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Contents:

1. Digital Magic
2. Painting By Numbers
3. A Revealing Image
4. As Easy as Calling Up a Log
5. A Lunar Odyssey
6. Fun with Fuzzy Filters
7. Building Color
8. A Digital Toolbox

As Easy as Calling Up a Log

When your friends wanted to see the faint, outlying parts of the galaxy better, they considered two possibilities: gamma scaling (sometimes called power-law scaling) and logarithmic scaling. In gamma scaling for deep-sky objects, the computer doesn't stretch the brightnesses uniformly across the full range. Instead it stretch-es the brightness more at the low end. That makes features with low pixel values (faint parts of galaxies for example) brighter on the screen. But the non-linear stretch keeps the bright parts such as the cores of galaxies from becoming too bright and burning out into one mass of white.

If you were to plot a graph of input pixel values versus output brightness, as most image-processing programs can, you'd see that the so-called gamma curve suitable for most deep-sky objects is not a straight line but bows upwards - the curve rises at the low end then flattens. Most of the time, this type of curve provides the most aesthetically pleasing rendition of deep-sky objects.

A gamma curve can also bow the other way, flat at first and then rising at the high end. This type of curve is useful for planetary images - it stretches the contrast between bright features on the planet while preventing the dark limb of the planet from becoming too dark and blending into the sky.

Logarithmic scaling acts much the same as gamma scaling but is harsher than even the most extreme gamma curve. A typical log curve rises steeply at the low end and then quickly flattens out. Your friends select log scaling when they want to bring out the faint haloes around galaxies without burning out the spiral arms. Although log-scaled images can reveal hidden details remarkably well, the resulting images often look unnatural.

Another stretching variation is "histogram scaling." Here you use the computer to construct a custom-made curve that force-fits all the pixel values onto a curve with a shape best suited for that object. Histogram scaling is even more dramatic than log scaling - it's usually too powerful for galaxies but does wonders in revealing impossibly faint features like the outer envelopes around planetary nebulae.