When we gaze at those iconic Hubble images, we witness a blend of scientific data and artistic interpretation. Here’s the cosmic backstory:
The Hubble Space Telescope captures celestial objects using its sensitive detectors. However, these detectors don’t perceive colour; they record the intensity of light. The initial images are essentially black and white, missing the vibrant colours that we associate with space.
Scientists enhance these monochromatic images by applying colour filters. These filters allow specific wavelengths of light to pass through. By capturing separate images through red, green, and blue filters, scientists create a colour composite.
These composites mimic what our eyes would perceive if we could see those wavelengths directly. The choice of filters is both scientific (to highlight specific features) and artistic (to create visually appealing representations).
Assigning colours to wavelengths involves creativity. For instance:
- Red: Often corresponds to hydrogen emissions (common in nebulae)
- Blue: Represents oxygen emissions (seen in star-forming regions)
- Green: May signify sulfur emissions. These mappings aren’t arbitrary; they align with the physics of light emission
Hubble images are accurate in the sense that they reveal real structures and phenomena. However, the vivid colours aren’t exactly what our eyes would perceive. They’re a visual translation of complex data. Scientists prioritize clarity and information over strict realism. Enhancements help us see details that might otherwise be hidden.
The artistic touch in Hubble images isn’t deceptive; it’s a celebration of discovery. These visuals inspire awe, curiosity, and scientific inquiry. They invite us to explore the cosmos. So, while the Hubble doesn’t “see” in colour, it gifts us a universe painted with wonder.
In summary, Hubble’s images are a harmonious blend of science, art, and cosmic storytelling. They ignite our imagination and deepen our understanding of the vastness beyond our blue planet.
RGB stands for Red, Green and Blue
Let’s unravel the captivating tale of Hubble images, tracing their lineage back to the 19th century and the ingenious work of Scottish physicist James Clerk Maxwell.
In the late 1800s, James Clerk Maxwell pioneered the world’s first colour photograph. His creation, aptly named “The Tartan Ribbon,” employed a groundbreaking technique.
To create this early colour image, Maxwell took three monochromatic images of a tartan ribbon. Each image was captured through a green, blue, or red filter placed over the lens. These filtered images were then printed onto glass plates and carefully aligned.
Maxwell used three projectors, each equipped with the same colour filters as the photographs. By superimposing these three images, Maxwell achieved a full-colour composite—a primitive but remarkable feat.
Blurry origins and shared principle
Maxwell’s “tartan ribbon” appeared blurry and lacklustre, far from the majestic Hubble images that now grace our screens. However, the core concept remained consistent. The essence? Combining different filter exposures to create a colour photograph. This fundamental idea persists in modern astrophotography.
But why those specific filter colours? Maxwell’s choice was deliberate: Red, Green, and Blue are the additive primary colours of light. When combined, they show white light.
Mix any two of these primaries, and you get secondary colours: cyan, yellow, or magenta. Our eyes, equipped with specialised receptors called cones, perceive colour. These cones are most sensitive to Red, Green and Blue light.
Hubble’s Monochrome Vision and Beyond, and how photographs of space are so colourful
Now, let’s turn back to the Hubble Space Telescope. Contrary to popular belief, the images shared with the public are not direct observations made by the telescope itself.
Hubble’s inherent vision is monochromatic (Black & White); it does not naturally perceive colour. However, its capabilities extend beyond visible light. Hubble detects ultraviolet radiation and a hint of infrared, both of which are imperceptible to our human eyes.
In essence, the enchantment of Hubble lies not only in its breathtaking visuals but also in the enduring legacy of trailblazers like Maxwell. Their ingenuity paved the cosmic path, seamlessly blending science, art and wonder.
Back to Hubble and why photographs of space are so colourful
You’ve likely suspected this for a while: the “public” images shared by the Hubble Space Telescope do not directly mirror what the telescope observes. Also, these images often diverge from the true colours of the celestial objects they depict.
The telescope itself does not perceive colour; its vision remains inherently monochromatic. However, Hubble’s capabilities extend beyond the visible light spectrum. It can detect ultraviolet radiation and even a touch of infrared—both imperceptible to our human eyes.
To create these mesmerizing images, Hubble employs specialized filters. These filters allow specific colours to pass through (known as wide-band imaging) or separate distinct wavelengths (narrow-band imaging).
The telescope captures pictures of celestial objects using various filters, one at a time. Astronomers on Earth then combine these individual exposures to build a “colour” image.
Some of these hues align with what you might perceive if your eyes were as keen as the Hubble Telescope’s, while others serve as colour maps, enhancing our understanding of the structures within these cosmic wonders.
Wide-band imaging
Wide-band imaging employs filters that selectively allow a broad range of wavelengths to pass through, much like the name suggests. Astronomers commonly utilize three primary filters: Red, Green, and Blue (RGB filters). Each of these filters covers a distinct third of the visible spectrum.
When capturing an image, each filter assigns a specific colour based on the wavelength it corresponds to. To create a full-color image, astronomers stack individual exposures taken with these filters. These wide-band images faithfully represent the true colours of celestial objects. For instance, galaxies and planets are often photographed using wide-band techniques.
Occasionally, astronomers opt for secondary colour filters—cyan, magenta, and yellow (CMY)—instead of the traditional RGB filters. If you’re curious to explore further, you can delve into the fascinating realm of RGB versus CMY colour imagery.
Narrow-band imaging
Narrow-band filters serve a distinct purpose—they allow us to capture an extremely narrow slice of the electromagnetic spectrum, pinpointing specific wavelengths.
These filters find their application in photographing emission nebulae—expansive clouds of gas and dust where new stars come to life. These celestial objects emit light due to the intense heat of the stars within them, which excites the gas particles in the nebula. The primary component of these emission nebulae is hydrogen, accompanied by traces of oxygen, nitrogen, sulfur, and other elements.
Photographing nebulae using narrow-band filters makes sense because it isolates specific spectral lines, emphasizing the unique characteristics of these cosmic structures. Unlike broad-band imaging, which covers a wider range of wavelengths, narrow-band techniques allow astronomers to focus precisely on the emissions from these nebulae.
The most frequently utilised emission lines include:
- Hydrogen Alpha (Ha) at 656.3 nanometers
- Twice-ionized Oxygen (OIII) at 500.7 nanometers
- Ionized Sulfur (SII) at 672.4 nanometers
Why are photographs of space so colourful? Here’s how those colours are assigned
Imagine we’ve taken three photos of a galaxy or nebula using filters specific to hydrogen, oxygen, and sulfur. Now, how do we decide which colours to assign to each exposure?
To our human eyes, the Ha (Hydrogen Alpha) and SII (Sulfur) lines appear reddish, while OIII (Doubly Ionized Oxygen) appears green. However, using this natural colour palette wouldn’t necessarily create a descriptive or visually pleasing image.
Instead, we take a different approach: we assign colours based on the relative positions of these wavelengths in the electromagnetic spectrum.
Here’s how it works:
- The longest wavelength (Ha) gets the colour red
- The shortest wavelength (OIII) is assigned blue
- The medium wavelength (SII) receives the colour green
Astronomers affectionately refer to this palette as “SHO,” or “the Hubble palette.” By colouring narrow-band images in this manner, we create strikingly rich and dynamic pictures.
For instance, consider the two images of “The Fighting Dragons of Ara” (NGC 6188 Nebula) below:
Both images: courtesy of Connor Matherne at https://www.cosmicspeck.com/
The first image represents true colours, while the second image is a colour map.
Which one do you think provides more insight into the intricate structure of the gas cloud? Colour maps make photographs of space so colourful to provide more visual context, and the fact they look incredible is almost secondary.
Now, you may ask…
The Hubble Telescope often combines more than three filters to create its captivating images. These photos utilize popular channels, including ionized Nitrogen NII and Hydrogen Beta. To generate colour images with more than three hues, astronomers follow a method that starts with three primary colours and then adds secondary ones.
Regarding the use of ‘the Hubble palette,’ it remains a widespread choice. However, many astrophotographers prefer alternative colour-mixing approaches. For instance, some reserve the colour red for the Hydrogen-Alpha line (which represents the natural colour of Ha).
Interestingly, images of the same nebula captured and processed by different individuals can look remarkably distinct.
If you’re curious about the filters employed in a specific Hubble photo and the corresponding colours assigned, you can visit either the NASA Hubble site or the Space Telescope Science Institute’s image gallery. The image descriptions there provide details on the filters used and the colour representation for each filter.
As for the choice between colour and monochrome cameras, using a ‘one-shot’ colour camera does offer advantages. However, in professional imaging, a monochrome camera with filters is often preferred due to its ability to produce higher-quality pictures.
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