What is CCD?

CCD stands for Charge Coupled device. A CCD camera consists mainly of a silicon chip that collects electrons which are excited into the conduction band. In other words, light from the object being imaged interacts with the silicon, thereby releasing electrons which are collected in pixels, and then read out to a computer.

The common analogy is that of a square array of buckets that collect rain during a rain storm. Then after the storm is over the buckets are emptied into a big tank, from which we can “see” the storm. We can think of the pixels of a chip as the buckets, and the computer as the tank.

When all the pixels are emptied to the computer, the computer displays a digital image of each of the pixels and the amount of light each collected, this produces a grayscale image of the object in the field of view of the telescope. This is the basic operation of a CCD camera.

How Does Noise Affect CCD Cameras?

Another important aspect to CCD cameras is noise. Noise is any signal that is not generated from the object being imaged. An example of noise is heat generated electrons. Such heat generated electrons can be reduced by cooling the chip, which the camera controls do electronically.

Of course, sometimes here in Minnesota during the winter, one might find the camera actually trying to raise it’s temperature to the set temperature. Noise is also reduced by processing the final image with other correction images such as dark frames and flat fields.

CCD Through History

CCD’s were developed in the late 1970s. Then during the 80s major observatories began using CCD’s for imaging. Now it is possible for many small observatories and amateurs to buy a CCD camera and take their own images.

Though CCD’s have not replaced regular astrophotography by any means, they do have many advantages. The main advantage of CCD’s is that they have a linear response to light which is in contrast to the non-linear response of photographic emulsions. Thus, it is possible to tell from an image how many electrons and thus photons were collected in each pixel.

This allows one to do such things as photometry or astrometry straight from a CCD image. Also, since CCD’s produce digital information, images need not be developed; they can be stored, displayed, and manipulated on any omputer with the right software. One can even make color images by taking images through red, green, and blue filters and then combining the images.

Another advantage of CCD cameras is that they have greater efficency than cameras using film, and therefore less time is required to take a picture using a CCD camera. One drawback, however, is that the field of view of a CCD chip is much smaller than that of a traditional camera. Though not the all around perfect detector, CCD’s have made a major contribution to the astronomical world and will undoubtedly continue to play an important role in this field in the future.

The following pages detail the basic images encountered when working with CCD cameras.