Waid Observatory

The CCD Camera

The heart or the modern CCD Astronomical Camera is its image array, the CCD chip.  The term CCD is an acronym for "Charge-Coupled Device".  This device is a silicon based microchip that is highly sensitive in its response to electromagnetic radiation over a broad band of the near visible to visible spectrum.

CCD imaging chips are made by depositing an oxide covering to a silicon substrate in a two-dimensional rectangular array of light sensitive elements.  These elements have the ability to store charges created by incoming photons of light.  These picture elements, or pixels, convert the photons to an electrical charge that can be accumulated and stored in a "potential well" for each pixel.  This charge is then transferred from each pixel to pixel through the array by changing the voltage on each one.  Pixels in adjacent rows are said to be "charge coupled".  The signal moves in parallel from row to row and when it reaches the bottom of the array it is read into a register where it is stored and processed in a computer to form an image.  Since the charge accumulated by the pixel elements is proportional to the numbers of photons falling on each pixel the chip response is linear.  This means the final image produced is directly related to the brightness of the object being imaged.  This is not true for photographic film and is one of the great advantages of using CCD for photometry work.1

One drawback of the CCD chip is "noise" generated by random motions of electrons within the chip itself.  Utilizing two techniques compensates for this "error" signal, or "dark current".  One is to cool the chip to a very low temperature.  The dark current noise is reduced by approximately half for each 5 degree Celsius reduction of temperature.  Professional astronomers typically use liquid nitrogen to cool their CCD cameras to extremely low temperatures.  A thermal electric device usually cools cameras available to amateurs. Operating temperatures for amateur grade cameras typically range in the neighborhood of -5 to -30 degrees Celsius.  The other technique used to compensate for dark current is to subtract a "dark frame" from the image.  Taking an image with the shutter closed for the same length of exposure and operating temperature as the "light frame" generates this dark frame.  The dark frame is then subtracted from the light frame by a computer.

Another drawback to the CCD camera is "blooming" of bright objects.  This happens when the electron well for a pixel element is filled to overflowing during an exposure.  If this occurs, the excess charge flows to pixels in adjacent rows and forms a vertical "spike" or "bar" above and below the bright object.  This obscures useful image data in these adjacent pixels.  Some CCD chips have been designed to "bleed off" this excessive charge as it approaches the well's maximum value.  These chips are designated "anti-blooming gate" chips, or "ABG" chips.  The resulting image is no longer truly linear and is drawback to this type of chip.  For this reason the ABG chip is not preferred for photometry work where precise brightness measurements are required.  They are well suited for "pretty picture" imaging where bloomed stars are a distraction.

The advent and use of the CCD camera is a quantum leap for astronomy and it has virtually replaced photographic film for serious scientific astronomical work as well for amateur astro-photography.

1Oxford Astronomy Encyclopedia, pg. 76 sub. "CCD"

Copyright Donald P. Waid