Quote:
Originally Posted by Mickoid
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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Again, I know what the f ratio is. I've been an amateur photographer for years - and am married to a professional photographer - so I really do understand the concept.
See the photos in my original post. There's very little difference in brightness between the two OTAs despite the reflector being ostensibly 2/3 of a stop faster.
There was an article posted here somewhere explaining how reflective coatings can be quite lossy. Add to that the 8% or so blocked by the secondary and you do indeed lose most of a stop.
Bottom line: Reflectors are advertised as being significantly faster, than refractors, but they (at least the SW/GSO/Saxons etc) aren't actually as fast as their f-ratio would suggest.