detections.

Each CCD frame contained a total of 30 minutes of observations, taken at two separate axial wavelengths. Each wavelength was exposed for 60 seconds a total of 15 times. This procedure was repeated twice in the case of MRC B1256-243 and four times for MRC B2158-206; the total exposure times at each wavelength are thus 30 minutes and 1 hour, respectively. Between each image, the telescope pointing was shifted slightly: this enabled the easy identification and subsequent elimination of diametric ghosts in the data.

### Objects at $z \sim 4.5$

The TTF was used at $f/8$ on the AAT in combination with the MITLL2 CCD. This resulted in a scale of 0.37” per pixel. After processing, the total useful rectangular field of view in the observations was 9’17” by 4’10”. The radial wavelength variation described in Section \[sec:wlvariation\] resulted in a shift of 5.1 Å at 2’ from the optical axis and 20.3 Å at 4’ from the axis. Conditions were photometric, and the seeing was on the order of 1.5". The full width at half maximum of the etalon transmission band was 9.5 Å. The AAT’s R0 intermediate-band order-blocking filter was used: this provided effectively constant transmission across 