DASI: A Degree Angular Scale Interferometer for Imaging Anisotropy in the Cosmic Microwave Background
N. W. Halverson, J. E. Carlstrom, M. Dragovan, W. L. Holzapfel, and J. Kovac

2. Array Configuration

The design of the array configuration was optimized to provide continuous coverage from the minimum to maximum baseline. The minimum baseline of 25 l (25 cm), which sets the maximum angular scale, was chosen to provide coverage of the first acoustic peak in the CMB power spectrum. The maximum baseline 115 l was chosen to overlap with the largest angular scales of the CBI.

The array elements are mounted on a single mount to allow an extremely compact configuration. To improve the u,v coverage and to provide redundancy checks, the entire array can be rotated along the line of sight. Note, the common mount prevents using standard Earth rotational synthesis techniques.

Rotational symmetry of the array configuration increases the redundancy of the array configuration under rotations along the line of sight. We chose a threefold symmetry to provide redundant baseline measurement every 60 degrees. Although the the array pattern repeats only under 120 degree rotations, the same Fourier components of the sky are measured under reflection of the array, since the sky is real and its transform Hermitian.

The array configuration resulting from the above considerations is shown in Fig. 2. Also shown is the beam pattern obtained with natural weighting of the u,v data.

Figure 2: A configuration for the DASI elements. A) The threefold symmetric feed horn arrangement. B) The resulting instantaneous u,v coverage is shown by the points. The u,v data at each point is convolved with the autocorrelation of the aperture illumination; this weighting is shown by the grayscale. C) The azimuthal average of the u,v coverage shown in panel B. The spatial frequenc y is shown as a function of the index l of the spherical harmonic expansion (l 2pu). Using mosaicing techniques, one can resolve spectral features over this range with a resolution set by the number of fields imaged. D) The resulting synthesized beam. The contour levels are 2.5, 5, 10, 20, 40, and 80% of the peak. The dashed circle delineates the half power response of the primary field of view.


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Last modified Apr 23, 1998