Annu. Rev. Astron. Astrophys. 1992. 30: 653-703
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3.3 Observations on angular scales 1-20°

There have been many searches for anisotropy on angular scales 1-20° since 1983. In keeping with the discussion of previous sections, we discuss these below in order of increasing angular size. It should be borne in mind that over this range of angular scales many models of galaxy formation predict angular spectra in the temperature fluctuations which are not well approximated by a Gaussian distribution. In particular cold dark matter models (e.g. Bond et al 1991) predict a strongly non-Gaussian form for W(k) over this range of angular scales. Thus, over this range of scales the observations must be compared to specific models in order to estimate the expected variance in sky temperature as given by Equation 3. In the absence of any accepted model for galaxy formation it has become common practice amongst observers to assume, for want of a better alternative, that the form of W(k) is Gaussian in order to give some idea of the sensitivity levels of their observations. We follow this practice below, but caution the reader that limits and expected values of the sky variance derived by this method cannot readily be compared to theoretically predicted levels of the sky fluctuations.

3.3.1 OBSERVATIONS AT SASKATOON Timbie and Wilkinson (1988, 1990) have carried out sensitive measurements at Saskatoon, Saskatchewan, of the isotropy of the microwave background radiation using an interferometer consisting of two horn antennas mounted on a rotatable table. The resolution (FWHM) was approx 1°.5 x 5°. The receivers used SIS mixers to achieve low noise levels operating at 43 GHz with a bandwidth of 500 MHz. Interferometric observations have a number of advantages over single dish observations. The most important of these are the elimination to high order of sources of systematic noise which are spread over large angles, such as ground spillover and atmospheric gradients on large angular scales.

The observations were carried out in January and February 1987. Timbie and Wilkinson observed fields straddling the North Celestial Pole at declination delta = 87°.5. The interferometer was switched in azimuth between the two positions 2°.5 from the pole every 10 seconds. The output was therefore equivalent to a double switching experiment, with the first level of switching being done by the interferometer at a frequency comparable to the reciprocal of the bandwidth. This double differencing eliminates long term drifts in the signal level caused by instrumental drifts in gain and offset and atmospheric changes.

Observations were made almost continuously over a 15-day period. The data were binned in 12 two-hour bins in sidereal time. In two hours a field 2°.5 away from the pole drifts about 1/3 of a beamwidth of the main lobe in elevation; so that the sky was slightly oversampled by this procedure. The symmetry of the observing technique ensured that any true sky fluctuations would average to zero when all the data were added together because any patch of sky was observed once with positive signal and twelve hours later with negative signal. In fact a mean value of 190 ± 50 µK was recorded, and was ascribed to non-uniform ground spillover. This has been subtracted from the data. The resulting measurements of this experiment are shown in Figure 3a.

For a fit to a constant DeltaT, chi2nu = 1.5, which implies that some ``signal'' has been detected. Timbie and Wilkinson do not interpret this result as a real detection of anisotropy, but they use it to place more conservative upper limits on the anisotropy using the likelihood ratio test. Their 95% confidence upper limits are given in Table 4. The power of the test is approx 55%.

A second set of observations has been carried out using a more sensitive receiver. The results will be reported in Jarosik et al (1992, in preparation).

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