astrometric calibration binary was observed. (c) GQ Lup A in a non-linear regime (not saturated). (d) GQ Lup A and HIP 73357 A in a non-linear regime (not saturated). Calibration (for 19 Feb 2007 GQ Lup data) obtained on 1 Mar 2007 with HIP 73357, see footnote 1.

Astrometric data reduction on GQ Lup
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One can combine all images taken within, e.g., one night by standard shift+add to obtain one image with very high S/N. We did this in N05, and have done the same for the six new observations from 2005 to 2007. In the final image, one can then determine the PSF photocenter of the bright star GQ Lup A and, after PSF subtraction, also the PSF photocenter of the companion. This results in measurements of separation in pixels and the position angle (PA) of the companion relative to star A. The companion is always slightly north of west. The error for the separation include errors from the Gaussian centering on A and companion, as well as the error in the pixel scale determined; the error for the PA includes centering errors and the error in the north-south alignment of the detector during the observation. In Table 