful check of all first priority candidates on the original spectral plates (scanning of 2 plates of the same region with cross-check - in the majority of cases) or additional visual inspection of the candidate spectra - in case of only one plate - to remove possible dust grains and noise hits.

These improvements allow to increase the discovery rate of BCGs by a factor of $\sim$ 2, as it is shown in a forthcoming paper by Pustilnik et al. ([1998]).