The technology behind MAP

Print edition : September 01, 2001
R. RAMACHANDRAN

THE Microwave Anisotropy Probe (MAP) will detect tiny fluctuations of about a part in a million in Cosmic Microwave Background Radiation (CMBR). The MAP instrument consists of a set of passively cooled microwave radiometers with 1.4 m x 1.6 m primary paraboloid reflectors to provide the desired angular resolution. Feeds from these back-to-back telescopes are coupled to 10 microwave receivers. The instrument has five frequency bands from 22 to 90 gigahertz (GHz) with comparable sensitivity to facilitate separation of galactic foreground signals from CMBR. Galactic signals are distinguishable from CMBR anisotropy by their differing spectra and spatial distributions.

The MAP design specification is a sensitivity in each of the five bands of 35 microkelvin (0.000035 K) per 0.3 x 0.3 pixel. However, the research team expects that if galactic emission is negligible at high latitudes above 40 GHz, the sensitivity achievable could be a higher 20 microkelvin per pixel. While recent balloon and other ground experiments have aimed at resolutions of less than 0.3 in measurements over limited sky regions, MAP will aim at resolutions ranging from 0.93 in the lowest frequency band to less than 0.23 in the highest band of 90 GHz. But the returns in a space mission are far greater as compared to ground and balloon experiments even with this relatively coarser resolution.

At the core of the MAP instrument is a complex set of state-of-the-art microwave receivers that turn the very faint microwaves from the early universe into signals that common electronic equipment can process. This is done with very low noise amplifiers and with minimal additional receiver artifacts. The receivers split the incoming microwaves into separate paths, each with its own independent amplification. Any difference in these signals are due to instrument changes, while common signals are from the sky. This automatically neutralises the adverse effects that would otherwise occur owing to naturally occurring changes of receiver gain. This MAP receiver approach was enabled by the creation of amplifiers that amplify microwave signals directly even at the highest frequency (100 GHz) with a high level of sensitivity. In the past it would have been necessary to convert the microwave signals to lower frequencies before amplification, a process that would have resulted in considerable loss of sensitivity besides other problems. The MAP receivers also retain the polarisation information of the original signal which provides additional constrains on cosmology.

MAP is a differential experiment. That is, MAP will measure the temperature difference between two points in the sky (separated by 141) rather than measuring absolute temperatures. Maps of relative sky temperatures will be reconstructed from the difference data using a modified form of the algorithm adopted by the Differential Microwave Radiometer (DMR) aboard the earlier COBE spacecraft (1989-92). This algorithm is an iterative one. The actual signal MAP measures is the temperature difference dT = T(A) - T (B) between the temperatures seen by the two feeds A and B of the two reflectors. If we knew T(B), say from some reference signal, we could determine T(A). But since T(B) is also just another measurement in the sky, a guess value is used from a previous sky map iteration. Thus the temperature in a given pixel of the map is the average of all observations of that pixel after correcting each observation for the estimated signal seen by the opposite feed.

The success of the above method requires a carefully designed sky scan strategy. The strategy adopted for MAP achieves this while simultaneously avoiding close encounters with the sun, the earth and the moon. At the observation post L2, MAP will execute a combined motion of "fast" spin about the spacecraft symmetry axis once every 2.2 minutes with a slow precession of once an hour about the Sun-MAP line at a fixed angle of 22.5 . Since each telescope line is about 70 off the symmetry axis, the path swept out in the sky by a given line of sight resembles a spirograph pattern that reaches from the north to the south ecliptic poles. The combined spin and precession will result in observing beams sweep an annulus. MAP will observe 30 per cent of the sky each day and will observe the ecliptic poles every day. As MAP orbits around the sun, the annular scan pattern continuously revolves around the sky so that full sky coverage is obtained after six months of observation. This coverage willbe repeated every six months for the duration of the mission. The redundancy of this coverage is intended to provide stability checks as several independent full sky maps based on six-month intervals can be compared for consistency.

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