paulhancock / friendlyvri

A Virtual Radio Interferometer application

The Friendly Virtual Radio Interferometer

The Friendly Virtual Radio Interferometer (VRI) is designed to simulate astronomical observations using linked arrays of radio antennas in a technique called earth rotation aperture synthesis. As the successor to the original Java-based VRI, it focuses on simulating the effect of combining different antenna layouts.

Installation:

The Friendly VRI is written in Python to work with both Python 2.7.x and 3.x. You will need the following modules installed:

• numpy
• matplotlib
• pil or pillow
• tkinter
• opencv (optional - enables web-cam image capture)

If you use the default system python interpreter, these can usually be installed from the command line on Linux or Mac OS X by executing the command

sudo pip install <module_name>.

If you are running Anaconda scientific python the command is

conda install <module_name>.

Once all required modules are installed, click on the 'View on GitHub' link at the top of the page and then the 'Clone or download' button.

A note about opencv on Mac OS X:

The default installation of opencv on Anaconda for Mac OS X seems to be broken (causes a segmentation fault). Some folk on the internet have reported success installing a working opencv module using the command

conda install -c https://conda.binstar.org/menpo opencv.

If this works for you and you want to enable the web-cam capture button for Mac OS X, comment out the following lines in the 'vriTk.py' code:

if sys.platform=="darwin":
    hasCV2 = False

Usage

Start the application by executing python vriTk.py on the command line from within the friendlyVRI/ directory. The interface is (hopefully) very intuitive and is split into a control window and a plotting window. The plotting window can be maximised and buttons at the lower-right enable jumping quickly between the two windows. The control window allows you to:

• Plot the layout of the antennas in an array configuration for a particular telescope.
• Create a list of observations to be simulated using different array configurations, over different time ranges (hour angle) and with different sampling cadence.
• Load in a model image and set its angular scale on the sky.
• Apply the uv-coverage of the observations to the model to simulate an observation.

The plotting window shows inputs, outputs and intermediate steps in the process: model image, fast Fourier transform (FFT) of the model, plot of uv-coverage, filtered model FFT, synthesised beam and final observed image.

See HELP or the help menu in the application for step-by-step instructions.

Note that the calculations underlying the graphical application are split into the file vriCalc.py to facilitate use with alternative interfaces. See CALCULATIONS for a simple example.

Defining Custom Arrays

The location and layout of antennas in each array configuration is defined in ASCII files under the arrays/ directory. You can define your own arrays by copying the existing templates and supplying a list of coordinate offsets in the East and North directions, in units of meters. The software will read these on start-up and they will appear in the list of available arrays. Note that the combination of 'telescope' + 'array' name must be unique.

Caveats

This software is designed to be a simple 'quick-look' tool and ignores lots of effects such as non-co-planar arrays, time averaging, multi-frequency synthesis etc. It also sets the sampling grid in the Fourier domain from the extent and pixel spacing of the input image, which can result in under-sampled synthesised beams and artefacts. At the moment, the observing simulations are assumed to be noise-free and no attempt is made at calculating sensitivities. Tasks to perform robust simulations of an interfometric observation exist in CASA, MIRIAD and AIPS, but are more difficult to use.

Other VRI Software

Other excellent virtual interfeometer software has been developed by staff at observatories around the world:

• APSYNSIM is a 'full-fat' simulator by Ivan Marti-Vidal at Onsala Space Observatory, Sweden.
• Pynterferometer is a public demonstration tool by Adam Avison and Sam George and has excellent documentation in the accompanying paper.

Credits and Contact Information:

The Friendly Virtual Radio Interferometer tool was written by Cormac Purcell and Roy Truelove at Macquarie University, Sydney. Questions, comments, feature requests and bug reports should be directed to 'cormac.purcell (at) mq.edu.au'.

Licence

Copyright (c) 2017 Cormac R. Purcell and Roy Truelove.

Released under the MIT licence.