Getting the Data Out

Photosites are read out, one row at a time, into a serial register along the bottom of the CCD array.  The serial register then transfers the charge from each photosite to an output node.  From there the charge is sent to the camera's electronics unit where the electrons are digitized. 

Above:  The first row of photosites is shifted into the serial register and read out.

Above:  Each row is shifted down to the next row of photosites.  The bottom row is transferred to the serial register and sent to the computer.

Above:  The process continues until all the photosites have been read out.

A numerical value is assigned to each photosite's charge, based on the number of electrons contained in the photosite.  This value is sent to the computer and the process repeats until each photosite's electrons have been converted to a pixel value and are displayed as a raw image on the computer screen.

Above:  An uncalibrated image of M51, the Whirlpool Galaxy.  The specks are due to "dark current", explained below.

This image is "raw data" because it is unprocessed by any software and there are artifacts present due to the nature of the CCD.  Image processing software allows the removal of most of these artifacts.  Below is a processed image.

Above:  A calibrated and enhanced version of the above M51 image.

Dark Current

Since astronomical CCDs are designed to image faint objects they must be extremely sensitive.  One drawback to this sensitivity is that the photosites also pick up electrons generated by heat within the camera.  To minimize this effect, astronomical CCD cameras are equipped with cooling systems.  These systems are capable of lowering the camera's operating temperature 20 to 60°C (about 40 to 100°F) below ambient temperature.  Even this is not enough to remove all the effects of this dark current.  For this reason CCD imagers take a dark frame.  A dark frame is an image taken with the camera's shutter closed.  This image detects only the heat-generated electronic noise.  The image below shows a dark frame of the same exposure duration as the raw image above.

Image processing software can later be used to subtract the dark frame (and hence the electronic noise) from the image.  Some newer CCD chips have much lower inherent noise and may not require a dark frame, but in general dark noise is still a problem with most CCD cameras.

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