Each FGS comprises two orthogonal white-light, shearing interferometers, their associated optical and mechanical elements, and four S-20 photo-multiplier tubes (PMTs). For clarity, we divide the FGS optical train into two sections: the star selection mechanism and the interferometer.
The Star Selectors
A schematic view of the FGS optical train is shown in the figure below. Light from the HST Optical Telescope Assembly (OTA) is intercepted by a plane pickoff mirror in front of the HST focal plane and directed into the FGS. The beam is collimated and compressed (by a factor of ~60) by an aspheric collimating mirror, and guided to the optical elements of the Star Selector A (SSA) servo assembly. This assembly of two mirrors and a five element refractive corrector group can be commanded to rotate about the telescope's optical axis. The corrector group compensates for designed optical aberrations induced by both the asphere and the HST Optical Telescope Assembly (OTA). The asphere contributes astigmatism, spherical aberration and coma to the incident beam. Aberrations from the OTA's Ritchey-Chretien design include astigmatism and field curvature.