Hues of the Heavens
Astrophotography by Ivan Bok
Is it Real?
Astrophotography and the Truthful Representation of the Universe
One of the questions I often get asked is whether the photographs of the cosmos taken by astrophotographers like myself are real. Or more specifically, are the colours real? The answer isn't as simple as it seems, and I'm not saying this because I'm being a politician trying to evade the question.
If we take photography in general as a starting point, it's important to know that cameras do not "see" the world around us the same way our eyes do. The reason photographs taken right out from most cameras look natural (or at least bear some semblance to how the scene looks to our eyes) is because there are algorithms within them that preprocess the raw digital signals recorded during capturing. There is therefore no such thing as an "unprocessed" digital photograph, as much as purists would like to think.
When we enter the realm of astrophotography, we begin approaching the frontier of what can be recorded on camera. The light from deep sky objects is so faint that cumulatively, many hours of exposure are required to create every single astronomical image. In contrast, the human eye cannot record light for more than 1/15th of a second before sending the electrical signals to our brain. If we follow from this logic, it stands to reason that no long exposure astronomical image is "real".
Of course, this would be too simplistic a view. If we instead imagine a world where the human eye is much more sensitive to light - on par with our best cameras, how would the Universe look?
True Colour Astrophotography
Unlike many telescope and imaging systems used in professional astronomy, where the main concern is with gathering scientifically-useful data, amateur astrophotography prioritises aesthetics (well, at least most of us do). In turn, this gives us the liberty, more so than for scientific use cases, to create true colour images.
What is a true colour image? Simply put, it is an image in which colours are blended and represented in a way that is consistent with how the human eye perceives colour.
It is also worth noting that humans can only perceive light in a very narrow range of the full electromagnetic spectrum, a range (for obvious reasons) referred to as "visible light". Within the visible light range, our eyes perceive colour by having cells that are sensitive to red, green and blue light respectively. These are also the primary colours of the additive colour scheme.
In order to replicate the way our eyes see colour, most consumer cameras have red, green and blue filters embedded onto the camera sensor matrix in pattern known as the Bayer pattern.
The Bayer Pattern
Photo Credit: Wikipedia
With different coloured filters on each pixel, the camera software can then reverse engineer each colour channel to create a full-colour image that is consistent with the human eye.
Indeed, many astronomical cameras also use sensors with a Bayer matrix, and these are known as one-shot colour (OSC) cameras, a term given because they can produce a full-colour image in a single shot.
However, many astronomical cameras are designed such that it ditches the Bayer matrix entirely. These cameras are essentially monochrome (black & white) in function because the sensors of such cameras can no longer distinguish differently coloured light. This of course begs the question - why create a monochrome camera? To answer this, it is important to first understand the drawbacks of using Bayer filters like those in typical colour cameras. Because the colour filters allow only the light of its corresponding pixel colour to pass, there is a significant amount of light that ends up being filtered out. In a use case like astrophotography where light is a scarce and valuable resource, not filtering it out is a sensible trade for colour data. Monochrome cameras in an astronomical application generally produce superior results in terms of sensitivity and resolution.
That said, monochrome cameras can and do produce true-colour images. Instead of the filters being embedded on the sensor, a filter wheel with different filters can be placed in front of the camera. By separately shooting images through Red, Green and Blue filters, we can reconstruct a true colour image.
Red
Green
Blue
Filter Wheel of a Monochrome Camera
Note: I'm not colour blind. The filter colours you see are due to the colour of light they reflect, not transmit. The red filter passes red light through, but reflects green and blue, making it look cyan.
False Colour Astrophotography
However, not all astronomical images follow the true colour approach in blending colours, and there are often good reasons for that.
Scientifically, true colour images of nebulae don't tell you a whole lot. Because Hydrogen is the most abundant element in the Universe, and glows red when ionised, most emission nebulae will just look overwhelmingly red when photographed in true colour, meaning that you can't represent much scientific information from the colours in an image. Additionally, the electromagnetic spectrum is far larger than just visible light - telescopes designed to detect infrared light, ultraviolet, X-rays and radio ways are all actively used in professional astronomy.
Of course, I'm not a professional astronomer doing science. I'm just here to take pretty photos. However, living in Singapore means that I am plagued with extreme light pollution. Instead of being forced to explore other hobbies, the same filters originally designed for scientific imaging also happens to be useful in blocking light pollution. These are known as narrowband filters, and they are designed to isolate light emitted by different ionised gases.
The most commonly used narrowband filters target ionised Hydrogen, Oxygen, and Sulfur:
Hydrogen Alpha (HA)
Oxygen III (OIII)
Sulfur II (SII)
Both Hydrogen and Sulfur emit strongly in the red, while Oxygen emits blue-green light. To represent the presence of different elements in a nebula in a colour image, we can (artificially) assign the data captured through the different filters into the red, green and blue. Because this assignment of colour is an artificial choice we make, the resulting image is known as a false colour image.
Indeed, we can choose to assign both Hydrogen and Sulfur to the Red Channel and Oxygen to both the Green and Blue colour channels. This will create a pseudo true colour image. If we go for this route, we could decide to drop shooting through the Sulfur II filter altogether since the Sulfur signal is often much weaker than Hydrogen. In this case, we can create a "bicolour" image, also known as the Hydrogen-Oxygen-Oxygen (HOO) colour palette where we map Hydrogen to red and Oxygen to both green and blue respectively.
However, because we're repeating Oxygen for both the green and blue channels, the resulting image can sometimes be rather bland.
A more popular way of depicting narrowband images is through the Hubble Palette, also known as SHO. In this case, Sulfur gets assigned to red, Hydrogen gets assigned green, and Oxygen gets assigned as blue. This is not entirely arbitrary - the Sulfur II signal actually has a slightly longer wavelength than the Hydrogen Alpha emission, which makes it closer to the red part of the spectrum. Oxygen III has the shortest wavelength, of the 3, making it the best candidate for blue. Hydrogen Alpha sits in between, and is therefore assigned as green.
So why is it called the Hubble Palette? This is because the Hubble Space Telescope uses this convention for many of its nebulae imagery. Let's now marvel at the famous "Pillars of Creation" image:
The Pillars of Creation, at the Pinnacle (pun intended) of False Colour Imagery
Photo Credit: NASA
The Pillars of Creation image was one of those images that truly captured my imagination when I was a child. If you're a purist though, I'm sorry to disappoint you by telling you that this is an entirely false colour image. If you're looking at any photo from the newer James Webb Space Telescope (JWST), those are also unequivocally in False Colour because it is optimised for infrared (the mirrors are plated with gold for this reason).
Given that many professional astronomical images from flagship space telescopes are in False Colour, I can at least have some semblance of moral ground to say that many of my images are in False colour, using the Hubble Palette.
Personally, I find the Hubble Palette aesthetically pleasing for depicting narrowband images.
Are False Colour Images Fake?
Going back to the question we originally posed, I want to make a more philosophical take on this. Sure, for most people, we could have left it as that - False Colour images show fake colours but with real structure. This is a valid way of thinking.
At the same time, is what's real really shaped or determined solely by the way our human senses perceive the world? Before you roll your eyes at me, hear me out.
Here are some images of the human hand:
The Human Hand in Visible Light, X-Ray, and MRI
Photo Credit: Wikipedia, Radiopaedia
Which of these is real and which are not? Arguably, all of them are real. We humans can only see in the visible spectrum, and we build tools like X-Ray Machines and MRI scanners to "see" beyond.
X-Ray machines function similarly to a photograph, just with wavelengths of light we can't see. MRI images, on the other hand, are created from a complex setup of powerful magnets that energise molecules in our cells, listening for the radio waves that then get emitted from multiple directions, and using some complex math to build a structural image. It is entirely synthetic.
The fact is that the underlying fabric of reality far exceeds the limits of human perception. Each of these machines take a slice of that reality to represent it in a way that we can understand. They all depict different "truths" about the world around us. In each case, we lose some aspects of the truth while seeing others - in an X-Ray image, we see the bones but not the skin. In visible light, we see nails and hair, but don't see the intricate blood vessels and segmented bone structure beneath.
When we consider False Colour images, we similarly trade the "truth" of a human eye-consistent colour palette with the information of the elemental composition of stardust.
And that's my excuse for creating False Colour images.