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Fermilab Builds 500-Megapixel Camera
heyitsme writes "Fermilab, a U.S. Department of Energy research lab, is part of a collaboration on an experiment to measure the properties of dark energy. The Dark Energy Survey would measure the history of the expansion rate of the universe more precisely than ever before, using the largest camera ever built with Charge Coupled Devices (CCD). The 500 megapixel Dark Energy Camera (DECam) would be placed on an existing 4-meter telescope located in north-central Chile at the National Optical Astronomy Observatory's Cerro Tololo Inter-American Observatory. The DECam together with the CTIO 4-meter telescope will allow for a survey of 15 percent of the sky to light levels faint enough to measure the colors of galaxies at redshift one."
The detector actually contains roughly 503,316,480 raw pixels, this amounts to slightly less than 500,000,000 effective pixels after initial processing.
The data will, of course, be stored directly to a large SAN storage system, probably from EMC or Hitachi.
The detector should generate single frame images of roughly 1.7G prior to post-processing, and roughly 700M single-frame image files after processing to TIFF or PNG format.
(Not an expert, they're all fast asleep right now.)
One of the things Fermilab is trying to do is get a measurement of the so called weak lensing effect. Matter distorts spacetime, and light is thus bent as it passes nearby a big cluster. This is gravitational lensing.
Famously, it is seen as "strong" lensing -- when the source is very close on the sky to the cluster, and the light gets bent enough that there are multiple images. Nobody really believed it could happen, but then in the last decade or so it's become an accepted and popular thing to play with and observe.
Weak lensing is when there are no multiple images, and instead only a slight distortion. Much harder to see and measure -- you basically look for a whole bunch of galaxies that are slightly distorted.
That means you need a very wide field of view -- to get enough galaxies quickly enough -- but also a very good resolution -- to be able to measure the slight distortions. Hence the need for such an insane[ly cool] device.
Why go through all this trouble? Well, weak lensing is one of the view ways to measure all the matter in the universe on very large scales. Because nearly all the matter is supposed to be invisible, in the past people have used various "tracers" that we can see. But there's a huge amount of debate as to how good the various tracers are, and, of course, you need a direct measurement to be sure you're not off in la-la land.
Weak lensing measures it all because all matter, regardless of how bright it is, bends spacetime in the same fashion. So, if you can get a good weak lensing measurement, you can theoretically create an unbiased map of the matter distribution. No need to cross your fingers and hope that some tracer is behaving properly.
It all fits into dark energy because dark energy is supposed to alter the extent to which matter can cluster (roughly speaking, dark energy behaves like antigravity, and pushes things apart, stopping them from falling together.)
Of course, weak lensing is just one of the things this guy is meant to do -- there are lots of other neat things that hopefully someone more awake than I can describe.
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Anyway, with 500MP you're going to be severely limited by the resolving power of the lens. It's quite difficult to get even 100 line pairs per mm with the best 35mm photographic lenses (lenses for larger formats tend to be much worse because it's harder to maintain accuracy over a large glass area, plus it's not as necessary with lower enlargement factors). A 500MP sensor needs a pretty exceptional telescope in front of it.
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So 960 million bytes per frame, which is only 915.5MB (1M = 2^20).
-- *My* journal is more interesting than *yours*...
CMOS is cheaper and can transfer the image faster off of the chip, but CCDs offer lower dark noise and lower reading noise, which means that your pictures are clearer and more scientifically usuable. CCDs are also INCREDIBLY more sensitive than CMOS, with the newer chips able to get upwards of 85% efficiency.
-Bill
-Bill