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Table 4

Mean plate scale and true north orientation for the SPHERE commissioning runs measured on the 47 Tuc field-stabilized observations in the IRDIS images in the H23 filter pair obtained with the APLC coronagraph (Sect. 2).

H2 H3
Com. date Plate scale (mas/pix) True north (°) Plate scale (mas/pix) True north (°)

July 2014 12.252 ± 0.006 1.636 ± 0.013 12.247 ± 0.006 1.653 ± 0.005
August 2014 12.263 ± 0.006 1.636 ± 0.013 12.258 ± 0.006 1.653 ± 0.005
October 2014 12.259 ± 0.006 1.788 ± 0.008 12.254 ± 0.005 1.795 ± 0.008

Notes. To align the IRDIS science images so that north is oriented up and east left, an offset accounting for the zeropoint of the derotator in pupil-stabilized mode of 135.87 ± 0.03° has to be added to the parallactic angles indicated in the fits header. This offset is measured on data of 47 Tuc acquired in July 2014 in both stabilization modes and assumed to be constant between the runs. For the astrometric calibration of the IFS science images, an additional offset accounting for the relative orientation between the IFS and IRDIS fields of 100.46 ± 0.13° also has to be added. This value is measured on simultaneous data of distortion grids of both instruments.

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