Caltech VLBI Analysis Programs
The Caltech VLBI Analysis Programs are a set of programs
for the planning and analysis of Very Long Baseline
Interferometry (VLBI) observations. The programs are available for
SunOS and Solaris only.
Index of Programs
- ARRGEN: generate antenna coordinates for a hypothetical interferometer array.
- FAKE: generate artificial data for a hypothetical VLBI experiment (in MERGE format; use MERGEFITS to convert to UV-FITS).
- FITSLIST: list headers of FITS files.
- GAINPLOT: graph polynomial gain curves.
- HAZI: compute uv-tracks for a hypothetical experiment.
- MAPPLOT: display FITS image as contour map or grey scale.
- MAPSTAT: statistics of a region of a map (FITS image).
- MERGEFITS: convert data in MERGE format to standard UV-FITS format.
- MODFIX: edit or modify model file.
- MODGEN: generate model file from physical parameters.
- MODMAP: convert model file to map (FITS image).
- MODPLOT: generate contour map from model file.
- MODSUM: combine model files with scaling or shifting.
- MTANAL: summarize contents of magnetic tape.
- MTDUMP: dump selected data from tape.
- PRECESS: precession and coordinate conversion.
- TSYS: convert various telescope log files to standard format.
- UPTIME: display source rise and set times at selected antennas.
The following programs associated with the Caltech Block-II VLBI Correlator
are now obsolete:
The following programs have been replaced by DIFMAP:
The following programs operate on data in the MERGE format and are no longer
supported. You should convert data in this format to UV-FITS using MERGEFITS
and then use the editing and display capabilities in DIFMAP:
The Caltech VLBI Analysis Programs are used for planning an experiment,
assisting in correlation, fringe-fitting, data display, calibration and
editing, model-fitting and mapping. The following paragraphs summarize
the uses of the major programs, excluding some of the less commonly used
ones. All the programs are controlled with keyword-directed commands
which can be used interactively or in batch command files. Sequences
of programs can be linked together using the command language of the
host operating system (UNIX or VMS).
Planning an experiment
UPTIME computes rise and set times of sources at one or more
telescopes, and displays the results graphically. It can also produce
graphs of source elevation and parallactic angle versus time.
HAZI computes and displays the u,v-tracks of a hypothetical
FAKE generates a Merge-format file containing artificial data
(computed from a model) for a specified source and antennas; noise can
be added. It is useful for testing programs, hybrid-mapping algorithms,
etc. FAKE includes capabilities for simulating antennas in earth orbit
and long baseline optical interferometers.
PRECESS converts radio source coordinates from B1950 or J2000 mean
position to B1950 or J2000 mean position, geocentric apparent position,
ecliptic longitude and latitude, or galactic longitude and latitude.
Correlating and fringe-fitting
MAKECCR creates a script to run the JPL/Caltech Block-II VLBI
B2PS summarizes the contents of output files from the correlator.
B2FITS converts the output of the Block-II correlator into standard
UVFITS format acceptable to AIPS. The first stage in the
post-correlation analysis is to calculate the fringe amplitude and
phase as a function of time on each baseline; this is usually done with
the AIPS system of the National Radio Astronomy Observatory.
Most of the programs operate on interferometer visibility data, which
are stored in data files in a standard format (the `Merge' format). The
following programs are used for converting data from other formats into
the Merge format: FITSMERGE (converts UVFITS data); EVNMERGE (converts
the European VLBI network format); MK3MERGE (converts export
data from Mark-III correlators). There are also programs to convert
Merge-format data into other formats: MERGEFITS (converts to AIPS/UVFITS
format), VLBEXPORT (converts to ASCII text format).
Display of visibility data
VISPLOT plots the visibility amplitudes and phases, closure phases, and
closure amplitudes (if requested) versus UT, GST or interferometer hour
angle, with a
superimposed model curve if desired. It can plot both calibrated and
LIST lists the contents of a Merge-format file at a terminal or on the
line-printer. It can list the file header, (u,v,w) coordinates,
amplitudes, visibility phases, closure phases and the errors in these
UVPLOT displays the u,v-plane coverage of the observation.
UVDIST plots amplitude versus radial distance or position angle
in the u,v-plane.
CLAMP calculates and lists the closure amplitudes.
VPLOT is an interactive program for data examination. It combines some
but not all of the functions of VISPLOT, UVPLOT, and UVDIST.
Operations on visibility data
VLBEDIT is used for removing bad data points from a data file or for
making corrections, e.g., for telescope mispointing. It can also be
used to change parameters in the file header.
IED is a convenient tool for interactive data editing, if an interactive
graphics device is available.
CAL uses measured system temperatures, gain curves, antenna temperatures
etc to convert the data from correlation coefficients to correlated flux
densities (or visibilities).
AVERAGE averages the data either coherently (complex average of
visibilities) or incoherently (rms of amplitudes).
UVCROSS can be used to identify crossing-points of tracks in the
u,v-plane, and for checking the calibration at these points.
UVCOPY extracts a subset of the data from a Merge file. Data can
be selected by time and by radius in the u,v-plane.
MERGE combines data from two or more Merge files into a single
In many cases parameters of the observed source can be estimated by
fitting a model of the source brightness distribution to the measured
complex visibilities. The Caltech programs allow the use of models of up
to 10,000 components, where a `component' is, e.g., a point source, an
elliptical gaussian, or an elliptical disk.
MODELFIT does a least-squares fit of a model of one or more components
to the visibilities and closure-phases. The output is
a model that can be compared with the data via program VISPLOT or
displayed as a contour map with MODPLOT.
QFIT computes the chi-squared agreement factors between a model and a
ERRFIT is a program for estimating the confidence
intervals of the fitted model parameters.
MODPLOT displays the models produced by MODELFIT or CLEAN as contour
MODSUM can add and subtract models.
MODFIX provides a number of operations on models, e.g., scaling,
Hybrid Mapping is an iterative self-calibration procedure, in which one
attempts to estimate the unknown phase and amplitude calibration
errors introduced at each antenna (Readhead and Wilkinson 1978; Readhead
et al. 1980; Pearson and Readhead 1984). These errors do not affect the
closure phases and closure amplitudes. Three programs are used:
AMPHI, INVERT, and CLEAN. AMPHI estimates the calibration corrections to
minimize the disagreement between the corrected visibilities and a
starting model provided by the user. INVERT makes a `dirty' map using the
normal aperture-synthesis equations; CLEAN cleans the dirty map
producing a `deconvolved' map---an array of point sources---that can
be taken as a new starting model. The agreement of this model with the
original data can be assessed qualitatively with VISPLOT or
quantitatively with QFIT.
VLBMEM is an alternative imaging program based on the Maximum Entropy
algorithm of Gull and Skilling.
MAPPLOT displays images as contour maps or gray-scale
MAPSTAT computes simple statistics (mean, rms, total flux) for regions
of an image. The images produced by the programs are stored in FITS
format and can therefore be analysed with many different image
The PGPLOT subroutine library
Many of the programs produce graphical displays using the PGPLOT
subroutine library. PGPLOT is a Fortran-callable, device-independent
graphics package for making simple scientific graphs. It is intended for
making graphical images of publication quality with minimum effort on
the part of the user. For most applications, the program can be
device-independent, and the output can be directed to the appropriate
device at run time. Although originally conceived as part of the Caltech
VLBI Analysis Programs, PGPLOT has much wider applicability and is now
widely used at astronomical and other scientific institutes throughout
the world. For further information, contact Dr T. J. Pearson at the
address given below.
The programs are written in Fortran-77 with occasional use of some of
the common VMS extensions to Fortran-77. As far as possible, machine- or
system-dependent functions are encapsulated in a small number of
subroutines. At Caltech, the programs run under the VAX/VMS, Sun UNIX,
and Convex UNIX operating systems. They can probably be ported to other
varieties of UNIX with a moderate amount of work.
Development of the programs is continuing, and updated versions of each
program are available on request. It is planned to release new versions
of the entire package approximately twice a year.
The Caltech VLBI Analysis Programs, including complete source code and
documentation, are available free of charge to non-profit astronomical
research institutes. For information, contact Dr T. J. Pearson,
Astronomy Department 105-24, California Institute of Technology,
Pasadena, California 91125, USA.
Bitnet: TJP@CITDEIMO; SPAN: 6021::TJP.
The development of the programs has been supported by the National
Science Foundation. Many people have
contributed to the programs, including M. H. Cohen, G. Comoretto,
D. Fort, S. F. Gull, M. W. Hodges, R. Linfield, D. L. Meier, T. J.
Pearson, G. H. Purcell, A. C. S. Readhead, D. H. Rogstad, J. Romney,
R. S. Simon, A. Tzioumis, S. C. Unwin, R. C. Vermeulen, R. C. Walker,
P. Wallace, P. N. Wilkinson, and J. Wrobel.
- Pearson, T. J., and Readhead, A. C. S. 1984, ARAA, 22, 97
- Readhead, A. C. S., Walker, R. C., Pearson, T. J., and Cohen, M. H.
1980, Nature, 285, 137
- Readhead, A. C. S., and Wilkinson, P. N. 1978, ApJ, 223, 25