Gnuastro: visualizing the full dynamic range in color images

TL;DR

The paper presents a method to visualize the full dynamic range of multi-wavelength astronomical images by applying a Lupton-based non-linear stretch to bright pixels and an inverse-gray mapping for faint regions via the astscript-color-faint-gray tool in Gnuastro. This approach enables simultaneous visualization of high- and low-surface-brightness features, demonstrated on M51 with J-PLUS data and complemented by $H\alpha$ channel mappings to probe star formation. The authors emphasize the importance of gray-background color mapping, parameter tunability, and reproducibility through Maneage and public code repositories, highlighting practical benefits for deep imaging and survey data. Overall, it provides a concrete, adaptable workflow for generating informative color images that preserve scientifically meaningful dynamic-range information.

Abstract

Color plays a crucial role in the visualization, interpretation, and analysis of multi-wavelength astronomical images. However, generating color images that accurately represent the full dynamic range of astronomical sources is challenging. In response, Gnuastro v0.22 introduces the program 'astscript-color-faint-gray', which is extensively documented in the Gnuastro manual. It employs a non-linear transformation to assign an 8-bit RGB (Red-Green-Blue) value to brighter pixels, while the fainter ones are shown in an inverse grayscale. This approach enables the simultaneous visualization of low surface brightness features within the same image. This research note is reproducible with Maneage, on the Git commit 48f5408.

Figures (1)

• Figure 1: M51 galaxy group color images from J-PLUS DR3 using the iSDSS, rSDSS, gSDSS (R-G-B), and J0660 (H$\alpha$) filters. Top-left: traditional color image, where the background regions become black. Top-right: modified weights of the channels balance to obtain a bluer image. Bottom-left: gray background color image; this is the default mode of astscript-color-faint-gray. The separation between color, black, and gray regions are defined from surface brightness cuts (see text) of the G channel (rSDSS). The use of the gray background colormap reveals diffuse low surface brightness structures that would otherwise remain unveiled. Bottom-right: color image using the H$\alpha$ narrow band filter (J0660) for the intermediate (G) channel instead of rSDSS. The use of this filter reveals interesting features such as the star-forming regions that are shown in green.