Interferometer Characteristics

The left panels of Figure 2 show the DASI instantaneous u-v coverage and the corresponding synthesized beam in the image plane. Besides yielding spectral information, the 10 1-GHz channels allow frequency synthesis mapping, since for any given baseline, the corresponding u-v radius varies by 30% over the 10 GHz band, hence the radial pattern in the u-v coverage. The effective snapshot resolution when all 10 bands are combined is approximately $20\arcmin$, as shown in the central panel.

Figure 2 also shows the effective sensitivity of the instrument in l-space. The right panel is an azimuthally collapsed composite of the l-space window functions for all independent baselines at a single frequency. Exact antenna spacings on the faceplate were numerically optimized to yield uniform u-v coverage over the angular range accessible to DASI. Note that the finite aperture introduces correlations between baselines at neighboring u-v points, particularly between visibilities of the same baseline at neighboring frequencies.

With the three-fold redundancy, the pattern of u-v coverage repeats with each $60\deg$ of rotation. Although the antenna locations repeat only every $120\deg$, the same Fourier components of the sky are measured under reflection of the array, since the sky is real and its transform Hermitian.

 

Figure: Fourier-plane and imaging characteristics of the DASI instrument. The instantaneous u-v coverage of the interferometer (left), and the resulting synthesized beam (center) for natural weighting, with approximately $20\arcmin$ FWHM. (right) The corresponding l-space coverage at 30 GHz. Note that the three-fold symmetry results in only 26 independent baseline lengths at each frequency.

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