MAPS Simulator

Use of the simulator

1. LOsim, which creates a pristine visibility set based on sources specified by the user, or a FITS image, or both;
2. Array Config, which with various specifications for number of stations, arm lengths, min and max baseline lengths (etc.) will create an array of stations;
3. Main Simulator Script, which takes the array and the pristine visibility set and creates an array-dependent visibility data set for use in other packages.

Registration

Access to the simulator is via a web interface. An overall cgi programme passes parameters to a server on the supercomputer, which then allocates jobs to the supercomputer on a load basis, and returns data and plots to the web. Because we are running these scripts on the Centre for Astrophysics and Supercomputing main machines, the web interface is password protected. Users from academic institutions can register for a username and password for access to the web interface.

If you want to register, then it is a simple process. You will then be e-mailed with a username and password.

History of the simulator

MAPS started out as code written by MIT Haystack for the LOFAR (Low Frequency Array) project. It was designed to simulate typical low-frequency skies and used as a comparison tool for different LOFAR projects.

In July 2003, at an SKA meeting in Geraldton Australia, Steven Tingay, the manager of the SKA group at Swinburne University approached Colin Lonsdale at MIT with a proposal that Swinburne provides support and resources to host and maintain the LOFAR MIT simulator (as it was known then), with the return that it could be used in simulating various SKA designs.

Various modules of the simulator were 'shipped' to Swinburne University over the next 6 months for pre-installation on the Swinburne's Supercomputer and in January 2004 Ramesh Bhat from MIT and Richard Ogley from Swinburne installed the simulator on the supercomputer. This was no small feat since the simulator utilised numerous supporting packages including Miriad, AIPS++, Python all of which had to be meshed together in one package.

Once the simulator was installed, Richard Ogley started beta-testing the software, a process which took a number of months mainly due to differences between LOFAR (a low-frequency telescope design) and the SKA (a high-frequency design with very long baselines). There were initial problems due to the software not working on the scales that the SKA was designed for, many of which were ironed out with early-morning teleconferences to MIT and help of other beta-testers such as Kirsty Rhook and Dave Barnes from Melbourne University.

At the end of April 2004, MIT Haystack decided to change the name of the LOFAR MIT simulator to one which represented a more active role of the simulator to the SKA community. Talks at the time were lively, but one of the conditions from MIT is that the name or acronym shouldn't be rude! (Much to the 'disappointment' of the Swinburne people). It was decided that the simulator would be called the MIT Array Performance Simulator or MAPS.

Breakdown of MAPS modules

LOsim module

The job of this module is to create a pristine uv visibility set, without the Earth present. This visibility set is then convolved with an Earth array, specified by the array generation module, to create a telescope-dependent sky.

The LOsim module takes inputs from either a pre-created FITS file, or a list of Gaussian sources, or both.

Array generation

Once the sky has been decided on and created by the LOsim module, one can create different arrays to observe the sky with. This is the job of the array generation module.

5 different arrays can be modelled. Four of these are log spirals with various parameters, and one is an exponential shell. All arrays take their inputs from the user as supplied parameters (entered on the web by a form). There are a number of common parameters, and also a couple of parameters which are unique to either the log spiral or the exponential shell case.

The overall array generation is described in the following diagram.

An array generation form is available for testing of different array parameters.

Simulator script

With a sky and an array, the main Simulator script can create a visibility set for the particular telescope defined as the array. The main module is called visgen and generates the visibilities as seen by the array, based on the sky visibilities generated by LOsim.

A diagram of the visibility generation is given in the diagram below.