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An 80mm APO Triplet (f7 typically) with reducer corrector become f5,53 . Plenty fast enough for imaging, circa 440mm focal length. Most have good imaging R&P focusers. I have one of these.
Then there are the new wave of 60mm ED (FPL-53) scopes around f6 with flattened, from WO, TS, Astrotech, etc, circa 360mm focal length. I have one of these. Of course if you have the money there is the TAK 60.
Generally speaking, the 80mm is a better all-rounder imho. You will get good field coverage, for example the Rosette Nebula fits the field perfectly, as do many other nebulae.
The 55mms are too small into to be of much use.
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