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When a satellite takes measurements of the radiation from different patches of the sky to construct a map of the thermal radiation of the universe on the surface of a sphere, it would also contain radiation from the Milky Way, for one. A big patch of the map will have noise due to the objects in the Milky Way or from other sources.

How does one know where and how much noise to remove while constructing the map?

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The contribution from different sources.

Above I have plotted few Planck's radiation contribution: The violet is due to CMBR, Green is due to Milky way Galaxy, and Blue is due to contribution from both.

One way to resolve out CMBR is by modeling milky way from higher frequency of the graph since CMBR contribution is negligible at higher frequency. At particular direction one can associate temperature due to Milky way by setting a high pass frequency. Then one can subtract this temperature contribution at lower frequencies where CMBR contribution is significant. This is the one of the way probably not the best, but doable.

Note1: The graph is not from actual data. It is just for explaining how to go about it.

Note2: What should be high pass frequency will be a good question. With good arguments one can decide what should be the high pass frequency. And there's always way about it.

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