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According to the TS link, that 0.73 reducer (F5 reduced to F3.65) is also a coma corrector.
This coma corrector and reducer turns your Newtonian telescope into a high-speed astrograph. The focal length is reduced and the field is corrected.
Image circle is 30mm diag, however its fully corrected field is only 22mm (ASI-1600 or ASI-294 cameras).
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