The solar distortion telescope has been used to measure the limb shape and latitude dependence of the limb brightness during the summer months for a 5-year period-a time base sufficient to detect solar cycle trends in the data. Comparison of these observations with spaceborne measurements of the solar constant suggests that a significant part of the solar cycle variation in irradiance is a result of temporal changes in the latitude-dependent surface temperature of the sun. P RECISE SPACEBORNE MEASUREments of the solar constant, that is, the solar irradiance as determined from the earth's orbit, have shown that it changes slightly with the changing level of magnetic activity on the sun (1). These measurements, specifically those made by the active cavity radiometer irradiance monitor (ACRIM) experiment aboard the Solar Maximum Mission (SMM) spacecraft, have shown that during a typical solar maximum the solar irradiance is 0.1% higher than at solar minimum when the sun's magnetic activity is low. Such changes may measurably affect the earth's climate-a decrease in solar activity in the 16th century was coincident with a period of cool temperatures in Europe known as the "little ice age" (2).The irradiance variation is a result of temporal changes in the flux and the surface brightness distribution of the sun. Earlier work has shown that dark sunspots on the visible disk do diminish the solar constant; other indirect (proxy) indicators also show that the solar irradiance increases because of bright faculae (3, 4). These studies also suggest that solar cycle variations in the spots and faculae alone cannot account for the total variability. Identifying the cause of this variation is a difficult problem-for example, temporal changes in differential rotation in the interior of the sun, a solar dynamo magnetic field near the base of the convection zone, or large-scale convective cells might all produce such an effect. However, observations of the surface temperature distribution of the sun can be used to distinguish the effects of each of these mechanisms. For example, a latitudinal temperature distribution characterized by large-scale quadrupolar variations could be related to solar differential rotation. Furthermore, brightness observations combined with helioseismological splitting data may allow the determination of the aspheric structure of the solar interior (5, 6). Sensitive observations of temporal and spatial brightness variations will thus lead to a better understanding ofthe solar convection zone. In this report, we combine precise relative photometric measurements of the solar disk with spaceborne absolute total irradiance measurements to investigate the latitude-dependent and solar cycle-dependent surface temperature of the sun.Total irradiance measurements, such as those made with the ACRIM experiment, do not provide data on the brightness variations across the visible solar disk. For this information the space experiments are well complemented by ground-based observations. Ground-based relative photometry...

No abstract

No abstract

The eclipse image of Figure 3 was provided to the authors by Jackob Strikis of the Elizabeth Observatory, Athens, who claimed authorship. However, shortly after publication the authors discovered that this eclipse image was in fact a preliminary version of an image belonging to Prof. Miloslav Druckmüller, taken during the 2006 total solar eclipse from Libya at 30 56.946 0 N, 24 14.301 0 E, and at an altitude of 158 m. This image can be found at http://www.zam.fme.vutbr.cz /~druck / Eclipse/index.htm. We extend our gratitude to Prof. Druckmüller, from Brno University of Technology, Czech Republic, who brought this incident to our attention, and who has graciously accepted our apology for this unintentional mishap. A forthcoming article in collaboration with Prof. Druckmüller is in preparation.

high-bandwidth communication, civil space surveillance technologies, wireless optical communication systems (UV and free-space systems), hyper-aperture multimirror structures, geoengineering (space mirror), and astronomical systems. [1,2] In particular the light-gathering power of an optical telescope, its "light grasp" or aperture gain, is one of the most important features of a telescope, [3] which requires very precise glass mirror technology.Recently, we have proposed a "World's Largest Telescope" for achieving highcontrast observations that could use this technology. [4][5][6] Such an optical system will be limited by the cost and manufacturability of large mirrors. The work described here, optics fabricated from an optimized electroactive polymer (EAP), could enable optics like these. The new approach will extend conventional active mirror technologies to larger smooth optical surfaces, without abrasive polishing. This means it will be possible to create precisely shaped low scattered light mirrors-suitable to astronomical applications-faster and at lower production costs. Our long-term vision for the new technology is to decrease the mass density (and cost) of mirrors by an order of magnitude.The idea of using force actuator-sensors fabricated from EAPs [3,4] is developed in this work in order to achieve active mirror surface shape control. By manipulating EAPs as active supports, integrated into the mirror structure allows correcting the mirror shape with a continuous actuator force distribution. Figure 1 illustrates how EAPs behave as elastic electromechanical deforming springs. This "electrical polishing" can correct surface shape errors that would be conventionally removed by abrasive grinding. To achieve high optical mirror quality surfaces with a thickness of a few millimeters the EAP glass deformation must have a dynamic range of few microns corresponding to actuator forces of about 1 N. The technique we propose could potentially be achieved using only additive manufacturing via 3D-printing technology.In this article we aim to present the EAP concept for Live-Mirror active optics. We evaluate different polymers in specific actuator designs in order to test their mechanical actuation properties for mirror-actuator prototypes. This article focuses on EAP-based actuator basic properties. Future work will explore how EAP sensors can be integrated into mirror systems.