Figure 1.2 shows the HST focal plane positions of the FGSs as projected onto the sky.
Figure 2.10 is similar, but includes the addition of two sets of axes: the FGS detector coordinate axes, and the POSTARG coordinate axes (used in the Phase II proposal instructions to express target offsets).
The (X,Y) location of the IFOV in the pickle is calculated from the Star Selector Encoder Angles using calibrated transformation coefficients. Each FGS has its own detector space coordinate system, the (X,Y)DET axes, as shown in
Figure 2.10. FGS2 and FGS3 are nominally oriented at 90 and 180 degrees with respect to FGS1r, but small angular deviations are present (accounted for in flight software control and data reduction processing). The FGS detector reference frame is used throughout the pipeline processing.
The POS TARG coordinate axes, (X,Y)POS, should be used to express offsets to target positions in the Phase II proposal (Special Requirements column). See Chapter 5: Writing a Phase II Proposal for more details.
The approximate U2,U3 coordinates for the aperture reference position (default placement of a target) for each FGS and the angle from the +U3 axis to the +Y
DET and +Y
POS Axis are given in
Table 2.3, Approximate Reference Positions of each FGS in the HST Focal Plane. The angles are measured from +U3 to +Y in the direction of +U2 (or counterclockwise in
Figure 2.10). Note that the FGS internal detector coordinate reference frame and the POS TARG reference frame have opposite parities along their respective X-axes.
