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Parfocal is really marketing BS. To be parfocal filters have to be virtually the same thickness. In reality there can be subtle differences in thickness.
Mind you my latest Astrodons are fairly parfocal but others I have had were not but often very close.
Who knows, maybe the seeing was worse or some wind came up when you did your red subs or the scope was lower in the sky where stars will get fatter anyway no matter which filter is being used.
One solution is to do some deconvolution on the red master to get the same FWHM size on the stars before colour combine to match them and prevent this.
Greg.
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