Deconvolving the Beam in Small Angular Scale Cmb Experiments

This paper is concerned with experiments which measure CMB anisotropies on small angular scales. A certain coverage, a beam structure and a level of uncorrelated noise define each experiment. We focus our atention on the reversion of the beam average. In each experiment, we look for the best pixelization for reversion, namely, for the pixelization that –after reversion– leads to good maps containing right spectra for the most wide range of angular scales. Squared pixels having different sizes ”smaller” than the beam radius (θ FWHM ) are considered. For a given size, the following question arises: How well can we assign a temperature to each pixel? Various mathematical methods are used to show that, in practice, this assignation –beam reversion or deconvolution– only leads to right spectra for pixel sizes greater than a certain lower limit close to θ FWHM /2. This limit is estimated for negligible and relevant levels of noise and also for spherically symmetric and asymmetric beams. After this general study, we focus our attention on two feasible detectors (which have been proposed to be on board of PLANCK satellite). For each of them, we estimate the size of the most appropriate pixelization compatible with beam reversion, difraction, observational strategy et cetera and, then, we answer the following question: Which is the part of the angular power spectrum which can be extracted from appropriately pixelized maps after deconvolution? Subject headings: cosmic microwave background—cosmology:theory— large-scale structure of the universe—methods:numerical PACS: 98.70.Vc, 95.75.-z, 95.75.pq, 95.75.St