the wavelength range under consideration.

Each CCD frame contained a total of 30 minutes of observations: ten at each of three axial wavelengths. Eight CCD frames were recorded, resulting in a total of 80 minutes exposure for each axial wavelength. As before, the telescope position was shifted slightly between images.

![On-axis etalon transmission bands for each of the three fields observed shown relative to the relevant order-blocking filter used on the telescope. Away from the optical axis the etalon transmission shifts to shorter wavelengths (§\[sec:wlvariation\]).[]{data-label="fig:trans"}](fig1)

Data reduction and catalogue construction
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Data reduction proceeds broadly as for standard broadband imaging. A full consideration of the issues surrounding tunable filter data is given by @Jones012 and @Jones02. The various different images of each field at the same axial wavelengths were aligned by a marginal centroid fit on bright stars and then combined. Wavelength calibration was performed through an emission line, as described by @Jones02; xenon and copper-helium arc lamps were used for the $z \sim 2.2$ fields, and a neon arc lamp for BR B0019-1522.

After the data had been reduced, object detection and fixed aperture photometry were performed on each image using [SExtractor]{} [@Bertin96]. The object detection parameters were defined 