The anisotropy of the microwave background to l=3500:: Mosaic observations with the cosmic background imager

Using the Cosmic Background Imager (CBI), a 13-element interferometer array operating in the 26 - 36 GHz frequency band, we have observed 40 deg(2) of sky in three pairs of fields, each similar to145' x 165', using overlapping pointings (mosaicking). We present images and power spectra of the cosmic microwave background radiation in these mosaic fields. We remove ground radiation and other low-level contaminating signals by differencing matched observations of the fields in each pair. The primary foreground contamination is due to point sources ( radio galaxies and quasars). We have subtracted the strongest sources from the data using higher resolution measurements, and we have projected out the response to other sources of known position in the power spectrum analysis. The images show features on scales similar to6' - 15', corresponding to masses similar to(5-80) x 10(14) M-. at the surface of last scattering, which are likely to be the seeds of clusters of galaxies. The power spectrum estimates have a resolution Deltal approximate to 200 and are consistent with earlier results in the multipole range l less than or similar to 1000. The power spectrum is detected with high signal-to-noise ratio in the range 300 less than or similar to l less than or similar to 1700. For 1700 less than or similar to l less than or similar to 3000 the observations are consistent with the results from more sensitive CBI deep field observations. The results agree with the extrapolation of cosmological models fitted to observations at lower l and show the predicted drop at high l (the damping tail).