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Astron. Astrophys. 318, 729-740 (1997)

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2. Hitchhiker camera observations

The data consist of deep B- and R-band images of four fields obtained with the Hitchhiker parallel CCD camera on the 4.2m William Herschel Telescope on La Palma. The camera, described in Driver (1994), performs imaging in an off-axis field, 7 arcmin from the optical axis, while the telescope is used for its normal spectroscopic programmes. Using a dichroic beam splitter, data are recorded from the same field in two colours simultaneously.

The data used here were collected over several observing periods in 1991 and in 1993. They are summarised in Table 1. Example R band data are presented in Fig. 1, where the candidate [FORMULA] and faint blue galaxies of Sect. 5.1 are marked. The 1991 data are described in detail by Driver (1994) (and one field by Driver et al. 1994a). Total integration times on the fields ranged between 32 and 125 minutes through each filter. Individual exposures were typically of 5-10 minutes duration. The fields used were all at moderate or high galactic latitude. Seeing was around 1.5 arcsec (full-width at half-maximum) in the R band; image size measurements made with the KAPPA software package, supplied by the Starlink project, are presented in Table 1. Reduced frames were all about [FORMULA] in extent; this means that clustering on scales up to [FORMULA] Mpc can be investigated at the distances at which we expect to see [FORMULA] galaxies. Note that if merging takes a few (say 5) dynamical times, only neighbours within [FORMULA] Mpc of a galaxy with rotation/dispersion velocity [FORMULA] could be expected to merge in the [FORMULA] Gyr since [FORMULA] Typically 200-300 galaxies are detected in each field.


Table 1. Observational data

[FIGURE] Fig. 1. Example reduced CCD frame. The R-band data frame is shown for the right ascension [FORMULA] declination [FORMULA] (epoch 1950.0) field of Table 1. The candidate [FORMULA] (large circles) and faint blue galaxies (small circles) from the samples of Sect. 5.1 are indicated
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© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998