The interferometric response of the Koesters prism arises from the difference in optical path lengths of photons entering one side of the prism to those entering the other side (and therefore to the tilt of the wavefront). A photon transmitted by the dielectric surface within the prism is re-combined with one which has been reflected by the surface. Both of these photons were incident on the prism’s entrance face at points equidistant from, but on opposite sides of, the dielectric surface. The degree to which they constructively or destructively interfere depends solely on their difference in phase, which by design, should depend only upon the wavefront tilt. Any optical aberration in the incident beam that does not alter the phase difference of the recombining beams will not affect the interferometric performance of the FGS. Such aberrations are considered to be symmetric.

No correction for the HST’s spherical aberration is incorporated in the original or refurbished FGSs. Though the Koesters prisms are not sensitive to symmetric aberrations (e.g., spherical aberration), small misalignments in the internal FGS optical train shift the location of the beam’s axis of tilt (“b” in Figure 2.2 and in Figure 2.3) effectively breaking the symmetry of the spherical aberration. This introduces an error in the phase difference of the re-combining photons and degrades the interferometric response.

With HST’s 0.23 microns of spherical aberration, a decentering of the wavefront by only 0.25 mm will decrease the modulation of the S-Curve to 75% of its perfectly aligned value. If the telescope were not spherically aberrated (i.e., if the wavefront were planar) misalignments up to five times this size would hardly be noticeable. The impact of HST spherical aberration and the improved performance of FGS1r are discussed in the next sections.