Millimetric Ground-based Observations of Cosmic Microwave Background Anisotropy

First results of a cosmic microwave background (CMB) anisotropy experiment conducted at the Observatorio del Teide (Tenerife, Spain) are presented. The instrument is a four channel (3.1, 2.1, 1.3, and 1.1 mm)³He bolometer system coupled to a 45 cm diameter telescope. The resultant configuration is sensitive to structures on angular scales ~1°-2°. We use the channels at the two highest frequencies for monitoring the atmosphere and apply a simple method to subtract this contribution in channels 1 (3.1 mm) and 2 (2.1 mm). The most intense structure at these two frequencies is the Galactic crossing, with peak amplitudes of ~350 μK. These crossings have been clearly detected with the amplitude and shape predicted. This demonstrates that our multifrequency observations allow an effective assessment and subtraction of the atmospheric contribution. In the section of data at high Galactic latitude we obtain sensitivities ~40 μK beam^-1. The statistical analyses show the presence of common signals between channels 1 and 2. Assuming a simple Gaussian autocorrelation model with a scale of coherence θ_c = 132 for the signal, a likelihood analysis of this section of data reveals the presence of fluctuations with intrinsic amplitude C₀^1/2 = 76₋₃₂⁺⁴²μK (with a 68% confidence limit including an ~20% calibration uncertainty). Since residual atmospheric noise might still contaminate our results, we also give our result as an upper limit of 118 μK at a 95% confidence limit.