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Total wavefront should be additive. So if the primary is 1/14th and you add a 1/14th 2ndary, the system will be 1/7th.

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That's true for uncorrelated RMS errors. It's certainly not true of peak to valley errors, and certainly not if one is expressed on the wavefront and the other on the surface (by the way, path length difference is 1.4x smaller when you go from surface to wavefront at 45° than at 90°, another source of "apples vs. oranges").

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400mm f/4.46 self made Dobsonian on Tom Osypowski equatorial platform
Orion Starblast (114mm f/4 reflector, Alt/Az)

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