In Miriad, flagging a correlation means you set a bit in a mask. There
are no explicit weights like in AIPS or ascii flagging tables.
When you list visibilities with uvlist
(see below), flagged
are indicated by an asterisk to the right of the phase.
There are three main flagging tasks in Miriad:
There are a number of other flagging tasks which occupy some niche
which are described in the next section.
- a plot-based interactive flagger. This normally plots
one baseline at a time, with a variety of possible axes (e.g. time against
amplitude), and you click on bad data to flag it out. This is similar to
tha AIPS IBLED task.
- a ``TV''-based, interactive flagger. This displays
one baseline at a time on a ``TV'' display, with the axes of the display
being channel number and time. You select regions of bad data in the display.
This is similar to AIPS task SPFLG.
- a non-interactive general flagger, where you
specify the data to be flagged by the select keyword. This is like
the AIPS task UVFLG.
Although there is some personal taste involved, a recommended scheme for
flagging is as follows:
- For 20 and 13 cm observations it is strongly recommended
that you start the flagging process with tvflag. Narrowband
interference is common at the wavelengths, and tvflag
is a good
tool at finding and flagging this interference. tvflag
performed on a multi-source file, straight after the data are loaded into
Miriad. NOTE: one shortcoming of tvflag
is that it ignores any
calibration tables that happen to be present in a dataset.
- Using blflag
is generally quick compared to tvflag,
and some forms of bad data are more apparent with blflag.
However, bad data is not that apparent when attempting to use it
with datasets containing multiple sources. So we recommend that you
split the data into single source datasets (using uvsplit)
before using blflag. Using blflag
even if you use have already use tvflag
on you data, because
is relatively quick and painless.
- Normally blflag
will apply any calibration tables
to the data before generating its plot. Thus it is just as useful after
calibration as before. It is useful to look at calibrators using blflag
both before and after solving for calibration parameters. With the program
source, you will probably only use blflag
after you are happy
with the calibration.