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When it comes to telescopes for visual observation, bigger is better: a
larger aperture will gather more and the observer will see more at the eyepiece.
For CCD imaging, on the other hand, aperture is less important. What
really matters is focal ratio. The focal ratio determines how much light
is picked up by the CCD chip in a given amount of time. If you want
shorter exposures you need a faster (smaller) focal ratio. For example,
when imaging extended objects (i.e., non-stellar, deep-sky objects) a telescope
with a focal ratio of f/6 will gather more light during a given exposure than a
telescope with a focal ratio of f/10, regardless of the aperture of the
telescopes. Therefore, a 200mm f/2.8 camera lens will detect more
nebulosity in a 5-minute exposure of the Orion Nebula than will the 200-inch
Hale telescope on Mt. Palomar, because the 200-inch operates at f/3.3. The
200-inch will pick up more stars, because aperture matters for pinpoint light
sources, but capturing faint stars (rather than deep-sky objects) is rarely the objective for amateur CCD imagers.
So why do many CCD imagers have large-aperture instruments, even if they
rarely observe visually with them? The answer is
image scale. Even
though the 200mm camera lens in the example above has a faster focal ratio than
the 200-inch Hale telescope, the focal length
of the Hale is nearly 17,000mm. This means the image scale is 85 times
greater than the camera lens, and an object will appear 85 times larger on an image from the Hale.
Image scale is a function of focal length, and speed is a function of focal
ratio. To get a large image scale and a fast focal ratio requires a large
aperture. An 8" f/10 telescope has the same focal length as a 16" f/5
telescope; the image scale will be equivalent, but the larger scope is 4 times
faster.
Where it really gets tricky is when you throw in field of view. With a
given CCD camera, field of view is dependent only on focal length. This
means a telescope which gives a greater image scale also gives a smaller field
of view. Often this is not a problem since a large image scale is
typically used to image smaller objects such as galaxies. However, the
field of view can be made greater by using a larger CCD chip, so it is possible
to have both a wide field and a large image scale.
One more variable, then some example which will make sense of all this mess.
The number of pixels in the CCD chip will determine how large the final image
appears on a computer monitor (it also affects print size should you choose to
output your images to paper). More pixels means a bigger display size,
regardless of the physical size of the pixels. Therefore, a chip with
7.4-micron pixels in a 1600x1200 array will give a larger image on-screen than a
1024x1024 array with 24-micron pixels, although the second chip is physically
larger.
Examples
Sample images shown to scale at 1/8 actual display size
A) 
B) 
Using the same CCD chip on two scopes with different focal lengths affects
the field of view and image scale, but not the display size.
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CCD: 9-micron pixels, 765x510 array
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Telescope A: 500mm focal length
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Telescope B: 1000mm focal length
A)
B) 
Using the same telescope with different CCDs affects the field of view and
display size, but not the image scale.
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Telescope: 500mm focal length
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CCD A: 9-micron pixels, 765x510 array
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CCD B: 9-micron pixels, 1530x1020 array
A) 
B) 
Using different CCDs and different telescopes can lead to any number of
results, but it is possible to end up with equivalent fields of view. In
this case, a large CCD with a long-focal-length scope gives the same field as a
small CCD on a short-focal-length scope, but the image scale is
greater on the larger instrument. The display size is also considerably
larger in the second image due to the larger camera's greater number of pixels.
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Telescope A: 400mm focal length
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CCD A: 7.4-micron pixels, 640x480 array
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Telescope B: 1250mm focal length
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CCD B: 6.8-micron pixels, 2174x1482 array
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