Once an image of an object has been taken using a CCD camera it is then necessary to improve the data to produce the most accurate image possible. There are four main stages in this data reduction, these are dark frames, flat fielding, calibration using standard stars and dithering.
All CCDs have what is called a dark current. This is the amount of charge that would build up on the CCD without any light from an outside source. In order to correct for this current, images are taken with the shutter of the detector closed. These dark frames are taken for the same amount of time as those of the image and their average can then be taken away from the image of the source, removing the dark current.
The quality of the image may also depend on the efficiency of each pixel in the CCD. In order to compare individual pixels these variations need to be corrected for. A flat field is used to do this. An image is taken of a constant source, usually the twilight sky, so that each pixel should be identical. Any differences can then be seen. The flat field is then set so that the average value of the pixels is one, called normalisation. The source image can then be divided by this normalised flat, to account for the pixel differences.
Once the image has been flat-fielded and had its dark current removed it must then be compared to a standard star to get an absolute value for its magnitude. A standard star is one that has a known magnitude, so by comparing the observed brightness of an image to a standard, taken on the same night, the actual brightness of the image can be found.
All CCDs are likely to have some bad pixels, there is also a chance of a cosmic ray hitting a detector during an observation. To account for this, astronomers use a process called dithering. This involves taking several images of the same object each of which is offset slightly so that when they are combined the effect of any one bad pixel or a cosmic ray is cancelled out.
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Authors: Carolyn Brinkworth and Claire Thomas
Last updated: July 2001