Reading RPFITS Files

ATCA data will be initially in RPFITS format. This needs to be converted to MIRIAD 's format, by task atlod , before any further processing can be done. We will discuss the various input parameters to MIRIAD atlod .

• The in parameter gives the Unix name of the input file (or files) or device (e.g. exabyte). CDROMS can also be read (these appear as fairly standard files, although they are in a special directory). When giving several files, wildcards can be used.

  For exabytes, you should use the physical device name. This is usually displayed on the exabyte drive, typically being /dev/nrst0 or /dev/nrst1 (note you should always use the non-rewinding, raw interfaces to exabytes -- that is, the nrst device names).

  An alternative to directly reading from exabyte is to load the data to disk with the Unix command ( ansiread on Suns, and ansitar on the Convex). See the UNIX man pages for more information.

• out gives the output MIRIAD dataset name.

• nscans gives two numbers which are the number of scans to skip over (before processing), and the number of scans to save. The default is to save all scans.

• nfiles, like nscans, gives two numbers, which are the number of files to skip and process. The default is to process the first file only. This parameter is useful when reading directly from exabytes. Note, however, that the mechanism used to skip files is rather inefficient. If you wish to skip a large number of files, you should do this with the Unix command mt. A complication is that every RPFITS file appears as three files to mt -- so you will want to skip three times as many tape files as RPFITS files. For example, to skip 10 RPFITS files, you would use the Unix command

     mt -f /dev/nrst0 fsf 30
  

  See the Unix man page on mt for more information.

• The options parameter gives miscellaneous processing options. Several values can be given, separated by commas. Possibilities are:

  birdie:

  The ATCA suffers from self-interference at frequencies which are a multiple of 128 MHz. The birdie option flags out the channels affected by this self-interference. This option is strongly recommended.

  In 33 channel/128 MHz mode, the birdie option also discards some edge channels and every other channel. This operation does not incur a sensitivity penalty, as correlator channels are not independent in this mode. The net result is that the output consists of either 13 or 14 good channels.

  xycorr:

  In polarimetric correlator configurations, the ATCA makes an on-line measurement of the phase difference between the X and Y channels. Although it is generally only a few degrees (assuming the observation was correctly set-up), this phase difference should be corrected when doing polarimetric work. Historically the recommendation has changed as telescope hardware was improved and a better understanding of the instrument has developed. The current recommendation is that the on-line measurements should be applied, without averaging, using the xycorr option in atlod . Consequently this option is recommended.

  reweight:

  In 33 channel/128 MHz mode, a Gibbs phenomena (see Albert Bos in the Indirect Imaging proceedings) affects the ATCA. This introduces a non-closing error into the data. The effect is moderately subtle, and is probably not significant for dynamic ranges of less than about 500. The `reweight' option reweights the visibility spectrum in the lag domain to eliminate this problem. This option is strongly suggested for high dynamic range work. Reweighting, however, can reduce the ability of the `birdie' option to reject self-interference in continuum mode.

  compress:

  Normally the correlation data are stored as 32-bit floating point numbers. Alternatively, the compress switch can be used to instruct atlod to store the correlations as 16-bit integers, with a scale factor associated with each spectrum. This approximately halves the disk space occupied by a dataset, and so may be very advantageous for large spectral-line observations.

  samcorr:

  Correct the measured visibilities for bad sampler statistics. Normally the samplers adjust their various threshold levels to ensure optimal operation. However various transients can cause the sampler levels to be wrong. All is not lost -- you can correct the data for the imperfect sampler levels after the fact. Use the samcorr (sampler-correct) option for this. Since December 1993, sampler correction has been done automatically on-line. For this data, the samcorr option is (quietly) ignored.

  relax:

  Normally atlod discards data if it has not previously read the appropriate calibration record. Although calibration records generally precede the data, there are some instances when this is not the case, and the data are still good. In this case, the relax option causes atlod to be more lenient, and to save data even if the appropriate calibration records have not been read.

  unflag:

  Normally visibility records that are completely flagged are discarded immediately. This usually indicates that there was significant reason to suspect the data -- for example the telescope was not on source. The unflag option causes all data to be saved, although it is still flagged as bad.

  noauto

  and nocross cause atlod to discard any autocorrelation or cross-correlation data respectively. Its fairly unlikely that you will want to discard the cross-correlation data!

  bary:

  Use the barycenter (often called heliocentre and usually used for extragalactic work) as the rest frame when calculating velocity information. The default is to compute velocity information relative to the LSR frame (which is usually used for Galactic work).

  hanning:

  Hanning smooth the data, and discard every second channel. This can be useful for spectral line experiments. Task atlod

  will refuse to Hanning smooth data with 33 channels or less.

  noif:

  When there are multiple frequency bands being observed simultaneously, MIRIAD normally maps the frequency bands to its spectral windows (IF axis in AIPS terminology), and writes them out in a single record. Alternatively, using the noif switch causes atlod to write the simultaneous frequencies as sequential records, making it appear somewhat like a frequency-switch. You will need to use the noif option if you have two spectral windows which sample different polarisation parameters.

Typical inputs for atlod are given below.

MIRIAD 's atlod saves a number of on-line measurements as visibility variables. These measurements may by helpful in analysing and flagging the data. They can be plotted and listed with task varplt (see Section 9.4). These on-line parameter, and its MIRIAD variable name, are described below.

xyphase:

This is the on-line measurement of the XY phase. It is important to examine this measurement if you are doing polarimetry.

xyamp:

This is the amplitude of the correlation between the X and Y polarisation channel of a given antenna.

xsampler, ysampler:

These variables give the sampler statistics for the
MMX and Y polarisation channels. There are three numbers per antenna per IF, which reflect the sampler levels. They should have values of 17.3 , 50 and 17.3 . Thus for six antennas, the xsampler variable will consist of 18 numbers per IF.

xtsys, ytsys:

These give the system temperature, in Kelvin, for the X and Y polarisation channels. Unfortunately MIRIAD tasks were originally developed around a single polarisation model of a telescope, and so most tasks that concern themselves with system temperature do not handle dual polarisation systems. The partial workaround used in MIRIAD has been to store the geometric mean of the X and Y system temperatures in the variable systemp. It is this variable that MIRIAD tasks use when they need to use the system temperature value.