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Anonymous
Unregistered
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I haven't started astrophotography yet, but I'm curious about something. Why do low focal length telescopes require less time to take a picture than longer ones?
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Suk Lee
Post Laureate
Reged: 10/07/03
Posts: 4315
Loc: Pleasanton, CA
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Dan:
They don't.
Focal length isn't the key to photographic speed, focal RATIO is. The lower the f-ratio (focal length divided by aperture), the shorter the exposure because: for a given magnification (focal length), the larger the lens (more aperture, smaller f-ratio), the more light you're gather at that magnification.
So, take a 100mm focal length lens of 50mm aperture. That's f2. Now take a 200mm focal length lens of 100mm aperture, that's also f2. Imagine an image taken with the 100mm focal length lens. Now imagine taking a shot of the same thing with the 200mm focal length lens. The image on the 200mm focal length lens is twice as big, so the light is spread out, so it's dimmer by half. BUT, the lens is twice as big, 100m aperture vs 50mm aperture, so it's gathering twice as much light, so the dimming caused by doubling the image size is cancelled out by gathering twice as much light, so the image is the same brightness as the 100mm f2 lens.
The confusing thing is that this only applies to distributed objects, e.g. planets, nebulae. For stars the rule doesn't apply because stars are point sources. Regardless of f-ratio, the important thing for capturing stars is aperture - more aperture = ability to capture fainter stars. A 10" WIDE f15 telescope (150 inch focal length) will capture more stars than a 3" WIDE f3 (9" focal length) telescope.
Suk
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http://www.siliconvalleyskies.com
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Anonymous
Unregistered
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Thanks for the great explanation Suk Lee. Does that mean that a 100mm f/3 and a 100mm f/12 would work the same as long as the magnifications were equal? If so, I'm glad there's nothing confusing going on. That also suggests that most astrophotography is done at low magnification, what magnification levels are most popular?
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Suk Lee
Post Laureate
Reged: 10/07/03
Posts: 4315
Loc: Pleasanton, CA
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Quote:
100mm f/3 and a 100mm f/12 would work the same as long as the magnifications were equal?
Definitely.
The magnification ratio you're looking for is arc-sec/pixel. For deepsky the lowest you want to go is around 1 to 2 arc-sec/pixel because below that the turbulence of the sky is going to smear any addiitonal detail over the course of a several minute exposure. Too much more than that, say around 5 or more arc-sec/pixel, and your stars are going to star looking blocky (squarish).
For planetary, you want to be around 0.2 - 0.5 arc-sec/pixel because you'll be taking short exposures and sampling at twice the resolution of your scope.
Calculating arc-sec/pixel for your scope/barlow/camera combo is straightforward arithmetic but the following handy freeware calculator is great:
http://users.rcn.com/assne/ccd.zip
Cheers,
Suk
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http://www.siliconvalleyskies.com
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Anonymous
Unregistered
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Thanks for the link! I think I have everything I need to get started, so sometime soon I'm going to try this out.
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