[ Basic Info | References | User Guide ]
Basic Information on early
Task: early
Purpose: Flag data which has early/normal problems.
Categories: calibration, uv analysis
EARLY is a Miriad task which flags visibility data that it suspects
contains early/normal problems. The input must be a multi-channel
dataset (NOT channel 0). Early works by comparing the scatter of
data within a visibility record with that expected because of thermal
noise. If the scatter exceeds some number of sigmas, then the data
are assumed to be affected by the early/normal problem, and the
entire record is flagged.
Several assumptions about the data are make:
Firsly there is significant signal in the visibility
record so that the phase slope across the band causes a substantial
increase in the scatter. This will not be true if the signal is
weak, and the visibility records are noise dominated. If the signal
is noise dominated, no early/normal problems can be detected, and
the data will not be flagged.
Secondly the data have been bandpass calibrated. EARLY always applies
a bandpass correction to the data if there is one present.
Thirdly that the noise level is fairly constant across the band,
Fourthly that any end channels that might be bad have already been
flagged.
Fifthly there is no substantial variation of the signal across
the band. This will be true for continuum, but not spectral
line work.
Key: vis
The input visibility data-set to flag. Several files can be
given. Wildcards are supported. No default.
Key: select
Normal uv selection. This selects which data to check for early/normal
problems.
Key: sigma
Number of sigmas. Default is 10.
Key: log
Output log file for messages. The default is the terminal.
Key: options
Extra processing options. Several can be given, separated by commas.
Possible values are:
noapply Do not do any flagging -- just check the data, and
produce the appropriate messages.
verbose Give details about what is (or would be) flagged.