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The next generation of CCD imagers will undoubtedly be mosaics, in order to overcome 1) the low yield when fabricating large devices, 2) the limited number of readout channels that can be put easily on a single CCD, and 3) the physical limitation of the 4-inch silicon wafer. As a first step towards a large CCD mosaic, we have recently fabricated a 2 X 2 array of Loral 2048 X 2048 CCDs. The chips are of a 2-side-buttable design (by John Geary of SAO), and have 15$\mu$ pixels. A prototype mini-mosaic imager, using unthinned, front-illuminated CCDs has been used at the 0.9m and 4m telescopes. The prototype has gaps of approximately 40 pixels (0.6 mm) between chips, giving a filling fraction greater than 98\%. These gaps are easily filled by taking multiple offset exposures. The RMS flatness of the array is better than 15$\mu$. Using the new CTIO transputer-based controller, the entire array is read out and de-multiplexed in 150 seconds. Data obtained with this prototype as well as proposed reduction algorithms for achieving geometric and photometric calibration are displayed. Thinned versions of the mini-mosaic are intended for use at KPNO and CTIO by spring 1994. After the completion of this project, an 8192 X 8192 pixel mosaic is planned.
