The angular power spectrum of the CMB contains information on the structure in the Universe that existed at the epoch of decoupling at z ~ 1000. Angular scales less than 1° are particularly interesting, as many cosmological models predict acoustic peaks in the angular power spectrum at sub-degree scales, the amplitude and position of which are highly sensitive to cosmological parameters. Within the context of a given theory, cosmological parameters such as the Hubble Constant, H0, and the mass density of the Universe, W0 º r/rcrit, can be estimated from observations of CMB anisotropy. Although there have been many CMB experiments conducted in recent years which have detected a significant increase in power at degree and sub-degree scales, none has yielded a definitive confirmation of acoustic peaks in the CMB anisotropy.
The DASI experiment, a thirteen element interferometer being built by the Center for Astrophysical Research in Antarctica (CARA, a NSF Science and Technology Center) at the University of Chicago, will provide high signal-to-noise images of CMB anisotropy and sample the CMB power spectrum in the range l = 160-710 with Dl ~ 30 (see Figure 1). Together, DASI and the Cosmic Background Imager (CBI) currently under development at Caltech, will allow the CMB angular power spectrum to be measured continuously from 4¢ to 1.15°, covering the first acoustic peak through the damping tail in the standard Cold Dark Matter (CDM) model.
Interferometers have several features which make them an attractive choice for high sensitivity imaging of CMB anisotropy. An interferometer measures the Fourier transform of the sky signal, determining directly its angular power spectrum with well-defined window functions. In contrast to swept or chopped beam experiments with filled apertures, an interferometer may be used to produce two dimensional images of the sky in a straightforward manner; an interferometer is not limited to differencing the sky signal only along constant elevation scans. Rapid phase-switching schemes ( > 10 kHz) can be used to effectively eliminate instrumental offsets.
While the stability of interferometry and its natural rejection of unwanted noise is well appreciated, many of these desirable properties arise from the clear and fairly large separation between the individual elements of the interferometer. Their beams typically trace completely independent paths through the atmosphere and do not overlap significantly until beyond the troposphere (i.e., the far field of the interferometer). Furthermore the resulting differential Doppler shift of the individually mounted elements toward the source of interest (the fringe rate) provides an added level of rejection of sources of interference. For DASI, however, the far field of the entire array is only 100 m and it is much less for the shortest baselines. The fringe rates, which would be extremely small in any case, are set to zero by using a common mount for all the array elements. While this simplifies the design considerably, additional attention must be paid to shielding the array from sources of interference.
This paper reviews the design of DASI. Special attention is given to the unique aspects of the design that are required to meet the challenges of imaging large scale CMB anisotropy.