Concerning the alignment of modern telescopes: theory, practice, and tolerances illustrated by the ESO NTT

Abstract

A recent paper by Bhatia (1995, TNG Tech. Rep. No. 43) suggests from calculations based on the optical geometry of the NTT that the zero-coma condition is a necessary but insufficient condition for the alignment of modem telescopes and that analysis of separate decenter effects is necessary to cope with decentering astigmatism aberrations near the field center. Here we show that, while Bhatia's calculations are quite correct, their basis shows a misunderstanding of how the NTT was actually aligned and that this "passive" alignment was sufficiently good that the decentering astigmatism effects he indicates are negligible. More dangerous are the wide-field effects of decentering astigmatism discussed by McLeod (1996, PASP, 108, 217). However, we show that these effects, linear with field, are also negligible with the NTT alignment quality achieved, provided the correct effective shift of the astigmatic field center is allowed for in the active correction software. This is of great importance in the NTT because the image analyzer operates at the edge of the field and the correct field astigmatism must be subtracted to give the necessary active correction at the mechanical field center. This approach is valid for the technical use of the edge field of an aplanatic (RC) telescope limited by field astigmatism. Decentering astigmatism will be much more critical at the field edge for anastigmatic telescopes if a wide field is also used for astronomical observations.

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

“…Hopkins 1.2 m telescope by McLeod [28]. The theory and practice of the alignment for ESO telescopes with normal field of views like the NTT and the VLT is described in [29][30][31][32]. 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].…”

Generic Misalignment Aberration Patterns in Wide-Field Telescopes

“…McLeod's (1996) paper describing the use of coma and astigmatism patterns to align the Whipple Observatory 1.2-m telescope anchors the recent literature. Wilson & Delabre (1997) discuss the alignment of the ESO NTT. Gitton & Noethe (1998) describe the alignment of the ESO VLTs, and Noethe & Guisard (2000) give a more general description of the astigmatism patterns expected from two-mirror telescopes.…”

The VST alignment: strategy and results

“…Hopkins telescope. The alignment of ESO telescopes like NTT and VLT are described in [3]- [6]. The effect of misalignments is magnified in the wide-field survey telescopes, e.g.…”