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Astronomical Data Analysis Software and Systems VII
ASP Conference Series, Vol. 145, 1998
R. Albrecht, R. N. Hook and H. A. Bushouse, e
Ö Copyright 1998 Astronomical Society of the Pacific. All rights reserved.
ds.
Using Java for Astronomy: The Virtual Radio
Interferometer Example
N.P.F. McKay 1 and D.J. McKay 1
Nu#eld Radio Astronomy Laboratories, University of Manchester,
Jodrell Bank, Macclesfield, Cheshire SK11 9DL, UK
Abstract. We present the Virtual Radio Interferometer, a Java applet
which allows the demonstration of earth­rotation aperture synthesis by
simulating existing and imaginary telescopes. It may be used as an edu­
cational tool for teaching interferometry as well as a utility for observers.
The use of the Java language is discussed, and the role of the language,
within the astronomical software context, reviewed.
1. Introduction
In 1996, the Australia Telescope National Facility (ATNF) was awarded a grant
to upgrade its existing interferometer --- the Australia Telescope Compact Array
(ATCA). The ATCA is a 6 kilometre earth­rotation aperture synthesis telescope,
consisting of six movable 22 metre antennas located on an east­west railway track
(JEEEA 1992). Each antenna can be moved and placed at various locations
along this track, known as stations. Part of the upgrade involves the construction
of new stations on the existing east­west track and the addition of a north­south
track. These extra antenna positions will improve the imaging capability of the
instrument.
To help astronomers and engineers at the various institutions experiment
with and visualise the e#ects of di#erent positions of the new stations, a design
and simulation software package was required. This program had to be intuitive
to use, widely available and quickly developed. The resulting program was called
the Virtual Radio Interferometer 2 (VRI).
2. The Virtual Radio Interferometer
The idea behind the VRI project was to create an interactive, user configurable,
simulated interferometer, providing a tool for determining the synthesised aper­
ture of real and hypothetical telescopes. It fosters an intuitive feel for antenna
positions, uv­coverage and the resultant point­spread function. VRI also proved
1 Formerly: Australia Telescope National Facility, CSIRO, Paul Wild Observatory,
Locked Bag 194, Narrabri NSW 2390, Australia
2 http://www.jb.man.ac.uk/vri/
240

Using Java for Astronomy: The Virtual Radio Interferometer 241
to be a useful teaching tool, in addition to its original design role of investigating
new antenna arrays and experimenting with ``what­if '' scenarios.
As well as exploring the properties of several existing instruments, the user
has control of a number of telescope parameters, allowing the creation of hypo­
thetical interferometers. These include the latitude of the site and the number
of array elements which form the interferometer, together with their diameter,
elevation limit and position on a two dimensional plane. The declination of the
source, as well as the frequency and bandwidth of observation can also be speci­
fied. Once these parameters are set, a corresponding uv­plot is produced by the
program, showing the aperture­plane coverage.
Given the plot of the uv­coverage, we can simulate its e#ect on various
objects. An image of the source is loaded onto one of four display panels, a
Fourier transform performed on it, and also displayed. This can be then masked
with the uv­coverage. When the inverse Fourier transform is applied, the original
image of the source is replaced by the image obtained by ``observing'' it with the
simulated interferometer. Finally, uv­plots generated by a number of antenna
configurations, or even di#erent observatories, may be combined to generate an
image composed of multiple ``observations''.
3. Implementation
The development path of the project, and hence the choice of programming
language, was governed by several constraints. Firstly, the program had to be
easy to use and have a short learning curve. It was decided that VRI would be
interactive, present a predominantly graphical interface and be mouse­driven.
To encourage people to use the it, the application had to be easily accessible,
so the World Wide Web was chosen as the transport medium and, as we had
no control over the platform that our audience was using, the code had to be
portable. Above all, rapid development was required.
Java was chosen as the coding language, because it satisfied these require­
ments. As a language, it is high­level, strictly object­oriented, and easy to learn.
Classes which facilitate the writing of software for Internet distribution form part
of the standard Java Development Kit. Hence, setting a program up to run on
the Web is not a complex exercise. The source code is partially compiled into a
set of byte codes before released for use. These are platform independent, hence
eliminating the need for multiple versions of the software. The byte code is
downloaded onto the client computer and, at run time, is interpreted into native
instructions and executed by the host machine. This alleviates congestion on
the organisation's HTTP server, and once the program is downloaded onto a
local machine, provides a faster program for users.
4. Appraisal of the Project
On the whole, Java proved to be an ideal development language for this partic­
ular project. This is not to say that no problems were encountered.
Being a semi­interpreted language, programs run slower than their native
code counterparts. For example, a 256 â 256 point Fourier transform takes
15 seconds to be executed by the Java interpreter, on a Sun Ultra­1 running at

242 McKay and McKay
Figure 1. The VRI Java applet, run from a Web browser.
140 MHz, which can be compared with figures of less than one second for native
code. Other potential reasons for avoiding the language include the fact that
Java is still somewhat immature and evolving, the lack of complete control over
registers, memory, etc., provided by other languages such as C and C++ and
the problems posed by integrating a new language with legacy code.
Nevertheless, the portability of the language, the rich vocabulary of stan­
dard class functions, as well as Java's easily extended object­oriented design,
greatly outweigh these disadvantages.

Using Java for Astronomy: The Virtual Radio Interferometer 243
Table 1. Software development metrics for VRI
Java Development Kit Version 1.0.2
Software classes 26
Java instructions 1565
Development time 80 hours
Size of byte code 1.3 Mbyte
5. Future Development
VRI has proved to be a successful software project and users have provided
favourable feedback. Improvements have been planned, and subsequent versions
of VRI will include deconvolution, three­dimensional arrays and the ability to
import real astronomical data. As Java has demonstrated itself to be a model
environment for the coding of this application, it will continue to be utilised.
The authors are of the opinion that the Java programming language has
a useful role to play in general astronomical software. We would not, however,
recommend its use for all applications. Its strength lies in the regime of o#­line
applications, interactive tools and, of course, user interfaces.
Acknowledgments. The authors would like to thank Mark Wieringa for
his contribution to this project, as well as Andrew Lyne, Jim Caswell, John
Reynolds and Dave Shone.
The Australia Telescope is funded by the Commonwealth of Australia for
operation as a National Facility managed by CSIRO.
References
June 1992, JEEEA (Journal of Electrical and Electronics Engineering, Aus­
tralia), 12, 2