The design and layout of HST are shown schematically in
Figure 2.1. The telescope is powered by two solar arrays that are oriented towards the sun. Nickel-hydrogen batteries power the telescope during orbital night. Two high-gain antennae provide communications with the ground via the Tracking and Data Relay Satellite System (TDRSS). Power, control, and communication functions are carried out by the Support Systems Module (SSM) that encircles the primary mirror.
Scientific instruments (SIs) are mounted in bays behind the primary mirror. WFC3 occupies one of the four radial bays; it has a 45° pick-off mirror that allows it to receive the on-axis beam as shown in Figure 2.2. There are three Fine Guidance Sensors (FGSs), each in the other radial bays, that receive light that is 10 to 14 arcseconds off-axis in the telescope’s field of view. At least two FGSs are required to guide the telescope, therefore, it’s possible to conduct astrometric observations with the third FGS. The remaining SIs are mounted in the axial bays and receive images several arcminutes off-axis.
The coordinate system used for HST’s focal plane is fixed to the telescope; it consists of three orthogonal axes: U1, U2 and U3. As shown in
Figure 2.1, U1 lies along the optical axis, U2 is parallel to the solar array rotation axis, and U3 is perpendicular to the solar array axis. (Note: Some
HST documentation uses the alternative V1, V2, V3 coordinate system for which V1=U1, V2= –U2 and V3= –U3.)
Table 2.1 lists the rough effective locations of the SI apertures in the (U2,U3) coordinate system. Precise values, available in the
HST Instrument Handbooks (see
Section 1.3), depend on subtleties in aperture definitions and operational modes.
