We often make the case here that astronomical imagery created to support the dissemination of Hubble’s scientific results to the general public relies on a delicate balance of aesthetics and science. The image above provides a great study in how these two aspects of the image come together in the publication of a press release. The images produced from Hubble data have an inherent beauty that usually shows through even when looking at the raw data.
We use image processing techniques to highlight that beauty and draw attention to the science results. As we’ve shown in previous blog posts, the calibration and cleaning process of the raw data is largely quantitative, aiming to provide the best possible raw products from which to build a press-quality image. As we move from this quantitative step into the assembly of a color-composite image, the process subtly shifts into a more subjective, qualitative space where decisions regarding color balance, tonality, and composition dominate the workflow. The choices made here can have a dramatic impact on the look of the final image and are made with the intent of maintaining the scientific integrity of the data—supporting the science behind the image. For the particularly motivated reader, here is a selection of further reading on this very topic:
1. Aesthetics and Astronomy: Studying the public’s perception and understanding of imagery from space
2. The Aesthetics of Astrophysics: How to Make Appealing Color-Composite Images That Convey The Science
3. High Energy Vision: Processing X-rays
The Subtle Sting of a Planetary Nebula
While working on a recent press release image of the Stingray Nebula, this balance of science and aesthetics rose to the top of conversations with the research team as the image products came together in support of the release. The Stingray Nebula, Hen 3-1357, is an interesting case where astronomers have used optical imagery from Hubble, spanning nearly twenty years, to observe a dramatic fading of this planetary nebula.
A planetary nebula is so named because in early astronomical studies before telescopes were able to resolve small-scale details, the generally round shape of these nebulae resembled planets in the telescope. They originate from intermediate-mass stars (around 1 to 8 times the mass of the Sun) near the end of their lives, in their red-giant phase, casting atmospheric materials into their surroundings. Once enough material has been cast off, the hot inner core of the star is exposed and its high-energy radiation ionizes the surrounding gas causing it to glow brightly as a nebula.
Back to the matter at hand, accurately comparing the brightness of two images taken 20 years apart, on two different detectors, presents a challenge. The data need to be carefully calibrated to remove any and all instrumental artifacts that could impact the flux estimates, or brightness measurements, from one dataset over the other. The researchers accounted for this when processing the data for analysis and these carefully calibrated datasets were used in the creation of the press image.
One of the goals of an image processor is to maximize the information content available in the data. This can mean scaling the intensity or brightness of the data to reveal the faintest details of the nebula, while preserving details in the brightest parts. This particular technique has the effect of compressing the dynamic range of the image, reducing the contrast between the brightest and darkest regions within the image, sometimes referred to as flattening the image.
Even working from the calibrated data, if this dynamic range compression is taken too far it can obscure the main scientific finding here, namely, the unprecedented fading of the nebula over twenty years. Therefore, a careful scaling of the data applied equally to both the 1996 and 2016 datasets is required. As the image processor, I need to rein in my instinct to reveal as much as possible from the imagery. Faint details of the nebula, while visually interesting, are not at the heart of the science story in this press release. The science team and I iterated over a few different versions of the image before finally striking a balance between revealing interesting structures within the nebula while also clearly showing the dramatic drop in brightness over twenty years.
It’s not often that the dual goals of creating a visually striking image and telling the science story of the data compete with each other to the extent that I experienced while working on this image. It is a learning experience that I will take with me moving forward. Below, I’ve placed three versions of the Stingray showing the evolution of the processing of the image.