The word resolution has several meanings. In the days of film
photography it referred to the smallest details discernable by the film.
This same meaning is applied to telescope optics and so much confusion can
arise.
Nowadays, the most common meaning comes from the computer world. Screen
resolution refers to the size of a computer monitor in
pixels. In the
prehistoric days of computers (a few years ago) VGA resolution was
standard. This meant the screen had a display 640 pixels wide by 480
pixels high. Now 1024x768 is the most common, with 1280x1024 very rapidly
becoming the new standard on large screens, including larger laptops.
Monitors with resolutions of 1600x1200 pixels or more are available now.
A similar reference is made to digital cameras and astronomical CCDs. A
digital camera might be said to have a resolution of 3072x2048.
Astronomical CCD chips are often referred to by the same statistic: a Kodak
KAF-3200 chip, for example, has a resolution of 2174x1482.
Since astronomical CCDs must perform different functions than a digital
camera they must be designed differently. Much more stringent tolerances
are required for an astronomical CCD (less thermal noise, low read noise,
low-temperature operation, minimal pixel defects, etc) and so the price is
correspondingly higher. While a 6 megapixel digicam might cost as little
as $500, and a 6 megapixel digital SLR costs less than $1000, a 6 megapixel CCD
camera still costs over $5000 in mid-2007. This is likely to change with
new technology, but astronomical CCDs will always be more expensive than their
digital camera counterparts.
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What is a Megapixel?
It depends on who you ask. Digital camera manufacturers try to
squeeze every megapixel out of a camera as they can since this is often how
the camera is advertised. Take the camera mentioned above. It has
a resolution of 3072x2048. This is a total of about 6.3 million
pixels. The camera would be advertised as 6.3 megapixels. A
megabyte, the measure of file size or storage space on a computer, is
equal to 1024 (not 1000) kilobytes. And a kilobyte is 1024 (not 1000)
bytes. So, a megabyte is really 1024 x 1024 bytes, or 1,048,576
bytes. A megapixel works the same way, so that 6.3 megapixels is really
only 6,008,150 pixels! (This same counting method is used is storage
devices, so that a "256 MB" flash card for a camera really holds 256 million bytes
of data, or only 244 "true" megabytes.)
Astronomical CCD manufacturers usually quote the actual number of
pixels. The KAF-3200 chip above has about 3.2 million pixels and is
sometimes referred to as a 3.2 megapixel camera even though this does not
follow the standard digital camera procedure.
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Resolution can also refer to angular resolution, or how much sky is
covered by a given CCD pixel. This is handy information to have for
certain applications. The angular resolution is a function of the actual
pixel size and the focal length of
the telescope to which the camera is attached. Pixels sizes are given in
microns and focal lengths are normally given in millimeters. Angular
resolution is most often quoted in arcseconds/pixel. The formula for
calculating angular resolution is:
Angular Resolution = (Pixel Size * 206) / Focal Length
For example, a CCD with 7.4 micron pixels on a telescope with a focal
length of 600mm will have an angular resolution of 2.54 arcseconds/pixel.
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