The radial wavelength variation described in Section \[sec:wlvariation\] resulted in a shift of 1.4 Å at 2’ from the optical axis and 6.7 Å at 4’ from the axis. Conditions were photometric, and seeing was on the order of 1.5”. The full width at half maximum of the etalon transmission band was 7.5 Å.

The targets were scanned at etalon plate spacings corresponding to a series of wavelength steps of approximately 10 Å, the aim being to straddle the redshifted Ly $\alpha$. However, an intermediate-band order-blocking filter is necessary to eliminate unwanted wavelengths and other orders of interference. In this case, the AAT’s B1 filter was the best available. Unfortunately, the observed wavelengths were at the very edge of the filter transmission, as shown in Fig. \[fig:trans\]: the signal to noise ratio therefore decreases significantly with wavelength. Table \[tab:obs\] and Fig. \[fig:trans\] record observations of MRC B1256-243 at 3987.1 Å. When these data were analysed, it was clear that the reduced filter transmission had resulted in no useful results at this wavelength. These data are not considered further in this work. The MRC B2158-206 observations at 3989.0 Å and 3999.0 Å are included hereafter, but did not include any useful 