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Yes that is a workable truth about the aperture and F ratio and exposure time. But like any theory I am sure there are deviations due to other factors and not all chips have exactly the same characteristics but yes I agree it is a workable rule that two 200mm scopes will expose the same brightness for the same time even if different f ratios. So if you want faster exposure times you up aperture. Although I know from an actual test that a Tak ED180 at F2.8 exposed to same brightness an image 2.5X quicker than an F5 FSQ106. Now that is 74mm more aperture but there is also a central obstruction. Not sure the total area of the aperture is over 2.5X. Probably more like 4% more area.
Collimation is not that big a point either for both scopes. F5 Newt can be tricky with a star test but if you use a cheap laser collimator it takes about 4 minutes (I used to have a Vixen R200SS F4 200mm). The RC optics though I had (RCOS 12.5inch) did require several steps including aligning the mirrors rotation-wise and using a Tak collimating scope as it was pretty impossible without but not hard with it ($400 collimating scope). The GSO may be simplified. I would not scare off based on collimation as it is a skill that should be learned anyway.
I am not criticising the GSO which is great bang for your buck. The question was whether to change the F5 200mm Newt for one.
What my main point was imaging scale. F5 200mm will give a wider field of view and that makes everything easier and more pleasing an image to more people. But then a lot prefer to image at a closer up scale so its what type of image you want to produce.
Greg.
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