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4.6 Outliers, stacks with exceptions or anomalies
Several images show unusual properties with respect to the bulk sample of stacks. This is the case for
the 32 y-band stacks that are expected to have slightly different (photometric) properties compared to the
other 139 i-band tiles due to the slightly different bandwidth of that replacement filter after the original
one broke in 2007.
The most common technical anomaly is an amplifier failure during an observing sequence. Since all
CCDs have two outputs, several images show missing data from a half or a full CCD area. Usually, each
observing sequence is done only once, so the data from the missing CCD cannot be recovered. However,
since the CCD failures are intermittent, the r-band observations which have been split into two observing
sequences normally do not experience this missing data problem. We note furthermore that many of the
pointings originally affected by this problem have been re-observed by in Director’s Discretionary Time
(images with more than one full CCD missing have been rejected during the image selection stack.)
Problematic T0007 stacks are listed in Tables 15 and 16, with a short description. None of these problems
are critical for the scientific exploitation of the survey, but there are some half-CCD size regions of the
CFHTLS Wide where one or two filters are missing.
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