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TNO developed a Wave Front Sensor (WFS) instrument for the GAIA mission. This Wave Front Sensor will be used to monitor the wave front errors of the two main telescopes mounted on the GAIA satellite, which may be corrected by a 5-degree of freedom (DOF) mechanism during operation. The GAIA-WFS will operate over a broad wavelength (450 to 900 nm) and under cryogenic conditions (130 to 200 K operation temperature). The WFS uses an all reflective, a-thermal design and is of the type of Shack-Hartmann. The boundary condition for the design is that the focal plane of the WFS is the same plane as the focal plane of the GAIA telescopes. The spot pattern generated after a micro lens array ( MLA) by a star is compared to the pattern of one of the three calibration sources that is included in the WFS, allowing in flight calibration. We show the robust and lightweight opto mechanical design that is optimised for launch and cryogenic operation. Furthermore we give details on its alignment and commissioning. The WFS can measure wave front distortions in the order of lambda/1000, and determines the focal plane with an accuracy of 50 µm

Basic properties of hydrogenated amorphous silicon-germanium ͑a-SiGe:H͒ alloys deposited by plasma-enhanced chemical-vapor deposition were studied. We found that there is an optimal growth temperature in the range 250-280°C. Infrared-absorption spectra measurements show that in this temperature range the alloys have optimal composition of hydrogen content and bonding, providing a minimum in dangling bond defect density N s and a high photoconductivity ph . Growth at lower temperatures results in hydrogen-rich alloys with high N s . Hydrogen in these alloys is mainly bonded as SiH 2 and as clusters on internal voids. Growth at temperatures above the optimal value gives less deterioration of the properties of a-SiGe:H than the growth below this optimal temperature. We found two different dependencies of ph on N s : in optimized alloys ph changes proportionally to N s Ϫ1 and in low-temperature alloys ph decreases more steeply with N s Ϫ1 due to a decrease in electron mobility. It is shown that annealing of a-SiGe:H causes a sharp increase in N s starting at annealing temperatures about 20°C below the temperature at which the alloy was grown. We observed a new phenomenon that during annealing N s increases much more ͑by two orders of magnitude͒ than the photoconductivity decreases ͑only factor of 5͒. The entropy-based model was applied to explain this experimental phenomenon. We found that annealing shifted the centrum energy of the D ϩ/0 dangling bond levels to the valence-band edge. The rate of electron recombination lowers due to this shift and the change in the effective electron capture crosssection of the defects, and therefore the decrease in ph is smaller than the increase in N s .

The Gaia payload ensures maximum passive stability using a single material, SiC, for most of its elements. Dedicated metrology instruments are, however, required to carry out two functions: monitoring the basic angle and refocusing the telescope. Two interferometers fed by the same laser are used to measure the basic angle changes at the level of µas (prad, micropixel), which is the highest level ever achieved in space. Two ShackHartmann wavefront sensors, combined with an ad-hoc analysis of the scientific data are used to define and reach the overall best-focus. In this contribution, the systems, data analysis, procedures and performance achieved during commissioning are presented Keywords: Astrometry, Gaia, metrology, interferometry, basic angle monitor, wavefront sensor, Shack-Hartmann, wavefront reconstruction, centroid, Cramér-Rao, spectral resolution • produce stellar images in a combined focal plane. Image credit.

A calibration procedure for determining the model input parameters of standard a-Si:H layers, which comprise a single junction a-Si:H solar cell, is presented. The calibration procedure consists of: i) deposition of the separate layers, ii) measurement of the material properties, iii) fitting the model parameters to match the measured properties, iv) simulation of test devices and comparison with experimental results. The inverse modeling procedure was used to extract values of the most influential model parameters by fitting the simulated material properties to the measured ones. In case of doped layers the extracted values of the characteristic energies of exponentially decaying tail states are much higher than the values reported in literature. Using the extracted values of model parameters a good agreement between the measured and calculated characteristics of a reference solar cell was reached. The presented procedure could not solve directly an important issue concerning a value of the mobility gap in a-Si:H alloys.