Separation of the effects of astigmatic figure error from misalignments using Nodal Aberration Theory (NAT)

Abstract

We present the nodal aberration field response of Ritchey-Chrétien telescopes to a combination of optical component misalignments and astigmatic figure error on the primary mirror. It is shown that both astigmatic figure error and secondary mirror misalignments lead to binodal astigmatism, but that each type has unique, characteristic locations for the astigmatic nodes. Specifically, the characteristic node locations in the presence of astigmatic figure error (at the pupil) in an otherwise aligned telescope exhibit symmetry with respect to the field center, i.e. the midpoint between the astigmatic nodes remains at the field center. For the case of secondary mirror misalignments, one of the astigmatic nodes remains nearly at the field center (in a coma compensated state) as presented in Optics Express 18, 5282-5288 (2010), while the second astigmatic node moves away from the field center. This distinction leads directly to alignment methods that preserve the dynamic range of the active wavefront compensation component.

The astigmatic aberration field in active primary mirror astronomical telescopes

“…Figure 5 illustrates the response of the astigmatic field to the introduction of astigmatic figure on the primary mirror. The theory that supports the real ray-based results shown in Figure 5 is provided by Schmid et al in [ 6 ]. For a two-mirror telescope in the VST class that is an essentially Cassegrain form (when not considering its field widening lenses), only spherical aberration is corrected and not the field linear coma or field quadratic astigmatism.…”

“…Until recently, the NAT, discovered by Shack [ 4 ] and developed by the author [ 5 ], has been limited to optical imaging systems made of rotationally symmetric components, or offset aperture portions thereof, that are tilted and/or decentered. Recently, the special case of an astigmatic optical surface located at the aperture stop (or pupil) (e.g., telescope primary mirror) was introduced into the NAT by Schmid et al [ 6 ] and analyzed for the case of a primary mirror in a two-mirror astronomical telescope. This work is directly relevant to the active primary mirror implementation that has become a baseline in modern ground-based telescopes.…”

Review and analysis of avionic helmet-mounted displays

“…53 In the decentered/tilted optical system, there is a leveraging binodal astigmatism given by 54 53 In the decentered/tilted optical system, there is a leveraging binodal astigmatism given by 54…”

Removing static aberrations from the active optics system of a wide-field telescope

“…The knowledge of field aberrations for alignment discussed in [33] is also attractive for wide field telescopes, where the astigmatism due to misalignment is large on the edge of the field, and therefore easily measurable and comparable with theory, e.g., in VISTA [34]. The differences between astigmatic figure errors and astigmatism induced by misalignment are also extensively discussed in [35][36][37]. The essential concepts and the strategy that has been followed for the VST are summarized below.…”