FGS Apertures
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 arcmintues. 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. Figure XX shows a schematic
representation of the FGSs relative to the HST focal plane.
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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. 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
"Writing a Phase II Proposal" for more details.
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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.
Table 2.2: Approximate Reference Positions of each FGS in the HST Focal Plane
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Aperture
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U2
(arcsec)
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U3
(arcsec)
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Angle (from +U3 axis)
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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.
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