This page contains a description of how the data is collected and sent back to Earth. It is also an explanation of how the scientists behind MAP can be sure that the radiation fluctuations they detect are actually fluctuations in the cosmic microwave background (rather than galactic or stray foreground signals).
MAP does not measure the actual temperature at each point in the sky. Rather, it measures the difference in temperature between two points in the sky. This is the reason for the back-to-back arrangement of the MAP instrument, illustrated below. The MAP telescopes and detectors sit back-to-back and collect data from independent spots on the sky. The instrument then combines these signals in a way to ensure that the measured temperature differences are relatively free of systematic error.
MAP is like the Earth in its orbit in that it both rotates and revolves. MAP rotates once every two minutes around its spin axis, with the aid of three "reaction wheels" symmetrically placed around the base of the satellite. The spin axis of MAP always maintains a fixed angle of 22.5 degrees with respect to the Sun-Earth line. This axis revolves around the Sun-Earth line every hour, and rotates annually with the Earth around the Sun. Anyone who has ever done a science project knows that one of the keys to obtaining reliable data is multiple observations, and with this scan pattern, MAP can see one point in the sky from many different perspectives. In this way, MAP gets a complete picture of the sky several times over, and compare the data it receives so as to eliminate any instrumental errors. The scan pattern that MAP will follow across the sky is as follows:
This pattern is analogous to the pattern traced out by a point on a spinning top. As it spins, it also wobbles (precesses). The blue line depicts the path of the spin axis in one hour (one precession), the green line depicts the path of one line-of-sight after one spin (2.2 minutes), while the black line depicts the path of the same line-of-sight after one hour.
One of the ways that MAP can be sure that the signals received are actually from CMB anisotropy is by covering the sky at multiple frequencies. There are three physical mechanisms acting in our own Galaxy that produce radiation that could be confused with CMB anisotropy: synchotron radiation, from electrons orbiting in magnetic fields; free-free radiation, radiation from hot ionized gas, and thermal radiation from interstellar dust.
These three mechanisms all emit radiation with a certain frequency spectrum. CMB anisotropy also has a characteristic frequency spectrum that is different the Galactic sources. Knowing what these are, MAP can measure the radiation fluctuations at these frequencies, and then subtract the values for the foreground radiation, leaving only the data for CMB anisotropy. However, high-resolution, large-scale templates for the free-free emissions are not available at this time, so MAP needs to employ other techniques in order to get rid of this possible source of error. To do this, scientists will combine the multi-frequency observations in such a way that the foreground radiation cancels. These techniques were successfully employed by the COBE satellite.
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Last updated: Friday, 05-21-1999