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Astron. Astrophys. 334, 969-975 (1998)

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2. Observations

Speckle polarimetric data at J, H, K were obtained in 1995 March 16 (epoch 1995.20) at the German-Spain astronomical centre at Calar Alto. The images were taken using the Blue MAGIC (M PI for A stronomie G eneral-Purpose I nfrared C amera, Herbst 1993) detector at the 3.5-m telescope with a pixel scale of 0:00072. During these observations, the seeing was about 1:005.

For the derivation of the linear polarization (degree and position angle), the source was observed at four polarizer settings corresponding to position angles of 45, 90, 135, and 180 degrees. A total integration time of 250 s was achieved per polarizer setting (8 data cubes each with 128 specklegrams with exposure times of 0.25 s). A nearby reference point source was observed in the same manner. For the estimation of the instrumental polarization standard stars from the UKIRT list were measured. The comparison of the derived polarization parameters of these stars with the tabulated values indicated that instrumental polarization is negligible.

From observations of Orion Trapezium Cluster stars, we deduced the exact pixel scale of [FORMULA] per pixel and a chip orientation of [FORMULA] (position angle of the 'north-south axis' of the chip).

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© European Southern Observatory (ESO) 1998

Online publication: June 2, 1998