Digital Imaging A Useful Cybersketching Tool

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A CCD (charge-coupled device) is a small, flat chip - about the diameter of a match head in most digital cameras - made up of an array of tiny light-sensitive pixels. CCD chips in low-end digicams, including those featured in a number of PDAs, may have an array of 640 x 480 pixels; a more expensive digicam may have a 2240 x 1680 pixel CCD chip (around 4 megapixels), while a high-end digicam may boast a chip of more than 8 megapixels. When light hits a CCD pixel it is converted into an electrical signal whose intensity corresponds with the brightness of the light that struck it. This information is then processed into an image and stored in the camera's own flash memory or on a removable flash card.

Electronic imaging devices have revolutionized astronomy in both amateur and professional arenas. Thanks to the little CCD chip, amateur astronomers of relatively modest means can capture stunningly detailed images of the Moon and planets. Faint deep sky objects once considered the visual 'property' of big professional observatories are now within easy reach of the amateur astronomer's sensitive electronic eyes.

Digital imaging is often said to be in competition with visual observation. This is a sentiment expressed by those who don't really understand the nature of amateur astronomy and the rewards to be found in applying and mastering age-old observational techniques. Those same people might look down on those unfortunate mortals who enjoy sketching what they see through the telescope eyepiece, confident in the belief that their unblinking chips are the way of the future and that science has left the visual observer and sketcher of things celestial to revel in an arcane pursuit of relevance only to a dwindling few of artistic bent. However, astronomical sketching has as much value and relevance today as it did before the first CCD chip bathed in its first photons.

One potent method of cybersketching, particularly applicable to lunar and solar observations, is to use a live or almost-live digital image taken with a digicam, DSLR, webcam, or astronomical CCD device as the basis for an observational drawing. This kind of sketching technique is a 'simultaneous cybersketch' - an electronic drawing made with the aid of a simultaneously displayed digital photographic image that is used either as a reference and/or as a template. Alternatively, it may be a traditional pencil sketch made using a low-contrast printed digital image template - a 'cybertemplate,' if you will - and these kinds of observational drawing might be termed 'digitally assisted' sketches. The techniques involved in these cybersketching methods are discussed in more detail later.

Digicoms

Most low-end digital cameras have a fixed optical system, with non-removable lenses, and some may not even have an LCD display. Astronomical photography through these cameras must be done afocally, by aligning the digicam lens with the telescope's eyepiece (Figure 2.34).

Figure 2.34. The author's Pentax Optio S30 digicam, set up for afocal imaging through a 100-mm refractor (photo by Peter Grego).

As a general rule, the digicam with the highest pixel rating will produce the clearest, highest resolution images. Most digicams offer a number of resolutions.

Figure 2.35. The Optio S30, a 3.2 megapixel compact digicam (photo by Peter Grego).

A low-resolution image will take up less space in the camera's memory, allowing more images to be stored, but their quality will be poorer. Digicams provide instant results, and the images can be viewed on the camera's small LCD screen (if it has one). LCD screens are usually on the small side, and the display is coarser than the captured image itself. Each image can be individually reviewed to decide whether it's good enough to keep or whether to delete it and free the memory for a better image.

Because of their basic nature, most digicam astrophotography is restricted to big, bright subjects full of detail, such as the Sun and the Moon. Digicams are designed for everyday use, and their automatic settings may pose considerable problems when attempting astrophotography, so it's essential to experiment with the digicam's various settings to produce the best results. One of the most important settings to come to grips with is the digicam's exposure settings, as many afocal lunar images tend to be overexposed, the bright part of the Moon appearing washed out and lacking in any detail. The digicam's automatic exposure works best if there is a uniformly bright image across the entire field. A digicam may judge exposures perfectly fine when the Moon is centered in the field of view or when taking close-up shots.

Color images of the Moon taken with digicams may show vivid hues that can't be seen visually through the telescope eyepiece. While color can enhance the aesthetic quality of an image, it can also be undesirable. Computer processing by reducing color saturation levels can easily tone down an image. Capturing the Moon's colors in an exaggerated or visually realistic fashion may produce a pleasing image, but the same amount of topographic detail can be recorded in black and white. If your camera has a facility to take black and white images, try it out; the results may be noticeably sharper than those taken in color. Black and white images will also take up less space in the camera's memory, too.

Using the digicam's zoom facility (if it has one) can eliminate the problems of image vignetting that tends to plague afocal photographs. Zooming adjusts the position of the camera's internal lenses; the magnified image becomes progressively dimmer, and vibrations in the telescope will show up more. When 'digital zoom' comes into play at high magnifications, the quality of the image begins to degrade, and the advantages of zooming are completely canceled out. Note that optimum zoom is not the same as maximum zoom. The best amount of zoom to use depends on the seeing conditions, the resolving power of the CCD chip, and the telescope, as well as the stability of the system and the accuracy of the telescope drive.

A low-end digicam is perfectly capable of producing an astronomical image -a 'cybertemplate,' if you will - for use as the basis for an observational drawing of a small area of the Moon or an interesting region of the Sun. It's not necessary for a lunar or solar cybertemplate to be perfectly framed, gloriously detailed, and as sharp as a pin to be used in a digitally assisted sketch. All that's required are the most basic visual prompts to the main features under observation; the observer can fill in all the detail to whatever level is desired using his or her own drawing skills and visual acuity. One of the main practical points of using a cybertemplate is that the observer need not spend a great deal of time in laying out all the broad detail by placing features in their correct relative size and positioning them accurately with respect to one another. This is already done, and all that the observer need do is delineate these features boldly and concentrate on the finer detail.

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