An intersting article appears on page 1 of the 2 February 2004 issue of
Electrical Engineering Times (www.eet.com), a trade rag, titled "Roving eye for
Mars forces CCD rethink", by Richard Goering. The basic story is that no
commercially available CCD image was suitable for the nine cameras on the Mars
rovers, so NASA had JPL develop a full-custom CCD image. The specs on this
CHIp give an idea what can be achieved; such things inevitably find their way
into consumer products after a few years.
The full story is at
<http://marsrovers.jpl.nasa.gov/mission/spacecraft_rover_eyes.html>.
The cameras are better described in
<http://europa.la.asu.edu:8585/PGG/greeley/courses/pdf/squyres_2003.pdf>.
Image is 1034x1024 pixels (monochrome), with each pixel being 12 microns square.
The full-well linear capacity is 200,000 electrons, and the noise is 3
electrons in a half-full well (100,000 electrons). 20log10(200000/3)= 96.5 db,
or 16 bits, with 99% linearity. (This compares to the 12 bits we have been
seeing in high-end digital cameras.) The temperature range is -90 to +20
degrees centigrade, and no mention is made of a chip cooler. (Mars is cold to
start with, so why bother.)
The article reports that developing the charge-to-voltage circuit (converts the
charge in a pixel well into an output voltage) was the big challenge. The
output is 4.8 microvolts per electron, so the noise level is 3(4.8)= 14.4
microvolts, which is pretty good.
Joe Gwinn
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