The MAP instrument consists of two back to back, off axis Gregorian telescopes that produce two focal planes, A and B, on opposite sides of the spacecraft symmetry axis. A set of 10 corrugated feeds lie in each focal plane and transport power to the amplification electronics. The microwave system consists of 10 4-channel differencing assemblies that are designed to eliminate low frequency gain instabilities and amplifier noise in the differential signal.
The reflector design incorporates two back-to-back off-axis Gregorian telescopes with 1.4 x 1.6 m primary reflectors and 0.52 m secondary reflectors. Each primary is an elliptical section of a paraboloid, while the secondaries are nearly elliptical. This arrangement produces two slightly convex focal surfaces on opposite sides of the spacecraft spin (symmetry) axis with plate scales of ~15'/cm. The 99.5% encircled energy spot size diameter is less than 1 cm over a 15 x 15 cm region of the focal plane, and less than 0.33 cm over the central 8 x 8 cm region.
In order to limit diffracted signals to less than 0.5 uK, diffraction shields are employed above, below, and to the sides of each secondary. In addition, the deployable solar panels and multi-layer insulation guarantee that the secondaries remain at least 6 degrees into the Sun's shadow during observing.
The feed design calls for as small an aperture as possible consistent with a primary edge taper requirement of -25 dB, and a length that places the throat of each differential feed pair in close proximity to the other. The feed aperture diameters scale inversely with frequency, while the primary is equally illuminated at each frequency, leading to a frequency dependent beam size. The feeds are corrugated to produce beams with high symmetry, low loss, and minimal sidelobes: the extremely low loss HE_11 hybrid mode dominates. The phase center of each feed is kept as close as possible to its aperture, resulting in a frequency-independent beam for each feed. Since the distance from the focal plane to the spacecraft symmetry axis is nearly the same for all the feeds, the high frequency feeds are extended with low loss corrugated waveguide, while the low frequency feeds are "profiled" to reduce their length, while limiting excitation of the TE_11 mode to less than -30 dB.
The microwave system consists of 10 4-channel differencing assemblies, one for each pair of feeds. One assembly operates at 22 GHz, one at 30 GHz, two at 40 GHz, two at 60 GHz, and four at 90 GHz. The base of an A-side feed in the Focal Plane Assembly (FPA) is attached to a low-loss orthomode transducer (OMT) which separates the signal into two orthogonal polarizations, A and A'. The A side signal is differenced against the orthogonal polarization, B', from the corresponding B-side feed, and vice-versa.
The differencing is accomplished by first combining the two signals A and B' in a hybrid tee to form (A+B')/sqrt(2) and (A-B')/sqrt(2), then amplifying each in two cold HEMT amplifiers and sending the phase-matched outputs to the warm receiver box (RXB) via waveguide. In the RXB the two signals are amplified in two warm HEMT amplifiers , phase switched between 0 degrees and +90 or -90 degrees, respectively, at 2.5 kHz, then split back into A and B' in a second hybrid tee. At this point, the two signals are square-law detected, amplified by two line drivers, and sent to the Analog Electronics Unit for synchronous demodulation and digitization. The other pair of signals, A' and B, are differenced in the same manner giving a total of four amplification channels per differencing assembly.
The splitting, phase switching, and subsequent combining of the signals enhances the instrument's performance in two ways:
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Last updated: Friday, 05-21-1999
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