An ALMA simulator in GILDAS

We have developed an imaging simulator for the ALMA project, which is distributed as part of the GILDAS software package. The simulator was developed to test the wide field imaging performances of ALMA, and in particular, the usefulness of an array of smaller dishes (12 antennas of 7-m, called ACA for Atacama Compact Array) to measure the short spacings in the UV plane. The simulator includes the following steps:

• Simple simulation of ``on-the-fly'' single-dish observations;
• Simulation of ALMA interferometric observations; Several different layouts of the array configurations can be selected;
• Simulation of ACA interferometric observations;
• Simulation of the atmosphere allowing a coherent modeling of atmospheric phase noise and anomalous refraction;
• Phase calibration;
• Detailed modeling of pointing errors and simple modeling of errors of amplitude calibration;
• CLEAN-based, joint deconvolution of data coming from both arrays (ALMA and/or ACA);
• Quantitative comparison of input (model) images with the deconvolved ones.

A thorough description of this simulator can be found in the ALMA memo #398 available at http://www.eso.org/projects/alma/. Compared to the previous release (see IRAM Newsletter 48), we improved i) the treatment of atmospheric phase errors (we thank F. Viallefond of LERMA for useful discussions on this point), ii) the robustness of the deconvolution step and iii) the comparison of the model image with simulated output. The next foreseen improvements are a more realistic simulation of single-dish observations and a web interface.

The simulator has been extensively used for the evaluation of the ACA, which was found to improve significantly wide field imaging precision. A similar conclusion was reached from independent studies, based on Maximum Entropy deconvolutions, performed at NRAO and in Japan. Following these studies, the ALMA Scientific Advisory Committee has recommended ACA as a first priority enhancement to the baseline ALMA project. Although developed with a specific goal, the simulator can be applied to ALMA as specified in the baseline project.

The most recent version of the simulator can be obtained upon request. Please feel free to contact us at alma-simulation@iram.fr for any comments, questions or suggestions.

Jérôme PETY, Frédéric GUETH and Stéphane GUILLOTEAU