Final Processin

The Wavelet page is, undoubtedly, the Registax page where really excellent images are created. This is where you can endlessly "tweak, tweak, tweak, tweak, tweak" the image until you are diagnosed as having an obsessive compulsive disorder and the men in white come to take you away. What in God's name is a wavelet anyway? Well, the term wavelet originates from the world of DSP, or digital signal processing. In electronics, as well as in planetary imaging, the challenge is often to extract signal from the noise, an especially tricky task when the noise frequency and signal frequency are virtually the same. This is where long focal lengths help, provided you have sufficient light grasp. If your finest planetary details span numerous pixels they will have a lower "frequency" than the finer pixel-to-pixel noise. Thus, noise can be suppressed while detail is enhanced. We are in the world of mathematical transforms here where signals can be reduced to digital numbers and mathematical series (like sine and cosine). Fortunately, we do not need to get involved with the math. Thank goodness for that! Registax' genius inventor, Cor Berrevoets, has done that for us. The wavelet layer sliders on the Wavelet page correspond to increasingly coarse detail as we move from layer 1 to layer 6. The default "Initial" and "Step Increment" values of 1 and 0 can be used as well as the default wavelet filter and wavelet scheme (linear) settings. The first time you use Registax you can leave all the clever bits at their default values; you don't need to understand what they do to use the software and get a good result. Once you get to the Wavelet page, if you are taking planetary images at a nice, large image scale of around 0.1 or 0.2 arc-seconds per pixel you will find that using the sliders for layers 1 and 2 just increases the image noise and nothing else. So you can leave these untouched. Indeed, it is at this stage that you will discover whether the optimizing routine has enhanced any electrical noise pattern in the image. You may want to scrap the wavelet processing at this stage and stack the images all over again, with "Optimize" turned off. Sometimes this can reduce a noise pattern, but if noise is not a big problem leave "Optimize" turned on.

How does wavelet processing compare with traditional unsharp mask processing? Well, the effects are remarkably similar although the Registax layer sliders system is a much slicker, more powerful system. With the original photographic unsharp mask technique, a blurred version of the whole image was used as a filter to project the original negative through. The filter, being blurred, suppressed low-frequency bright information (such as Jupiter's white zones) revealing higher frequency, more subtle data, which was previously hidden. In the digital unsharp mask a radius (in pixels) is specified which tells you how blurred the mask will be. A strength is usually specified which is equivalent to the density of the blurred mask from the photographic era. The Registax layers are roughly equivalent to different unsharp mask radii, so all six layer sliders are like applying six unsharp masks of different radii to one image simultaneously—powerful stuff! Essentially, you endlessly tweak the layer sliders until you get the most aesthetic image. The trick is to know when to stop, as too much sharpening leaves an image looking noisy and unnatural.

There are other options on the Registax Wavelet page that are almost as important as the wavelet layer sliders themselves and can make a crucial difference to the final appearance of a planetary image. Apart from the simple brightness and contrast controls, the most critical options are the RGB shift and the gamma function. As we have already seen, in North America and northern Europe the Moon and planets are rarely at a decent altitude. When these objects are at their highest northerly declinations, i.e., above +20 Dec, they can achieve altitudes of 60 or 70 degrees from latitudes of 50 and 40 north, respectively. However, Mars, Jupiter, and Saturn spend half their time below 0 Dec and when below 40 degrees altitude atmospheric dispersion splits up the colors from these planets noticeably. Registax' RGB shift tool is a partial solution to this problem and it works very well, effectively realigning the red, green, and blue channels of the color image. Obviously this is not a complete solution: dispersion occurs within these color bands, too, and seeing is always worse at lower altitudes. Also, filtering the colors before they hit a monochrome webcam chip works far better. However, images look remarkably improved after applying the Registax RGB shift. Northern planetary limbs are no longer surrounded by a blue rim and southern planetary limbs are no longer surrounded by a red rim (and vice versa in Australia, South Africa, and New Zealand). Registax' RGB shift tool is pretty self-explanatory, with buttons giving you the option of shifting the red or blue channels, up, down, left or right a pixel at a time, until the planet appears not to be fringed with red or blue.

The final Registax page (Figure 9.4) contains a number of image tweaking features that, while useful, can be found on virtually any image processing package, such as JASC's Paint Shop Pro, or Adobe's Photoshop. These features allow altering the precise color hue and degree of color saturation (reducing color saturation can substantially reduce the noise, especially in a one-shot color image), resizing the image (useful for when Saturn's rings appear blocky), and rotating the image such that north (or south) is at the top. Most modern image processing packages also feature an auto-color balance feature, which attempts to give a scene a red/green/blue color balance that is correct for objects illuminated by sunlight. As everything in the solar system is illuminated by sunlight, auto-color balance can work well on the planets, too.

Figure 9.4. The final Registax page allows the image to be rotated and the Color hue, saturation and lightness to be adjusted.

CHAPTER TEN

Imaging the Moon

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