9. Appendix: A Modified Logarithmic Visualization Method

The dynamic range in the surface brightness of a galaxy can range several orders of magnitude, from the bright nucleus to the faint disk light. Grey-scale or 3-color realization of the Large Galaxy Atlas images presents a considerable challenge. The standard solution is to use logarithmic "stretch" of the color or grey-scale table. But the contrast for both high and low surface brightness features tend to wash out. We have found that a slight modification of this scheme enhances both surface brightness components. The key is to exploit our knowledge of the background noise, thusly:

P'= sqrt{ log(1 + P / n*sigma) }

where P is the pixel intensity value, sigma is the image RMS "noise", and n is a threshold throttle (with satisfactory values between 5 and 10). This method simultaneously amplifies low SNR pixels (P<10n*sigma), while damping the dynamic range between the bright and faint pixels, thus showing both the nucleus and faint extended emission with comparable contrast. This method appears to work as well or better than histogram equalization, particularly in extending the dynamic range of the high surface brightness galaxies. We employ this technique to produce individual (gray-scale) JHKs images, "super" J+H+Ks images (see Figures 1-5), and 3-color RGB combinations (e.g., Figure 6).

Contents     back     next