a total of 9h VLT/UVES spectroscopy of SDSS1228+1040 between 2007 and 2009 using the Blue390 and Blue437 setup with a $0.9\arcsec$ slit, covering both the Ca and Mg features with a resolving power of $\simeq40\,000$. The data were reduced in Gasgano using the UVES pipeline. The individual spectra were of relatively low S/N, and we analysed only the error-weighted average spectrum, binned to 0.05Å, with $\mathrm{S/N}\simeq35$.

The optical spectra around the [Ca]{}K and [[Mg]{} 4482Å]{} lines are shown in Fig.\[f-camg\]. We note that while most previous studies of metal-polluted white dwarfs have focused on the [Ca]{} H/K lines, their strength for a given abundance decreases strongly with increasing temperature, as [Ca]{} is ionised to [Ca]{}. For temperatures ${\mbox{$T_{\mathrm{eff}}$}}\simeq20\,000-25\,000$K, [[Mg]{} 4482Å]{} becomes a more sensitive probe of metal pollution (e.g. @gaensickeetal07-1 [@farihietal12-1]).

[lrr]{} Object &  \[K\] & $\log g$ \[cgs units\]\
\
optical, [@liebertetal05-1]& $23\,870 \pm 392$& $7.90 \pm 0.05$\
HST, this paper & $23\,095 \pm 230$& $8.17 \pm 0.06$\
\
optical, [@liebertetal05-1]&$19\,540 \pm 305$ & $8.04 \pm 0.05$\
optical, [@koesteretal09-2]&$19\,948 \pm  33$ & $7.925\pm0.006$\
HST, this paper &$19\,200 \pm 180$& $8.22 \pm 0.06$\
\
optical, [@eisensteinetal06-1] &$22\,125 \pm 136$& $8.22 \pm 0.02$\
optical, [@gaensickeetal07-1] &$22\,292 \pm 296$& $8.29 \pm 0.05$\
