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We report preliminary results from the third year of observations from Amundsen-Scott South Pole Station with the Python microwave background anisotropy experiment. The instrument is a five channel bolometer array, with detectors operating at 50 mK, cooled by a hybrid $^{3}$He-Adiabatic demagnetization refrigerator. Single-mode waveguide optics center the passband at 90 GHz, and couple the detector system to the 0.75m off-axis parabolic primary mirror of the telescope. The measured beamwidths are $0.75^\circ$. We combine a fast 5 Hz 3-beam chop of a vertical flat plate with slow .025 Hz telescope beamswitches to produce a gradient-free 4-beam pattern on the sky. The chop amplitude is $2.75^\circ$.
In each of the first two years of observations with this system we scanned the same 22 fields, centered at $\alpha$ = 23.32, $\delta$ = -49.5. In both years we detected statistically significant fluctuations with an amplitude of $\Delta T / T \sim 3.3 \times 10^{-5}$ for an anisotropy model with a Gaussian autocorrelation function with a coherence angle of $\theta_c = 1^\circ$. Our new results incorporate observations made in December 1994 that intersperse these fields and sample more fully this region of sky. The entire three year data set will be used to make a difference map of the structure of anisotropy at intermediate angular scales across a $6^\circ \times 22^\circ$ region of sky.
This work was supported by the National Science Foundation under a cooperative agreement with the Center for Astrophysical Research in Antarctica (CARA), grant number NSF OPP 89-20223, M.D.'s PYI grant NSF AST 90-57089, and the James S. McDonnell Foundation. CARA is a National Science Foundation Science and Technology Center.
