An alternative to an optical wedge prism solution and to digital RGB channel splitting is to image the planets through narrow-band or separate red, green, and blue filters and combine the image into its final color solution. This might seem like a backward step when color webcams are cheap and easily available and, indeed, it is certainly more hassle. However, it has big advantages when a planet is at a low altitude. Color webcams have an array of filters covering the CCD pixels, from which they deduce the final color. As we have seen, in a typical example every 2 x 2 block of pixels might have a red-filtered pixel, a blue-filtered pixel, and two green-filtered pixels. Thus, the sensitivity of the webcam is significantly reduced by the filters. In a monochrome webcam, full-size glass filters have to be purchased to create the colors, reducing the sensitivity as before. However, an additional "luminance" signal can be taken with a monochrome camera, with no filters in the way, and this luminance signal will have much better signal-to-noise than the color images or the color webcam. The technique that is used to combine the excellent luminance signal with the color information is known as LRGB (luminance-red-green-blue) and it is a very powerful technique indeed. In fact, Registax has an LRGB mixing tool incorporated into the software that enables the color channels from a color webcam with a Bayer filter matrix to be split up and recombined (see Figures 8.2 and 8.3) but this not as powerful as true filtered LRGB with a monochrome webcam where we typically have 24-bit color (3 x 8 bit) information to play with. We will see shortly that even more powerful luminance techniques can be created with specific filters.
The LRGB feature in Registax, when used on a color webcam AVI, works slightly differently in that it constructs the luminance (brightness) signal of the final image from a mixture of the red, green, and blue channels in the existing image. The default ratio for the mix, i.e., the one that gives the most natural appearance from a ToUcam webcam, produces a luminance value with the following ratios: luminance = 0.299 x red + 0.587 x green + 0.114 x blue. Not surprisingly, green is given the highest weighting as the human eye is most sensitive to green, and blue is given the least weighting as the blue end of the spectrum has already been boosted to noisy levels by the webcam's YUV image compression system. Altering the red, green, and blue sliders in Registax can make significant changes to the appearance of a planet imaged by a web-
cam. With Mars, increasing the red content significantly increases image contrast, whereas with Saturn, increasing the blue content (although noisy) enhances the subtle Saturnian belts. However, true LRGB imaging can only be achieved using filters on a monochrome camera. A cheap way of minimizing dispersion, for the complete beginner, is to employ a UV-IR block filter that at least restricts the spectrum to the visual band and knocks out the near infrared and ultraviolet regions. However, there are better solutions when monochrome cameras are used.
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