Abstract 9619374 Meyer The TopHat long-duration balloon experiment is designed to measure the spatial variations in the cosmic microwave background radiation (CMBR). It utilizes a novel configuration of the balloon payload, with the telescope itself being mounted on the top of the balloon in order to eliminate many of the sources of systematic errors caused by the balloon's structures and the Earth being in the sidelobes of the telescope antenna, while the electronics and control equipment will be suspended below the balloon in the usual fashion. The resulting map of the cosmos in five spectral bands with wavelengths between 0.45 and 2.3 mm, will provide unprecedented detail in the CMBR anisotropy over a large (1800 square degree) patch of the sky. The variations in the CMBR carry the imprint of the distribution of matter in the early universe, approximately 300,000 years after the big bang. The nature of the anisotropy can be used to sensitively test various models of the nature of the universe, both early-on and now; hence, TopHat will contribute to the resolution of such questions as whether the universe is open or closed and what is the nature of the missing (dark) mass in the universe. This experiment will be flown from McMurdo, Antarctica. The project is jointly sponsored by the Danish Space Research Institute, NASA and NSF. ***
