Image Analysis for Hubble

Since the early 1990s, the Hubble Space Telescope has collected stunning visuals of countless celestial entities, igniting the passion of both professional scientists and hobbyist astronomers alike. Among its most mesmerizing visuals are those of nebulae, such as the renowned Pillars of Creation situated within the Eagle Nebula (M16), depicted in vivid shades of green, blue, and gold. Though observing these same celestial objects through a telescope’s eyepiece doesn’t provide quite the same spectacular view, astrophotographers can capture Hubble-like images utilizing a specialized set of filters along with precise post-processing techniques.

Wideband Vs Narrowband Imaging :

Wideband filters, like LRGB filters intended for monochrome cameras, allow broad segments of the optical spectrum to pass through, encompassing red, green, and blue as perceived by our eyes. In contrast, narrowband filters focus on a limited slice of the spectrum, just a few nanometers wide, centered around particular emissions from excited gases present in space.

The three most prevalent narrowband filters include Hydrogen-alpha (Hα), oxygen-III (OIII), and sulfur-II (SII). The numeric and Greek symbols used with the elemental names denote specific energy transitions of electrons within the atom, which correlate to particular wavelengths of light. Emission from Hα is at 656.28 nm, or deep red; OIII represents a doublet (two close, related spectral lines) at 495.9 and 500.7 nm, appearing as blue-green; whereas SII registers at 672.4 nm, also deep red. Given that both Hα and SII are variations of red, the creators of Hubble images opted to assign false colors to these emissions for more visually appealing representations: SII, having the longest wavelength, is represented as red; Hα, with the next-shortest wavelength, is designated as green; and OIII, possessing the shortest wavelength, is depicted as blue. This color scheme is known as the Hubble palette. Astrophotographers can also emulate Hubble-like images through the use of these narrowband filters. An added advantage is that narrowband filters significantly diminish light pollution due to their narrow bandpass, making them invaluable when capturing images amidst urban illumination.

Real • Image of Hubble Telescope Reveals Huge Galaxy Explosion •

Balancing Color :

Since hydrogen is the most prevalent element in the universe, merely assigning these colors without further adjustment would lead to a predominantly green and uninteresting image. 

Therefore, one needs to modify the relative strengths (or weights) of each color along with their hues to create a visually pleasing final photograph. Beauty is subjective, and every astrophotographer will process their images uniquely, yet certain common principles should be kept in mind while adjusting color balance.

Firstly, don’t eliminate all the green completely! Green is a crucial element in creating other colors, like teal and yellow. Completely removing green results in a dull, two-tone blue and orange image, a common pitfall for many astrophotographers. On closely examining Hubble images like the Pillars of Creation or the Crab Nebula (M1), you will discern a wealth of green interspersed, enriching the artist’s color palette. 

Second, color balancing should be treated as an iterative endeavor. Instead of merely adjusting red, green, and blue sequentially, utilize color masks to concentrate on specific segments of the color wheel, modifying shadows, mid-tones, highlights, and vibrance/saturation.

After that, revisit those colors and fine-tune them as you adjust the other hues. In PixInsight, I employ the updated Color Mask tool to isolate a particular color segment and then tweak the red, green, and blue components of that color, alongside the vibrance (c component) and saturation, via Curves Transformation. 

I recognize that an image is finished when I attain a precise shade of teal cyan for the blues and gold that transitions into red, giving the image an impressive depth. Achieving perfection takes time, but the stunning results justify the effort! For astrophotographers aiming to enhance their craft, investing in a monochrome camera and quality narrowband filters is certainly worthwhile. 

Numerous emission nebulae and supernova remnants in the cosmos align beautifully with the Hubble palette, and with diligent processing, they can transform into the highlights of breathtaking and vibrant images.

--Bhautik Thummar