Transcript CCDs

CCDs:
Charge-Coupled Devices
Kathy Cooksey
European Space Agency
Raw CCD Image
Dust Donuts
Bad Column
Hot Pixels
Examples of CCDs
How CCD Works
Human CCD
Confetti…
How CCD Works
• Photons (packets of light) hit CCD
• Their energy release electrons
• Those electrons are trapped in individual pixels (also
called “wells”)
• The collection of electrons in each pixel is called “charge”
• After exposure is done, charge is transferred as we just
showed: row to row and then pixel to pixel (“read out”)
• Computer measures charge and makes digital image,
which we display
Problems with Images:
Extrinsic to CCD
• Point spread function: summary of distortions to perfect
disk due to telescope optics, pointing, atmosphere
• Saturation: too much light fills up pixel (bucket) and
charge (confetti) “spills” over [take shorter exposures]
• Dust donuts: dust on lens or filters makes shadow on CCD
[flat field]
• Cosmic rays: high energy particles that strike CCD and
leave fake star or streak [take multiple images]
Problems with Images:
Intrinsic to CCD
• Hot pixel: pixel that is always full of charge [careful
positioning]
– Dead pixel (or row): pixel (or row) that never collects any light [careful
positioning]
• Bias: some charge is permanently stuck in all pixels [subtract
baseline]
• Detector response: (variable quantum efficiency) when pixel (or
group) doesn’t record all light that falls on it [flat field]
• Read noise: computer doesn’t accurately “count” amount of
charge [take longer exposures]
– Low charge transfer efficiency: some of charge gets lost while reading
out (charge gets stuck in pixel)
Color Images with CCDs:
Astronomy
M42 “Great Orion Nebula” - Copyright Graham Pattison
Color Images with CCDs: Digital
Camera
What have you learned?
• What do you think was important?
• What was new news to you?
Summary
• CCDs are basic tools of astronomical
imaging
• CCDs are grids of light buckets
– Read out properly to reconstruct image
• Raw images are ugly
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Flaws in CCD (bad pixels, etc)
Read out problems (noise, CTE, etc)
Whole telescope problems (dust, etc)
Exposure problems (saturation, cosmic rays, etc)
• CCDs are color blind
– Filters combine to give colored images