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

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The following programs have been replaced by DIFMAP: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 experiment. 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 correlator. 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.

Visibility data

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 uncalibrated data. 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 quantities. 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 file.

Model fitting

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 dataset. 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 maps. MODSUM can add and subtract models. MODFIX provides a number of operations on models, e.g., scaling, rotating, windowing.

Hybrid mapping

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 images. 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 processing programs.

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. Internet: TJP@Deimos.Caltech.Edu; 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.