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Posted by Clitherow on February 3, 2005 11:14:29 UTC

Hi,
No-one answered this back in September so I suspect no-one will read this either, but if you are still out there....
The resolution of a CCD camera does not depend on the number of pixels in the camera, rather it depends on the size of the pixels making up the chip and the focal length of the lens forming the image. A standard webcam has pixels of about 5 or 6 microns in size and with a 640 by 480 array has a total of some 307,200 pixels (less than a third of a megapixel). A larger chip made from the same substrate will have identical pixel sizes, just more of them. If you put either chip at prime focus of the same telescope the image formed by that scope will fall onto the chip and in each case will cover exactly the same number of pixels (since the pixels are the same size). Thus both chips resolve exactly the same amount of detail.
The larger chip will be able to 'see' more to each side of the central object being imaged, that is it will have a wider field of view, but it wont have better resolution.
In digital SLRs the lens focal length of the standard lens is chosen to ensure that the expected field of view just fills the physical chip size. If you upgrade to the next camera model up, which has more pixels, then a different focal length is chosen to just fill the bigger chip. In each case the view through the viewfinder may be identical in which case the camera with the larger number of pixels will have a higher resolution than the other i.e. more coloured dots to make up the same picture. From this you can see that image size formed by the lens also has an effect on resolution. A longer focal length produces a larger prime-focus image, which in turn covers more pixels;thus there is finer detail per pixel and higher resolution.
With a fixed focal length telescope we don't have that luxury, in which case smaller pixel size rather than larger chip size is the only way to increase resolution.
The highest resolution CCD cameras for astronomical-use available are actually the cheap and cheerful webcams, which is why they are so popular for capturing the delicate detail on nearbye planets, unfortunatly small pixels are also relativly insensitive which is why CCDs designed for deepsky work tend to have larger pixels, typically 8 to 10 microns.

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