What mechanical feature in Newtonian and SCT designs blocks some light and reduces the actual light gathered compared to a refractor of the same aperture?

Answer

Central obstruction

The central obstruction, caused by the secondary mirror and its support structure in reflecting types, blocks some of the light path, meaning a refractor gathers more actual light for the same aperture size.

What mechanical feature in Newtonian and SCT designs blocks some light and reduces the actual light gathered compared to a refractor of the same aperture?

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