Having given a description of the theoretical context, we now consider the experimental considerations that go into making measurements of the CMB. These include both the observing modes that are used to gain the sensitivity required and the various contaminants that emit at the frequencies of interest to CMB astronomy.
There are a number of foregrounds that need to be well understood, or at the very least minimised, when making CMB measurements. One of the main foregrounds that is seen in CMB data originates from extra-galactic sources. These are usually unresolved point sources such as quasars and radio–loud galaxies. The best analysis of the contribution by unresolved point sources to CMB experiments has been produced by Franceschini et al.  and de Zotti et al. . They used numerous surveys, including VLA and IRAS data, to put limits on the contribution to single beam CMB experiments by a random distribution of point sources. This analysis assumes that there are no unknown sources that only emit radiation in a frequency range between 30 GHz and 200 GHz. This range of frequency has not been properly surveyed and therefore there is still a cause of concern in the CMB community. The problem with using past surveys to predict the effects of point sources is that a large fraction of these sources are highly variable. Therefore, by far the best way to subtract the sources from the data is to make simultaneous observations with a higher resolution telescope at the same observing frequency (the Ryle Telescope at Cambridge is used by CAT for this purpose).
At the higher frequency range of the microwave background experiments, dust emission starts to become dominant. This is the hardest galactic foreground to estimate, as it depends on the properties of the individual dust grains and their environment. At lower frequencies Galactic synchrotron and free–free emission become important. Each of the Galactic foregrounds are extremely difficult to eliminate, and so far the best method has been to observe in regions where their emissions are expected to be small and at frequencies where they are less dominant. With the multi–frequency observations it is possible to subtract the effect of these foregrounds if their spectral dependencies are known (see Section 6.1).
The final foreground that is seen with experiments looking at the microwave background is closer to Earth than those already discussed. This is the atmosphere. Fluctuations in the atmosphere are hard to distinguish from actual extra–terrestrial fluctuations, when limited frequency coverage is available. There are three ways to overcome this problem. The first method is to eliminate the atmospheric effect completely. Space missions are the best way to do this, but their main problem is cost. High altitude sites (either at the top of a mountain or in a balloon) can reduce the atmospheric contribution as can moving the experiment to a region with a stable atmosphere. A cheaper alternative to physically moving the experiment is to observe with the experiment for a long time. As the atmospheric effects occur on a short time–scale, compared with the life-time of the experiment (typically of order a few months for each data set taken with ground based CMB experiments), and the extra-terrestrial fluctuations are essentially constant, by integrating over a long time the contribution from the extra–terrestrial fluctuations are increased with respect to the atmospheric effects. Stacking together data points (taken from separate observations) will reduce the variable atmospheric signal with respect to the constant galactic or cosmological one by a factor of (providing that they are independent with respect to the atmospheric signal and any long term atmospheric effects that affect the gain have been removed). The third way, which can also be combined with both the first and second way, is to design the experiment to be as insensitive as possible to atmospheric variations.
The first obvious design consideration is to make the telescope sensitive to frequencies at which the atmospheric contribution is a minimum. By avoiding various bands in the spectrum, where much emission is expected (for example water lines), the atmosphere becomes less of a problem. Above a frequency of about 100 GHz the atmospheric effect is too large to allow useful observations from a ground based telescope. Taken with the increasing foreground contamination from the Galaxy at low frequencies (It is expected that the Galaxy dominates over the CMB signal at frequencies below 10 GHz.), this reduces the observable frequencies for ground based CMB experiments to between 10 and 100 GHz. This narrow observable range results in the need for balloon or satellite experiments so that a larger frequency coverage can be made to check the consistency of the results and to check the contamination from the various foregrounds that are expected. Figure 6 shows the expected level of the various foregrounds for a typical CMB experiment. Figure 7 shows a typical region of the sky over a range of frequencies covered by CMB experiments (no atmospheric emission has been added).
The largest atmospheric variations occur mainly on relatively long time scales, compared to the integration time of telescopes (typically of order a few minutes), as the variations are produced by pockets of air moving over the telescope. If an experiment could be insensitive to these long term variations, then it should effectively see through the atmosphere. An interferometer (e.g. CAT) extracts a small range of Fourier coefficients from the sky, reducing any incoherent signal (short time scale variations) or any signal that is coherent on large angular scales (long time scale variations), and so should see through the atmosphere very well. Similarly, an experiment that switches between two positions on the sky relatively quickly will also reduce the long term atmospheric variations. This technique is called beamswitching (e.g. Tenerife, MSAM). A variation on the beamswitching technique is to scan a single beam backwards and forwards across the sky (e.g. Saskatoon, Python).
© Max Planck Society and the author(s)