It is common to wish to process only a subset of the possible
In Miriad, the visibility data to be selected are usually specified by one
parameter - the select keyword.
This parameter is constructed from
a number of subcommands, each subcommand selecting or rejecting
visibility data which satisfies some condition. The subcommands, which can
be abbreviated to uniqueness, are as follows:
Selecting UV Data in Miriad
As noted before, all
subcommand names can be abbreviated to the minimum number of characters
needed to keep them unambiguous (minimum match).
- This selects visibilities observed between times
t1 and t2. Times are given in UT.
The second time (t2) is optional.
If missing, it is assumed to be 24 hours after t1.
The time is composed of a date and `time-of-day' portions, either of which
can be omitted (but not both at the same time!). When the date is omitted,
then the selection matches data, for the given time-of-day, regardless of
the day. This is most useful for files containing only a single day's data.
When the time-of-day is omitted, then 00:00 is assumed.
t1 and t2 are given in the form:
Here yy is the year, mmm are the first three letters of the
month's name, dd is the day of the month, fff is a fraction of
a day, hh is the hour (24-hour clock), mm are the minutes, and
ss.s are seconds and fractions of a second.
The `time-of-day' portion can be abbreviated. For
example, the seconds part can be omitted. Indeed (provided a date is given)
the `time-of-day' portion can be totally omitted if desired.
Note that only a two-digit year is given. These two digits give a year
in the century 1940 - 2040.
is 12:30 on 12 January, 1990, whereas
is midday on 3 June, 1978, and finally
is April Fools Day in the year 2000.
To give some examples:
will select data observed on 5th January, 1991, between 10:50 UT to 17:20 UT.
The form where no date is given:
will select data observed on any day between 10:50 and 17:20.
To give an example where only one time is given, the following
will select all data observed on 3rd January, 1991. This form is only
useful for a file containing several days of data.
- This selects according to the
antennas and baseline. Here a1,a2,... is a list of antennas to select.
The second list, b1,b2,... is optional. If present, only baselines
corresponding to the antennas pair (a1,b1), (a1,b2), ..., (a2,b1), ..., etc
are selected. For example, to select all visibilities using antennas 1 or 3,
To select all visibilities using baselines with antennas 1 and 2, or 3 and 2,
- This selects visibilities whose u-v
is in the range uvmin to uvmax.
If only one value is given, then uvmin is
taken as zero. The units of uvmin and uvmax are kilowavelengths.
- This is the same as the uvrange subcommand,
except that the units are nanoseconds.
- This selected data based on the hour angle of
the data. h1 and h2 are the hour angle (in decimal hours).
- This selects data based on the elevation
at which the data were observed. e1 and e2 give the elevation
range in degrees.
- This selects data based on the LST of the
observation. l1 and l2 are the start and end LST values, and can be given
in the form hh:mm:ss or as decimal hours.
- This selects visibilities numbered n1
to n2 inclusive.
- This selects only every inc'th visibility.
- Select visibility data within
a particular range of right ascension. Right ascension is given in hh:mm:ss
format, or as decimal hours.
- Select visibility data within
a particular range of declination. Declination is given in dd:mm:ss
format, or as decimal degrees.
- For data files containing several pointing
selects visibilities whose pointing centre is offset, in RA, from the main
pointing centre, by between p1 and p2 arcseconds.
- For data files containing several pointing centres, this
selects visibilities whose pointing centre is offset, in DEC, from the main
pointing centre, by between p1 and p2 arcseconds.
- For data files containing rms pointing error data,
this selects visibilities with the rms pointing error in the range p1
to p2 arcseconds. If only one number is given, p1 is taken as 0.
pointing error of a visibility is taken as the maximum of the rms azimuth and
rms elevation pointing errors of the two antennas.
- For data files which contain multiple
sources, this selects data according to source name. Several source names
can be given, separated by commas. The source name can include an
asterisk, which acts as a wildcard.
- This selects data according to the sky
frequency of the first channel. The loval and hival values
give the permissible range, in GHz, of frequencies to accept. If only a single
value is given, then this is used as a mid-frequency, and frequencies within
a tolerance of 1% are accepted.
- Any correlation, where the amplitude
is between x and y, is processed.
If only one value is given, it is assumed to be x, and
y is assumed to be infinity.
- This can be used when the dataset contains
multiple spectral windows (or IF bands in AIPS terminology). This is
discussed in Section 5.6.
- This selects data that would be shadowed by an
antenna of diameter d meters. If d is zero, then the actual
diameter of the antennas (if known) is used. If some data is shadowed, it
is advisable to use an antenna diameter value greater than the physical
antenna size (e.g. 20% larger).
- This selects auto-correlation data only. So the
negated form (-auto) selects cross-correlation data only.
- This is intended for use with beam switched single dish
observations, and selects based on the `on' variable in the visibility
The `on' variable indicates whether the telescope is pointing on or off
- This selects visibilities whose pulsar
bin number is in the range n1 to n2 inclusive. If n2
is omitted, just bin n1 is selected.
- This selects based on the polarization/Stokes
type of the visibility. The possible values for a,b,c, etc. are mnemonics
for the polarization type, as discussed in Section 5.8.
- This is discussed below.
Each subcommand can
be prefixed with a plus or minus sign (
-). A plus sign means to select the data given by the following
subcommand, whereas a minus sign means to discard the data. If neither
a plus nor a minus sign is present, a plus sign is assumed. For example
means to select all visibilities between 0 and 10 kilowavelengths, whereas
selects all data except visibilities between 0 and 10 kilowavelengths.
Several subcommands can be combined on the same line, separated by commas.
When combining several subcommands of different types,
the visibility must be selected by all the
subcommands to be accepted (a logical AND).
When combining several subcommands of
the same type, then the visibility is accepted if it is selected
after sequentially examining each of the subcommands (a logical OR).
selects data with a u-v radius in the intervals 0 to 10 kilowavelengths as well
as 20 to 30 kilowavelengths. The following uses a `select then discard'
approach to selecting the same u-v ranges as above:
The following selects the same u-v ranges, but only for the baseline between
antennas 1 and 3.
The following selects all baselines, with the exception of 1-2, 5-7 and 6-7:
Another way of combining subcommands, is with the or subcommand.
This allows you to OR together two `clauses' of selection commands.
For example, to select spectral
windows 1 and 2 for the times 0:00 to 1:00, and spectral windows 4 and 5 for
times 2:00 to 3:00, use:
By combining the various subcommands and the or subcommand, quite complex
selection criteria can be developed. For complex selections, an
@ file (as described in Section 2.5) may be preferred.
For example, consider a file, select.data, containing the text:
will use windows 1 and 2 for time 1:00 to 2:00, and windows 1, 2, 3 and 4 for
times 3:00 to 4:00. For time 2:00 to 3:00, it uses windows 1 and 2, but omits
data for baseline 1-3 which has a u-v range of 50 to 100 kilowavelengths.
There are a few limitations on the use of amplitude,
polarization and window selection. Some of these limitations
are mentioned in following sections, whereas others are not.
Few of these limitations affect normal practice,
and the selection software will inform you if there is an problem.