Selecting UV Data in MIRIAD

  It is common to wish to process only a subset of the possible visibility data. 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:

time( t1,t2)

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.

The times   t1 and t2 are given in the form:

yymmmdd.fff

or

yymmmdd:hh:mm:ss.s

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.

For example:

   90jan12:12:30

is 12:30 on 12 January, 1990, whereas

   78jun03.5

is midday on 3 June, 1978.

To give some examples:

   select=time(91jan05:10:50,91jan05:17:20)

will select data observed on 5th January, 1991, between 10:50 UT to 17:20 UT.

The form where no date is given:

   select=time(10:50,17:20)

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

   select=time(91jan03)

will select all data observed on 3rd January, 1991. This form is only useful for a file containing several days of data.

antennae( a1,a2,...)( b1,b2...)

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,   use

   select=antennae(1,3)

To select all visibilities using baselines with antennas 1 and 2, or 3 and 2, use

   select=antennae(1,3)(2)

uvrange( uvmin,uvmax)

This selects visibilities whose u-v

radius 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.

uvnrange( uvmin,uvmax)

This is the same as the uvrange subcommand, except that the units are nanoseconds.

visibility( n1,n2)

This selects visibilities numbered n1   to n2 inclusive.

increment( inc)

This selects only every inc'th visibility.  

ra( hh:mm:ss1,hh:mm:ss2)

Select visibility data within a particular range of right ascension. Right ascension is given in hh:mm:ss format, or as decimal hours.

dec( dd:mm:ss1,dd:mm:ss2)

Select visibility data within a particular range of declination. Declination is given in dd:mm:ss format, or as decimal degrees.

dra( p1,p2)

For data files containing several pointing centres, this selects visibilities whose pointing centre is offset, in RA, from the main   pointing centre, by between p1 and p2 arcseconds.

ddec( p1,p2)

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.

pointing( p1,p2)

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. The rms pointing error of a visibility is taken as the maximum of the rms azimuth and rms elevation pointing errors of the two antennas.

source( srcnam1,srcnam2,...)

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.  

frequency( loval,hival)

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.  

amplitude( x,y)

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.  

window( w1,w2,...)

This can be used when the dataset contains multiple spectral windows (or IF bands in AIPS terminology). This is discussed in Section 5.6.  

shadow( d)

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).  

auto

This selects auto-correlation data only. So the negated form ( -auto) selects cross-correlation data only.  

on

This is intended for use with beam switched single dish observations, and selects based on the `on' variable in the visibility dataset. The `on' variable indicates whether the telescope is pointing on or off source.  

bin( n1,n2)

This selects visibilities whose pulsar bin number is in the range n1 to n2 inclusive. If n2 is omitted, just bin n1 is selected.

polarization( a,b,c,...)

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.  

or

This is discussed below.  

Each subcommand can be prefixed with a plus or minus sign (+ or -). 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

   select=uvrange(0,10)

   select=-uvrange(0,10)

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). For example:

   select=uvrange(0,10),uvrange(20,30)

   select=uvrange(0,30),-uvrange(10,20)

   select=uvrange(0,10),uvrange(20,30),antennae(1)(3)

   select=-antennae(1)(2),-antennae(5,6)(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:

   select=time(0:00,1:00),window(1,2),or
          time(2:00,3:00),window(4,5)

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:

   time(1:00,2:00),window(1,2),or
   time(2:00,3:00),window(1,2),-uvrange(50,100),-antennae(1)(3),or
   time(3:00,4:00),window(1,2,3,4)

   select=@select.data

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.