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Old 05-12-2008, 06:58 AM
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F/Stop vs Aperture

Some day I hope to be answering these questions instead of always asking them.

I have a 6" Newtonian (F/5) and a 10" Reflector (F/10). I know that F/5 is better than F/10 for photography, but wouldn't the 10" aperture offset the higher F/Stop and kind of even the 2 scopes out as far as photography is concerned? (I hope that makes sense.)
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Old 05-12-2008, 07:57 AM
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The F5 6" has a focal length of 30 inches whilst the F10 10" has a focal length of 100 inches. So the image scale of the 10 is about 3 times that of the 6 and so photographically the same target will appear about 3 times larger, as a result, the 6" would have a wider field of view of the same area (about 3 times in each direction, or about 9 times the area).

The 10" gathers around 3 times the amount of light (10 squared:6 squared) per unit area, but this is spread 9 times more than the 6" and so the larger image will appear duller (about 3 times) than the same object seen in the 6".

Visually, the same eyepiece in the 10" will give 3 times more magnification than in the 6".

Does this make sense? The figures are of course "ballpark".
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Old 05-12-2008, 11:09 AM
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Thanks, Trevor. It does make sense, I think.

So, as far as photography goes, conditions and darkness could dictate which of these 2 scopes might do a better job on any given night. Under a light polluted sky, the 10" might actually produce a poorer photograph (albeit larger subject) because you might be expected to need longer exposures due to the higher F/stop. Whereas, shooting at a very dark site where longer exposures are less likely to be mucked up with light pollution, might as well use the 10", even with the higher F/stop, and take advantage of the higher magnification. Right?
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Old 06-12-2008, 11:03 AM
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I guess it also depends on the subject. The target may fit in the field with one scope and not in the other or be too small.
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Old 08-12-2008, 09:10 AM
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I hadn't thought of that. Thank you Trevor!
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Old 08-12-2008, 10:10 AM
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The other thing you need to remeber too Kirk is that if you double your f/ratio (f/5 > f/10) Then you increase the time required to achieve the same level of brightness by a factor of 4. Now this may not be a problem with widefield, but for narrow field work increasing an exposure from 5 min to 20 min puts a lot more demand on your mount's quality and autoguiding abilities.
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Old 09-12-2008, 07:57 AM
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Originally Posted by [1ponders] View Post
The other thing you need to remeber too Kirk is that if you double your f/ratio (f/5 > f/10) Then you increase the time required to achieve the same level of brightness by a factor of 4. Now this may not be a problem with widefield, but for narrow field work increasing an exposure from 5 min to 20 min puts a lot more demand on your mount's quality and autoguiding abilities.
Paul, I have a small correction to the above..
That depends on how f/ratio was increased, and what objects are in question.
The increase of f/ratio with barlow will have the effect you are mentioning (longer exposure time required), but on "spread" objects only, like nebulae (which have surface brightness) .. Stars, however, being spot-like light sources, will leave roughly the same number of photons per pixel (unless FL is increased such that star images become bigger than pixels, in which case they will start to behave like nebulae as well).

Increasing of f/ratio while keeping the same FL means actually smaller aperture. Then everything fades exactly as you described

Last edited by bojan; 09-12-2008 at 08:11 AM.
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Old 09-12-2008, 08:13 AM
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Oh I agree bojan. The number of stars recorded is determined by aperture not by FL. In other words it doesn't matter whether a 100mm scope has an FL of 500mm or 1000mm, both will record the same number of stars, though over a different field of view. The responsiveness to faint extended objects however is determined by f/ratio (FL/A). So to successfully image faint small extended objects you need a long focal length with wide aperture, ie M57 will look better though appear as "bright" imaged through 2000mm with 500mm aperture than through 200mm with 50mm aperture (both f/4).
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Old 10-12-2008, 07:44 AM
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Quote:
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Paul, I have a small correction to the above..
That depends on how f/ratio was increased, and what objects are in question.
The increase of f/ratio with barlow will have the effect you are mentioning (longer exposure time required), but on "spread" objects only, like nebulae (which have surface brightness) .. Stars, however, being spot-like light sources, will leave roughly the same number of photons per pixel (unless FL is increased such that star images become bigger than pixels, in which case they will start to behave like nebulae as well).

Increasing of f/ratio while keeping the same FL means actually smaller aperture. Then everything fades exactly as you described
Bogan is right on the money there! Any object that is extended shall react on how fast the lens system is (f/5 is 4 times faster than a f/10 system) but when we look at aperture and the airy disk in the same system we would have the same brightness (same aperture f/5- f/10 scope for instance) the airy disk will be 4 times brighter in say a 12" f/10 than a 6" f/5 and as long as the ccd is matched well to the system as well.....

The 6" f/5 shall need 1/4 the exposure of say the Horsehead neb (not including stars) as a 12" f/10 while if you imaged Omega Cent the 12" f/10 will be 4 times quicker in recording all those stars..
cheers Gary

Last edited by Garyh; 10-12-2008 at 07:56 AM.
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