Title: Detection of Cosmic Microwave Background Anisotropy by the Third Flight of MSAM

Abstract: The third flight of the Medium Scale Anisotropy Measurement (MSAM1), in June 1995, observed a new strip of sky, doubling the sky coverage of the original MSAM1 dataset. MSAM1 observes with a 0.5 deg beam size in four bands from 5-20 icm. From these four bands we derive measurements of cosmic microwave background radiation (CMBR) anisotropy and interstellar dust emission. Our measurement of dust emission correlates well with the 100 um IRAS Sky Survey Atlas; from this comparison we determine an effective emissivity spectral index between 100 um and 444 um of 1.46 +/- 0.28. Analysis of our measurement of CMBR anisotropy shows that for Gaussian-shaped correlation functions with theta_c = 0.3 deg, we place a limit on total rms anisotropy of 2.2 x 10^{-5} < Delta T/T < 3.9 x 10^{-5} (90% confidence interval, including calibration error). The band-power limits are <delta T> = <l (l+1)C_l/2 pi>^{1/2} = 50^{+16}_{-11} uK at l = 160, and <delta T> = 65^{+18}_{-13} uK at l = 270 (1 sigma limits, including calibration error). The corresponding limits with statistical errors only are <delta T> = 50^{+13}_{-9} uK and <delta T> = 65^{+14}_{-10} uK respectively. These measurements are consistent with a standard adiabatic cold dark matter model; we discuss constraints on h, n, and the redshift of reionization.