Chapter 9
REDUCING VLBI DATA IN

This chapter describes the reduction of VLBI data in  . A Step-by-step recipe, covering both simple and more difficult situations, is presented. See Appendix C for simpler and shorter recipes suitable for straightforward observations using only the VLBA and HSA (Arecibo, Effelsberg, GBT, and phased VLA) and the VLBA correlator. Procedures to simplify some of the VLBI reduction steps are mentioned and become available to you after you enter the command RUN VLBAUTIL  C R. We also include here some background information concerning the structure of VLBI data sets, the data reduction philosophy and a description of some of the effects for which corrections must be determined and applied. It is important to understand these aspects if you wish to reduce your data reliably. For more background information on VLBI data reduction consult VLBI and the VLBA, Astronomical Society of the Pacific (ASP) Conference Series No. 82, 1995.

Programs of particular interest for VLBI may be found in Chapter 13 or displayed from inside  by typing ABOUT VLBI C R or APROPOS VLBI  C R. Remember, the best and most complete information available on all  verbs and tasks may be found in their EXPLAIN files. A 15APR97 or later version of  is required to support full Space VLBI data reduction.

Most types of VLBI data, once read into , appear very similar in structure as far as the user is concerned. We shall concentrate on describing the reduction path for data produced by the VLBA correlator, but most operations also apply to MkIII and MkII data. This chapter contains no specific discussion of data from a MkIV correlator since we have little experience as yet with such data. Where appropriate, we shall draw the reader’s attention to any differences. In particular, §9.2.2 deals with reading data from a MkIII correlator into  and the steps necessary to prepare such data for calibration. The few extra steps necessary for calibrating phase-referencing observations are described in §9.4.8.4. Note that successful phase-referencing observations require careful planning before the observations. See VLBA Scientific Memo No. 24 by J. Wrobel, C. Walker, J. Benson, and A. Beasley.

Some of the VLBI-related tasks require the ability to read files resident outside  . To communicate to  the directory in which these files exist it is necessary to define a logical pointer or environment variable. Please refer to §3.10 to see how this is done.

While the majority of VLBI observations are continuum observations, more sophisticated data reduction techniques are increasingly common. Continuum VLBI observers sometimes also apply spectral-line VLBI techniques to improve the dynamic range of their data sets. For these reasons, this chapter is organised to make the discussion of data reduction techniques more uniform. The overview in this section of the steps involved for several type of VLBI data reduction is meant to guide the user through the rest of the chapter. It is strongly recommended that you read the overview carefully before proceeding.

The expected size of the output uv data file can be an important consideration in VLBI data reduction. The disk space required by  for a compressed dataset is given by the relation:

Note that certain operating systems are still subject to a 2-Gigabyte limit for any individual file, as a result of their 32-bit file systems. Larger  files are supported on DEC Alpha, SGI (running XFS), HP, Solaris (revision ≥ 2.6), and Linux (kernel ≥ 2.4.2). The last three require all ’ C code to be compiled with an additional option. The size of the output file can be reduced by IF selection or limited concatenation in FITLD or by time- or spectral-averaging later using UVAVG, SPLAT or AVSPC.

It is possible to construct data sets on disk that cannot be written to a single tape using FITTP because FITTP uncompresses the data when writing to tape. The task FITAB is designed to address this problem. FITAB writes data in compressed form to tape and can write data in pieces to multiple tapes. Note that FITAB is only available in 15APR99 and later releases and that versions of FITLD from earlier releases cannot read such data. Packages other than  may also be unable to understand these files.

One large point of divergence in the reduction of continuum polarization VLBI data is the question of whether or not to determine separate LL and RR phase solutions. The polarization-specific portions of the recipe given below are based upon the premise that L and R phase solutions should always be determined separately on the grounds that it is safer and should work with data from a wide variety of antennas. If the L-R phase offsets for antennas in your data set are small and constant in time, you may consider modifying the recipe in §9.1 by determining averaged LL,RR phase solutions everywhere except in step 7.