Each FGS aperture represents a quarter annulus at the outer perimeter of the HST focal plane with inner and outer radii of 10 and 14 arcmin respectively. The total area (on the sky) subtended by the FOV is ~ 69 square arcminutes. The entire FOV is accessible to the interferometer, but only a 5 x 5 arcsec aperture, called the Instantaneous Field of View (IFOV), samples the sky at any one time. A dual component Star Selector Servo system in each FGS - the components of which are referred to as Star Selector A (SSA) and Star Selector B (SSB) - moves the IFOV to a desired position in the FOV. The action of the Star Selectors is described in detail in Chapter 2 of the FGS INstrument Handbook, along with a more detailed technical description of the instrument. The figure below shows a schematic representation of the FGSs relative to the HST focal plane.
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 2.7. FGS2R2 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 "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 +YDET and +YPOS Axis are given in Table XX. The angles are measured from +U3 to +Y in the direction of +U2 (or counterclockwise in Figure 2.7). Note that the FGS internal detector coordinate reference frame and the POS TARG reference frame have opposite parities along their respective X-axes.
|Angle (from +U3 axis)|
While FGS 3 was initially the FGS instrument calibrated for science activities, currently only FGS 1R is calibrated and is being supported for FGS Science.