Quote:
Originally Posted by AlexN
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...
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Star brightness is only determined by aperture, not focal length, because it is a point source of light. You are confusing star brightness with the brightness of
diffuse objects whose surface brightness changes with focal length.
The mak would only show stars slightly dimmer than an equivalent aperture refractor because of the central obstruction, however I agree that the smaller field of view makes finding guide stars harder, and you risk mirror flop messing with your guiding.