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Peter, Yes I know the difference, I was referring to the quoting of what I imagined were spot sizes from geometric ray tracing ( just talking about on axis to simplify things) . Assuming you had pixels to oversample , the true spot size ( allowing for diffraction effects) achieved in an image would be somewhat larger than 5 micron due to the extra energy thrown out of the airy disc by the 50% central obstruction.
See two simulations I have done with Aberrator. When you add in the blurring effects of seeing the true blur spot diameter of the real instrument is going to be somewhat larger than the one calculated by geometric ray tracing. If the first dark minima is at 4.8 microns, the blur spot will be closer to 10 micron in a time exposure.
Unless you are using very small pixels the obstruction probably makes no difference to the quality of the images.
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