How severely do localized deformations affect RMS Spot Radius, and is that a useful metric?

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I'm modelling a telescope system with a deformable mirror, and I am providing the deformed mirror surface as a grid sag data file. For the case with an extremely localized deformity (0.0014 to 0.0018% of the total mirror surface area deformed) with a PV difference of about one wavelength, the RMS Spot Radius changes from about 27 micron, to a few hundred micron (200-400 micron). My questions are as follows:

1) Is it expected that such a small deformity should have such a large effect on RMS Spot Radius? How is RMS Spot Radius calculated, and why does increasing the number of rays decrease the RMS Spot Radius? Why does the spot diagram appear mostly symmetric, when the deformity is localized?

2) Are there alternative Analysis parameters to determine the deterioration in image quality due to a single highly localized deformity, that can be used as a more suitable alternative to RMS spot radius? Would RMS wavefront error be a better parameter? Is there anything else?

Thank you!

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