Multi-field alignment of the James Webb Space Telescope

When the secondary mirror (SM) of a Three-Mirror Anastigmat (TMA) telescope is misaligned with respect to the primary mirror (PM), optical wavefront errors are created. In general, the errors take the form of a dominant coma term, common to all field points, in addition to astigmatism and power terms which vary with field position. The magnitude of the field dependent wavefront is usually only a few percent of that of the common coma term, depending on the size of the field of view being considered. The architecture of the James Webb Space Telescope (JWST), however, presents a unique optical problem in that the common term created by misplacement of the SM is compensated by adjustment of the PM segments. As such, the residual field dependent terms become dominant and can be sensed at multiple field points using phase retrieval techniques. In this paper, we present a linear set of equations that describe the multi-field (MF) wavefront errors resulting from a misaligned SM. It is shown that inverting these equations yields corrections for the SM alignment that can independently control the field-dependent astigmatism and the focal-plane tilt. Computer simulations illustrating the correction are presented.