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Does it have to be a refractor? The Skywatcher MN190 (1000mm fl, f5.3) is within your budget and mount capacity, and a darn good astrograph with true colour rendition, no detectable coma, and no diffraction spikes. If you don't mind diffraction spikes there are many newt (& coma corrector) combos that will eat up less than half your budget.
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