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The ONAG guide star astigmatism is quite small at best focus. It is not a concern for auto-guiding and centroid calculation. As a mater of fact using OAG may lead to much more off axis aberrations, such as coma, yet without a significant issues on centroid, unless the aberration becomes extreme.
Fo auto-guiding the right figure of merit is the half flux diameter (HFD), not much the FWHM. It expresees how the starlight energy is spread around the central peak.
With an ONAG the guide star HFD diameters seen by the imager and guider cameras are very close, no worries.
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