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Figure 1. Side view cross section of the MAX gondola including optics and cryostat.

The combination of a 1 meter diameter primary mirror, a small secondary mirror, and the receiver forms a 1/2 degree FWHM optical beam. With the present MAX receiver, the beam illuminates one photometer, which means that only one pixel on the sky is measured at one time. This pixel is "chopped" back and forth on the sky by quickly modulating the secondary mirror. Differences in signal between the two chop positions represent anisotropy in the temperature of the sky on the scale of the chop amplitude (~ 1 degree). Many regions of the sky have a low enough level of foreground galactic interference that the measured temperature differences are believed to be those of the cosmic microwave background.

The rapid chop (5.5 Hz for all flights to date) reduces the effect of low-frequency atmospheric variations by comparing two positions on the sky before atmospheric structures can change. The rapid chop also reduces the effect of low-frequency noise in the signal amplifiers and the temperature of the cryogenic refrigerator.

The temperature difference is measured for a set of points that lie along a strip on the sky that constitute the "scan." For telescope guidance purposes each scan is centered about a guide star, and the scans are named for this star. The temperature difference as a function of scan angle for the star Gamma Ursae Minoris is shown in the following figure (reference).

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Figure 2. Temperature difference on the sky as a function of scan angle for the MAX4 scan of the GUM region.