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Based on my own calculations, the loss of light gathering reduction due to the secondary mirror and spider veins is quite insignificant.
My example can be explained by these facts: My 8 inch f 5 Newtonian has a primary mirror diameter of 200mm and a focal length of 1000mm which equates to a f ratio of 5, as stated by the manufacturer. Now rounding it off to the nearest whole number after compensating the area taken up by the secondary mirror and spiders, is just that. Not really a misleading figure but rather a simpler number to remember.
By calculating the surface areas of each component in the optical path we can arrive with the true f ratio and in my case, this is f 5.33, hardly the one stop loss someone stated it could be.
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