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Sub-nanometer distance stabilization and positioning of large optical structures is a key active control issue for future space telescopes needing picoradian precision. The paper presents solutions and results of a leading experiment, named COSI (Control Optics Structure Interaction), aimed at investigating Fabry-Pérot cavities as active optical links, capable of stiffening large and massive structures operating in vacuum. The optical length of each cavity is actuated by a PZT-driven mechanism and the cavity ensemble is controlled by a model-based Digital Control Unit. The first experiment stabilized the relative tip-tilt motion of two 6.9 kg annular plates, 0.5 m distant. A residual error, lower than 3t-½ pm (1s), t>0.1s, was achieved in presence of severe environment noise and artificial micrometer distance variations, thus fully demonstrating feasibility of COSI concept and technology.

The accurate measurement of the position of celestial objects is a fundamental step forseveral astrophysical investigations. For ground based instruments, the atmosphere is considered the basic limiting factor; in space, the knowlwdge of the instrumental parameters and/or their stability define the performance limits, but CCD cameras operated in time delay integration may take advantage of their operating mode to reduce significantly the calibration problem. We implemented a low cost laboratory experiment aimed at assessing the precision achievable in the location determinationwith a CCD camera, by evalueting the measurement repeatability throughout a set of images of a simulated stellar field. Our experiment provides an initial location dispersion of the order of 1/100 of the CCD pixel, with clear evidence of dominant common mode effects. After removing such terms with straightforward numerical procedures, we achieved a final location precision of 1/700 pixel on individual images, or 1/1300 pixel on co-added images. The scaling of precision with target magnitude is qin quite good agreement with theoretical expectations. The initial common mode systematics appear to be induced by the thermal control of the CCD camera head,which degrades the strctural stability. In actual implementations, such problems can be greatly reduced by proper design. Finally, our results show that residual effects, which could hamper the final astrmetric accuracy, can be calibrated out with simple procedures

This paper reports the fourth set of results of a series of grouped laser comparisons from national laboratories undertaken by the Bureau International des Poids et Mesures (BIPM) at the request of the Comite Consultatif pour la Definition du Metre (CCDM; now the Consultative Committee for Length, CCL) during the period July 1993 to September 1995. The results of this comparison, involving eleven lasers from eight countries and the BIPM, again meet the goals set by the CCDM in 1992 and adopted by the Comite International des Poids et Mesures (CIPM) the same year. The standard uncertainty of the frequency of the He-Ne laser stabilized on the saturated absorption of 127I2 at [lambda] approximately equals 633 nm is reduced to a level of 12 kHz (2.5 parts in 1011) when the lasers compared meet the recommended values of the parameters. The lasers were first compared with the BIPM4 laser with the parameters set to the values normally used in each laboratory; using the BIPM4 laser as a reference the results ranged from -15.4 kHz to 36.8 kHz. After checking and readjusting the values of all the parameters, the range was reduced to -8.6 kHz to 14.0 kHz. Under the latter conditions, the average frequency difference of the group of lasers, with respect to the BIPM4 laser, was 2.7 kHz with a standard uncertainty (1 [sigma]) of 8.1 kHz. The best relative frequency stabilities, with Allan standard deviations of about 5.5 parts in 1012 and 4.6 parts in 1013, were observed with sampling times of 1 s and 100 s, respectively. [Author abstract; 16 Refs; In English]