The figure above shows the coordinate system projected onto the sky for the POSition TARGet Special Requirement. The short lines indicate the blooming directions of the CCDs. Units are seconds of arc. See POSition TARGet <X-value>,<Y-value> for details.
Note that the POS TARG X- and Y-axes are not the same as the U2- and U3-axes, and that POS TARG axes are associated with each aperture selected (i.e., there is a POS TARG coordinate frame centered on each aperture).
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