CCD Cameras

Even webcams, some of them at least, use CCD chips, but "CCD" in amateur astronomy has come to refer to cooled long-exposure cameras designed specifically for astronomical use. CCD cameras are at the top of the heap when it comes to capability in astronomy picture taking, but may seem a little more complicated than the other choices—at first, anyway. DSLRs, at the most basic level, are used like film cameras: screw one onto the back of an SCT and snap away. CCD cameras require a computer to do anything at all and are more dependent on "calibration," plus the use of

"dark frames" and "flat-field frames" and other specialized processing techniques to perform at their best. There's no viewfinder or display screen, either. Targets are focused and finished images are displayed on the PC. When an integrating camera's exposure ends, it assembles the picture and sends it (usually via a USB connection) to the laptop for display and processing.

One other important difference between DSLRs and CCDs is cooling. Much of the noise seen in DSLR images is thermal noise. When light strikes a CCD or CMOS chip pixel, it liberates an electron that is "counted." The counting of electrons determines how bright or dim that pixel is in the final image. Unfortunately, heat (from camera electronics or the environment) can also free electrons. These "thermal electrons" get counted, too, and appear as white dots—"false stars"—in images. Some thermal noise can be dealt with by dark frames. Take an exposure with the dust cap on the scope's corrector, combine this "dark" with the image frame, and the computer can subtract out the thermal noise—most of it, anyway. Some doesn't go away, not in an uncooled camera such as a DSLR. CCD cameras are able to eliminate almost all the noise "specks" with a dark frame because they start out with less noise than an uncooled camera has. Chilling the CCDchip to at least below freezing (usually by a Peltier cooler, a solid state "heat pump") gets rid of enough of those dratted false stars to allow a dark frame to take care of the rest.

What about specific models? Unlike DSLRs, no single brand completely dominates the CCD arena. These cams are, instead, divided into three tiers, much like CATs: inexpensive, mainline, and top-of-the-line. As with telescopes, capabilities increase (bigger CCD chips) with price. Also like telescopes, though, a dedicated and talented user can make a less expensive camera perform like one costing much more.

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