The ten meter diameter primary mirror of the W. M. Keck Telescope is a mosaic of thirty-six hexagonal mirrors. An active control system stabilizes the primary mirror. The active control system uses 168 measurements of the relative positions of adjacent mirror segments and 3 measurements of the primary mirror position in the teleL'cope structure to control the 108 degrees of freedom needed to stabilize the figure and position of the primary mirror. Th_ components of the active control system are relative position sensors, electronics, computers, actuators that position the mirrors, and software. The software algorithms control the primary mirror, perform star image stacking, emulate the segments, store and fit calibration data, and locate hardwaredefects. We give an overview of the active control system, its functional requirements and test measuremeitts. Fig. I The locations of the actuator and sensors from the concave side of the primary. '3 tj 1lr II m' _r =mi == ' .... 11' " '" " " " I?1 ' ' ....

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A radiofrequency-gain total power radiometer measured the intensity of the cosmic microwave background (CMB) at a fr_uency of 1.47 GHz (20.4 cm wavelength) from White Mountain, California, in September 1988 and from the South Pole, Antarctica, in December 1989. The CMB thermodynamic temperature, TCMB, is 2.27 :!: 0.25 K (68% C.L.) measured from White Mountain and 2.26 :t:0.21 K from the South Pole site. The combined result is 2.27 :!:0.19 K. The correction for galactic emission has been derived from sealed low-fr_uency maps and constitutes the main source of error. The _tmospherie signal is found by extrapolation from zenith scan measurements at higher frequencies. The result is consistent with previous low-frequency measurements, including a measurement at 1.41 GHz (l.,¢vinet al. 1988) made with an earlier version of. this instrument. The result is-2.5 o (-1% probability) from the 2.74 + 0.02K global average CMB temperature.

We have measured the temperature of the cosmic microwave background radiation at wavelengths of 0.33, 3.0, 8.2 and 21.3 cm. These measurements represent a continuation of the work reported by Smoot e f al. (1985). The new results have a weighted average of 2.70 & 0.05 K and are consistent with past measurements. They limit the possible distortion of the cosmic microwave background radiation spectrum to less than 6%. The results of all measurements to date are consistent with a Planckian spectrum with temperature 2.74 f 0.02 K spanning a wavelength range of 0.1 to 21 cm. Subject heading: cosmic microwave background radiation

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