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The reason for the corrector plate is to alter the light path towards the edge of the mirror (spherical) in a progressive manner to correct for spherical aberation. To do this truly accurately the mirror the corrector and the secondary should be at a specific position relative to each other. However the SCT's get their primary focus by moving the main mirror and the differences are minute. It won't make any practical difference which way the corrector surface faces as long as it does the job.
That's my understanding of it.
Barry
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