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focal length and focal ratio are different beasts. The Mak will be F/10~F/12 where as the refractor is F/5. two optical systems of equal aperture, one F/10, one F/5, the F/5 will show a wider field of view, and be brighter... The extra aperture of the mak would be negated by the fact that it has over double the F/ Ratio than the refractor, not to mention the Mak will have a central obstruction due to the secondary mirror...
I can see no possible reason someone would chose a 90mm Mak Cass over a fast 80mm achro refractor for guiding. None whatsoever...
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