MAP Sky Coverage and Scan Strategy

MAP will observe the full sky every six months. The scan strategy adopted by MAP combines spacecraft spin and precession to achieve the following: 1) The MAP instrument observes more than 30% of the sky each day; 2) The spacecraft spin (and symmetry) axis maintains a fixed angle of 22.5 degrees from the Sun-Earth line to mitigate systematic effects; and 3) Each sky pixel is connected to thousands of other sky pixels to ensure high quality map solutions with negligible noise correlations.

Sky Coverage

Since a major goal of cosmology is to determine the statistical properties of the universe, it is clear that the largest possible number of sky samples improves constraints on cosmological models. The measurement of each individual position on the sky is an independent sample of the cosmology of the universe. Moreover, full sky coverage is absolutely required to accurately determine the low-order spherical harmonic moments. While the largest angular scales were observed by COBE, MAP will remeasure the full sky with higher resolution to:

• Avoid relative calibration errors when two or more experimental results are combined (eg., COBE and MAP).
• Provide greater sensitivity to the angular power spectrum.
• Independently verify the COBE results.

Scan Strategy

The goals of the MAP scan strategy include the following:

The angular separation between the two observing beams should be "large" in order to maintain sensitivity to signal at large angular scales.

This is important for comparing the MAP results to COBE, for properly normalizing the angular power spectrum, and for retaining sensitivity to the dipole which will serve as MAP's primary calibration source.

Observe a large fraction of the sky every day.

This guarantees that sky pixels will be observed at many different times in the mission which provides the capability to monitor instrument stability on many different time scales.

Maintain a fixed angle between the spacecraft spin axis and the local solar vector.

The provides for stable illumination of the spacecraft solar panels which lie normal to the spin axis, and provides a thermally stable environment to mitigate systematic effects.

Connect each sky pixel to as many other sky pixels as possible.

Since the MAP beam separation is fixed, this implies observing as many pixels on the differential ring of pixels as possible. This provides for high quality map solutions from the differential data, and renders negligible pixel-pixel noise correlations.

The beam separation adopted by MAP is 141 degrees: each beam axis points 70.5 degrees off the spin and symmetry axis of the spacecraft. The spin axis will precess in a 22.5 degree angle about the local solar vector. The combined spacecraft spin and precession will cause the observing beams to fill an annulus centered on the local solar vector with inner and outer radii of 48 and 93 degrees respectively. Thus MAP will observe more than 30% of the sky each day and will observe the ecliptic poles every day. The spin period will be 2.2 minutes while the precession period will be 1 hour. As the Earth orbits the Sun, the whole observing annulus revolves with it producing full sky coverage after six months.

The image below depicts the MAP scan pattern after one complete spacecraft precession (1 hour); the bold circle shows the path for a single spin (2.2 minutes). Click on the highlighted text to see the scan pattern superposed on a model sky map. The map is displayed in ecliptic coordinates in which the ecliptic equator runs horizontally across the map. Note that because of the large size of the annulus, the beams will always see a substantial modulation due to the CMB dipole.

The MAP Scanning Geometry:

[Click here to see the scan pattern superposed on a model sky map]

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Last updated: Friday, 05-21-1999