Preparing your Data in MIRIAD

Before you calibrate, you must prepare your data. This consists of loading, flagging, perhaps converting to ``channel-0'' datasets, and then splitting.

1. ATCA data will be initially in RPFITS format. As described in Chapter 8, MIRIAD 's task to read RPFITS data is called atlod . MIRIAD atlod has a few options to play with. See Section 8.1 for more information. Chapter 15 gives some extra tips for spectral-line observers. For historical reasons, some of the strongly recommended options are not invoked by default -- you will probably want to use these. These include:
   • birdie: A shortcoming in the ATCA hardware means that there is self-interference at multiples of 128 MHz. Channels at such frequencies are invariably compromised in spectral observations. Although the continuum observing frequencies are chosen to avoid these, the spectral sidelobes can cause the self-interference to creep into every second channel in the observing band. The birdie option flags channels that are likely to be affected by self-interference.

     In continuum mode, the birdie options also discards every second channel, as well as some end channels. Output will contain 13 or 14 channels, which are usually all good for imaging purposes. Because the channel bandwidth in continuum mode is twice the channel increment, discarding every second channel does not have a sensitivity penalty. Discarding these channels obviously also reduces disk usage and reduction time.

   • xycorr This option corrects the data with the on-line XY phase measurements. This is now the recommended way to correct the XY phase.
   Another option, reweight, also be considered for high dynamic range continuum experiments -- this significantly reduces the so-called Gibbs phenomena, which causes a non-closing error in ATCA observations. Unfortunately, the reweight option reduces the effectiveness of the birdie option to reject self-interference spectral sidelobes.

   Unlike its AIPS \ cousin, MIRIAD atlod

   does not write out any XY phase text files -- the on-line XY phase measurements are saved in the xyphase visibility variable (which can be plotted using varplt ). Unlike all other MIRIAD tasks, atlod can read directly from a tape device. Typical inputs to atlod are

   

2. You should now flag your data, and possibly convert to channel-0 (using uvaver ). Tasks for these operations are described in Chapter 9.

3.   As MIRIAD datasets can contain only a single set of calibration tables, it is rather poor at handling the calibration of datasets containing multiple sources and multiple frequency bands. For calibration purposes, it is best to work on datasets containing a single source and single frequency band. So, it is best to break the multi-source, multi-band dataset into a collection of single-source, single-band datasets. The best task to do this is uvsplit . Task uvsplit generates the names of the output datasets itself, forming these from the source name and the central frequency (in MHz) of the data. It is rather unforgiving if you already have a files with one of the names that it wants to use. Make sure your directory is free of files that may usurp uvsplit 's name choice. Task uvsplit allows you to perform extra selection if you wish, which may be convenient if you only want to deal with part of your observation at a time.

   

   For large spectral line data-sets, if disk space is low, it may be useful not to split off the program source. Rather you can split off the calibrators, determine the calibration tables from them, copy the calibration back to the multi-source file, and then image directly from the multi-source file. This way you avoid making a second copy of your program source data. For example, to avoid the source `` vela'' from being split off, use