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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.

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