Astro-Physics 6" f:8 apochromat (one of the early Christen scopes) with a Brandon Vernonscope 80mm f:6.25 apo as a finder/guide scope.

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The ether of general relativity therefore differs from that of classical mechanics or the special theory of relativity respectively, in so far as it is not 'absolute', but is determined in its locally variable properties by ponderable matter.

Albert Einstein, "On the Ether", 1924