Of Pixel Size and Focal Reducers
Before you purchase a CCD, weigh the options possible when focal reducers are added to the imaging system.
Nevertheless, when the shutter clicked open on the KAF-1000 camera, jaws dropped. "Now thats a CCD!" exclaimed one guest. Measuring 1 inch square, this chip offers only slightly less imaging area than a frame of 35-millimeter film. While everyone was predictably fascinated by this expensive bit of silicon real estate, blank stares followed my comment that, at a given resolution, I could capture more sky with the KAF-1600 despite its substantially smaller size.
How can this be? Even a quick glance reveals the KAF-1000 to be considerably larger 4.65 times, to be precise than the KAF-1600. The key to this paradox, however, was my qualifying statement that at a given resolution the KAF-1600 covers more sky.
Most of us photographers never think much about resolution. Todays emulsions have relatively fine grain, and we use the same film with telescopes big and small. As such, the larger the piece of film, the more sky will be captured up to the point where optical or mechanical considerations limit the field of view.
CCDs, however, are a different story. Pixels the individual, light-sensitive picture elements that make up the checkerboard array of a chips imaging area come in many sizes. The detectors found in todays popular cameras have square or slightly rectangular pixels ranging from about 7 to almost 30 microns (thousandths of a millimeter) across. The best results occur when a pixels size is matched to a telescopes resolution under a given set of observing conditions. For example, conventional wisdom suggests that the astronomical seeing conditions experienced by a typical backyard observer will produce excellent deep-sky images with pixels that cover about 2 arcseconds (2") of sky.
Consider the CCDs mentioned above. The KAF-1000 has 1 million 24-micron pixels arranged in an array measuring 1,024 pixels on a side. At 2" per pixel, the detector covers a field 2,048" (about 34') square. The KAF-1600, on the other hand, has 1.6 million 9-micron pixels assembled in a 1,552-by-1,032-pixel array. At the same scale, this chip covers a field measuring 3,104" by 2,064" (about 52' by 34'). The KAF-1600 has 60 percent more pixels than the KAF-1000 and should therefore cover 60 percent more sky.
There is a catch, however. Obtaining the same 2"-per-pixel scale for these detectors necessitates very different effective focal lengths. Indeed, the KAF-1000s larger pixels require an effective focal length of 2,475 mm (about 97 inches), while the smaller KAF-1600 pixels need only 928 mm (about 37 inches). The nomogram on the next page makes simple work of determining the relationships between pixel size, focal length, and a pixels image scale.