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Originally Posted by MrObvious
Hi Andy,
It seems we are pretty much agreeing but talking slightly apart.
What I'm saying is for the same size image of a particular object (on a CCD/film/eye) it makes no difference to the brightness whatever the native F ratio of the scope is.
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I think the problem is just talking about f-ratio confuses the issue because it reflects 2 things: The apeture and the focal length (f = focal length / apeture). 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 apeture constant. The
Takahashi Epsilon scope is f2.8, very fast. You get more data in less exposure time than other 500mm focal length scopes.
Quote:
Originally Posted by MrObvious
If you were to require a barlow lens on a shorter F ratio scope because the object required magnification, then the longer F ratio scope will take less time to image (all else being equal) because now there is one more lens in the light path. Assuming the object size on the CCD in both cases is the same.
If the object to be imaged requires a focal reducer on a longer F ratio scope then the shorter F ratio scope will provide the shorter exposure time (all else being equal) because the longer F ratio scope now has an additional lens in the light path.
IOW, for the same size image to be seen on a CCD, it will require pretty much the same time for both scopes to form the same brightness image. Since the addition of a single lens won't impact the time by a huge percentage.
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Addition of a lens will have little or no impact on the exposure. If you are creating the same size image then the focal length is the same. If the focal length is the same, the scope with the larger apeture (and therefore lower f-ratio) will have a shorter exposure time.
Quote:
The shorter F ratio shouldn't be called brighter, it should be called wider.
Only the diameter of the main lens/mirror determines the exposure time given that the formed image will be the same size.
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The apeture and the focal length determine exposure time and brightness. F-ratio encompasses both, so it confuses the issue to talk about it in isolation from the other specifications of a telescope.
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E.g. I noticed on your site where you described your experience at imaging at F10. The image wasn't as good as you'd wanted it, but, it should also be noted that it would also be larger than if you used F6, almost twice the size in fact.
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Larger, but also harder to image. And the seeing problems are larger as well. That's why I will stick with f6.
Clear skies,
--Andy