Using a Charged Coupled Device CCD

CCD stands for charge coupled device. A CCD is a digital camera. The camera contains a chip consisting of a mosaic of light-sensitive electronic micro-cells called photodiodes or pixels, a contraction of picture element. The chip has a rectangular or square shape and it can be about the size of a dime. The pixel mosaic is called an array. As in photography, you can take exposures lasting several minutes with a CCD since each pixel retains the amount of light received within its memory.

Most astronomical detectors in use today at professional observatories as well as with many amateur telescopes are CCDs. This fact will probably give you the impression that there must be something very special or useful about CCDs. It has been said, almost too much, that CCDs have revolutionized modern astronomy. Without a doubt, they will take their place in astronomical history along with other important discoveries such as the telescope, photographic plates, prisms, and spectroscopy.

For those of you who seek the challenges of CCD observations, you will be pleased to know that they are innumerable. I will not attempt to provide a complete description of CCD operation and CCD use in this book. It would simply be too much for one book.

However, I will provide some things to consider before you make a final decision to move on to CCD observations, followed by a brief description of what a CCD can do when used properly. It seems to me that it is a perfectly natural progression to move from visual observations to instrument observations. Instruments, such as CCDs, allow you to observe things that you will miss when using just your eyes.

First, consider the expense. CCDs can be expensive, at least several hundred dollars and several thousand dollars is normal. Then you probably need additional equipment such as flip-mirrors, filters, and filter mounts. All of this can easily double the cost of just the CCD.

Second, your telescope must be able to track accurately. This means that your telescope must be able to follow a star, and keep it centered on the CCD chip, throughout a long exposure. By long exposure, I mean ten seconds or more. If your telescope cannot do that, it will cost more money to correct that problem.

Third, there is a learning curve - for some it can be quite steep - when you first begin using a CCD. It takes time, patience, more time and much more patience, to begin to properly use a CCD. The challenge is exciting; I am not trying to scare you away. Just be prepared for a time-consuming challenge and budget several weeks to properly learn how to use a CCD for variable-star observing. For some, it may take several months.

The magazine Sky and Telescope maintains a Web site specifically for CCD topics. You can find it at http:// At the Web site, you will discover articles such as Starting out Right in CCD Imaging, Optimizing a CCD Imaging System, Image Processing Basics, and a few others. This is really a nice place to get a basic start in CCD imaging.

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