Thanks for responding Andy.
you said;
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I think the problem is just talking about f-ratio confuses the issue because it reflects 2 things: The aperture and the focal length (f = focal length / aperture). If you keep your focal length constant, a lower f-ratio scope does produce a brighter image. That is why the image gets darker when you use a barlow lens -- you have increased the focal length while holding the aperture constant. The Takahashi Epsilon scope is f2.8, very fast. You get more data in less exposure time than other 500mm focal length scopes.
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Thats correct. The key thing is that you keep the focal length constant. IOW, it works like a camera. A smaller F ratio will automatically imply a bigger aperture.
It's when people compare two scopes of the same diameter/aperture and then claim the one with the lower F ratio is a brighter scope, that is the problem I'm referring to.
This is what comes across confusing to beginners.
Claims that smaller F ratios make brighter images is wrong. Its larger main lens's that make brighter images.
If we buy a scope that has the same focal length as another scope then the F ratio will also determine the main lens size. Smaller ratios mean bigger lens.
If however most people are likely to buy a scope buy its lens diameter and compare two scopes of equal diameter main lens/mirror. Then claims of one being brighter than another because its F ratio is lower are just plain wrong.
To see what I mean your link states:
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This fast field photo visual astrograph is perfect for the new mega pixel digital 35mm cameras and CCD cameras. The great speed will enable the imager to record faint nebulosity’s invisible in slower instruments.
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This is misleading. It's claiming its due to the F ratio, but its really a due to the lens diameter.
One can envision a smaller object requiring a barlow to get a reasonable size in this scope. If this is done so that the recorded image is the same size on both instruments the scopes will have the same exposure times. Likewise a F/5.6 instrument with a 2:1 reducer will give exactly the same image size, FOV and exposure times as this scope.
It is the lens diameter that determines the result. Not the F ratio.
I think the problem here is that they provide half the information. The rest is left to the reader.
In your case you know how it will all work out. A beginner will just assume that because the scope has a lower F ratio it is brighter simply by virtue that its the F ratio. I've had salesmen tell me that this 200mm scope is brighter than this 200mm scope because it has a faster mirror. This propagates the misconception.
If he said this scope provides wider views for the same size eyepiece, that would be correct and not misleading.
Summing up:
Two scopes of equal main lens size give exactly the same image brightness for the same magnification. (this is obvious)
Two scopes of the same focal length but different F ratios will have a different sized main lens. (this too is obvious)
Conclusion:
It's the size of the lens that is the key parameter, not the F ratio, when determining brightness.