IRAM Technical Reports

The following technical IRAM reports have recently appeared. The full versions in pdf format are available on our web pages. Please follow the ``science users'' link to ``Letters, results and reports'', and then to ``Technical Reports''.

2009-4: Averaging spectra with CLASS

Authors: S. Bardeau (IRAM) and J. Pety (IRAM)
Keywords: AVERAGE, ACCUMULATE, resampling, weighting, concatenation
Related Documents: CLASS manual, IRAM memo 2005-1: CLASS evolution: I. Improved OTF support

Abstract:
CLASS90 (hereafter CLASS) provides a set of commands capable to average two or more spectra. They provide many averaging modes, presented hereafter in this document. The different modes often imply to perform internally and silently some important computations, namely resampling and weighted average. Combining these operations at the same time may imply some non-trivial effects described here. On October 2008, it appeared that some particular combinations of the tunable modes were not behaving as expected (either in CLASS90 and in CLASS77). A complete cleaning and factorization of the algorithm was performed, associated to exhaustive tests of all combinations. A test suite was also provided to check the output of each commands and modes. Following this maintenance of the code, and the use of these capabilities to concatenate the new EMIR spectra, it was decided to write this document in order to keep a trace of all the methods applied. This was also the occasion to investigate deeply in the code and to examine the effects which can occur during all the possible processings.

2009-3: A simulator of interferometric On-the-fly observations

Authors: N. Rodriguez-Fernandez (IRAM), Frederic Gueth (IRAM), Jerome Pety (IRAM)
Keywords: interferometric wide-field imaging, on-the-fly observing, simulations

Abstract:
We have developed a simulator of interferometric on-the-fly observations in the framework of our research on image synthesis for mosaics observed in the on-the-fly (OTF) mode, in which the interferometer takes data at the same time that the antennas move continuously across the source. This simulation tool has been developed on the basis of the IRAM/GILDAS ALMA+ACA simulator (Pety, Gueth & Guilloteau 2001).

2009-2: Imaging of interferometric On-The-Fly observations (I): context and discussion of possible methods

Authors: N. Rodriguez-Fernandez (IRAM), Jerome Pety (IRAM), Frederic Gueth (IRAM)
Keywords: interferometric wide-field imaging, on-the-fly observing

Abstract:
We discuss the measurement equation for interferometric observations of fields larger than the primary beam of the antennas, both for standard ``pointed'' mosaics and for mosaics observed in on-the-fly (OTF) mode. The main advantages of using the OTF mode are a gain of observing time and a higher homogeneity of the dataset. OTF mosaicing is similar to classical stop-and-go mosaicing but the effective beam when observing OTF is not exactly the primary beam of the antennas. We show that the effective beam is similar to the primary beam when the scanning rate is better than Nyquist. We review different techniques to image and deconvolve mosaic data, in particular the Ekers & Rots 1979 (ER79) scheme, which consist in Fourier transforming the visibility function with respect to the scanning coordinate. We discuss how to implement an OTF-optimized imaging algorithm to deal with the mosaic data as a whole based on the ER79 scheme. Finally we discuss observing time and mosaic size constrains for OTF observations.