Question: How many kinds of foreground pollution [are there] in the CMB data, and how [does one] remove them so as to get the real CMB signal? What role does the improvements of sensitivity and resolution of the telescope play on the identification of foreground signals? — Hongmin
Answer: Foreground sources of emission which need to be extracted from CMB measurements in order to get the pure CMB signal include dust, free-free emission, synchrotron radiation, and point sources. Extracting these signals from CMB data is quite complicated, usually involving identification of the background signals by their known frequency dependence. Measurement sensitivity is important in these measurements. An example of a procedure for extracting these signals can be found in Tegmark and Efstathiou 1996.