We recently demonstrated that the correlator offset problem present
in 25 MHz bandwidth spectral line observations is being triggered by
the so-called self-test in the correlator. We were unable to detect
any offset in a three hour run taken with self-test turned off. As a
result, self-test has been turned off automatically for all 25 MHz
bandwidth spectral line observations taken after November 8, 2007.
Note that these offsets have never been observed at bandwidths other
than 25 MHz, and for those other bandwidths self-test will remain on,
as before. Because the correlator self-test is now
turned off during all 25 MHz spectral line observations observers need
to check their data very carefully, and notify us if there are problems
as soon as possible.
For observations taken before that date, the effects of the offset can
be removed during post-processing provided there is no emission of
interest in the center of the field. See the relevant section in the
post-processing chapter for more details.