as described in the next section.

Photometry {#sec:photo}
----------

The observations of the standard stars were reduced in the same way. For each star, [SExtractor]{} was used to perform aperture photometry yielding a count $C_\mathrm{s}$. This corresponds to a known magnitude $m_\mathrm{s}$, based on @Hamuy92 for the lower redshift fields or from the ESO Standard Star Catalogue for that of BR B0019-1522. If the exposure time on the standard is $t_\mathrm{s}$ and that on an object in the field is $t_\mathrm{Obj}$, the AB magnitude of the object is

$$m_\mathrm{AB} = m_\mathrm{s} - 2.5 \log_{10} (C_\mathrm{Obj}t_\mathrm{s})/(C_\mathrm{s}t_\mathrm{Obj}).$$

The AB magnitude system [@Oke74] is defined by $m_\mathrm{AB} = -2.5
\log_{10} f_\nu - 48.60$ where $f_\nu$ is the flux in units of . The monochromatic flux $f_\lambda$, in units of , is then

$$\label{eq:abtoflux}
f_\lambda = (c \times 10^{-\left(m_{\mathrm{AB}} + 48.60\right)/2.5})/\lambda^2.$$

Conversion from $f_\lambda$ to the total flux in the band, $f_\mathrm{total}$ is performed by multiplying by the effective width of the etalon transmission. The etalon transmission band may be taken as Lorentzian, normalised to 1 at the wavelength of peak transmission, thus:

$$\label{eq:ttfpass}
T(\lambda) = (\lambda_{\nicefrac{1}{2}}^2 / 4)/((\lambda - \lambda_\mathrm{c})^2 + \lambda_{\nicefrac{1}{2}}^2 / 4)$$

where $\lambda$ is the wavelength, $\lambda_c$ the central wavelength 