What is the Hubble Palette?
The Hubble Palette refers to a specific technique used in astrophotography and was made famous in the processing of images captured by the Hubble Space Telescope (HST), which photographs objects in very narrow wavelengths of light using various filters.
This technique involves assigning colours to different wavelengths of light that are not visible to the human eye, to create detailed and visually striking images of deep sky objects, such as nebulae, galaxies, and supernova remnants.
Normal coloured images exist of the blend of RGB (red, green and blue) while the Hubble Palette means that you create false-colour RGB images. (Link to my article where I explain in short how to create a false-colour RGB image => https://beatriceheinze.com/my-first-tlpod-the-rosette-nebula-in-sho/)
In astrophotography, many celestial objects emit light beyond the visible spectrum, such as infrared or ultraviolet, in wavelengths that humans cannot see. With the Hubble Palette, you can take advantage of this by using narrowband filters to capture images in these wavelengths, specifically focusing on the narrowband emissions from sulfur, hydrogen, and oxygen atoms.
To capture sulfur, hydrogen and oxygen, you’ll need a mono-astronomy camera and a filter wheel with HSO-narrowband filters (HSO = hydrogen, sulfur and oxygen).
Below is a picture of my colour astronomy camera, the ZWO ASI 533MC Pro with a ZWO Mini filter wheel for 5x 1.25” filters. It is just to give you an example of what an astronomy camera with a filter wheel looks like as I don’t have a mono camera to show.
A mono astronomy camera with a filter wheel looks similar but creates monochrome images instead of RGB images. There are also filters in a bigger format, for example, 36mm or 2” thus in a bigger filter wheel. A mono camera is more expensive than a colour camera because you need to buy additional filters and a filter wheel. A filter wheel for a colour camera is not necessary because you already get RGB images from it.
Once captured, the images are then processed by assigning each of these very specific wavelengths to a colour that we can see. Traditionally, the Hubble Palette assigns the colour red to sulfur emissions (SII), green to hydrogen-alpha emissions (Ha), and blue to oxygen emissions (OIII). This method of colour assignment, also known as colour mapping, is somewhat arbitrary and does not reflect the true colours of the objects as they would appear to the human eye. However, it is chosen to maximize contrast and detail, revealing intricate structures and compositions of the celestial objects that would otherwise be invisible.
The resulting images are not only scientifically valuable, providing insights into the chemical composition, density, temperature, and other physical properties of astronomical objects, but they are also aesthetically captivating. The Hubble Palette has become a popular choice among astrophotographers and astronomers for its ability to highlight subtle details and differences in the gas clouds of nebulae (see picture below, The Rosette Nebula in SHO, the Hubble Palette (with annotations)) or the intricate structures of distant galaxies, making the invisible universe visible breathtakingly and colourfully.
Although I don’t like all nebulae in the Hubble Palette, I do think that the Rosette Nebula is an excellent choice to create an image of it in SHO (Hubble Palette).
Let’s take a closer look at the 3 elements of the Hubble Palette
- Sulfur (SII): Sulfur emissions occur at a wavelength of 672.4 nanometers. In the Hubble Palette, these emissions are typically assigned the colour red. Sulfur is often found in the outer regions of nebulae and in the remnants of supernovae. By isolating the sulfur emissions, astrophotographers can highlight these areas, revealing structures and features that contribute to our understanding of the composition and dynamics of celestial objects.
Sulfur (SII) => 672.4 nanometers => Red
- Hydrogen-alpha (Ha): Hydrogen-alpha emissions are found at about 656.3 nanometers. This wavelength is associated with the green colour in the Hubble Palette. Hydrogen, being the most abundant element in the universe, is a critical component of many astronomical objects, including stars and nebulae. Capturing hydrogen-alpha emissions allows for the visualization of the distribution and movement of hydrogen gas within these objects, providing insights into star formation and the lifecycle of stars.
Hydrogen-alpha (Ha) => 656.3 nanometers => Green
- Oxygen (OIII): Oxygen emissions occur at roughly 500.7 nanometers and are assigned the colour blue in the Hubble Palette. Oxygen, particularly ionized oxygen found in space, emits light at this wavelength, revealing the presence and conditions of oxygen within celestial objects. This can be particularly illuminating in regions of active star formation or in the delicate filaments of planetary nebulae, where oxygen plays a role in the complex chemistry and physics at play.
Oxygen (OIII) => 500.7 nanometers => Blue
If you want to know how I won the TLPOD (Telescope Live Picture Of the Day) with the image here above and if you want to see the image without annotations, click on this link to my blog post “My first TLPOD: The Rosette Nebula in SHO => https://beatriceheinze.com/my-first-tlpod-the-rosette-nebula-in-sho/
Here is the heart (as I like to call it) of the Rosette Nebula in SHO, the Hubble Palette, with annotations (the center part from the image above, zoomed in, rotated 90 degrees downward):
Is it possible to create an SHO image from OSC data?
Although, I wrote in the first paragraph, „What is the Hubble Palette?“, that you’ll need a mono-astronomy camera with a filter wheel with HSO-narrowband filters to create an image in the Hubble Palette (SHO)… these days it’s possible to create an image in the Hubble Palette with just an OSC (One Shot Colour) camera with a Duo-Narrowband filter a filter screwed onto it. I own the 2“ STC Astro Duo-Narrowband filter and the 1.25“ ZWO Duo-Narrowband filter (it’s in the filter wheel of my colour camera in the picture above -> see picture: Astronomy camera with filter wheel). Both work great for this purpose.
- Here is my blog post about M42 The Orion Nebula where I explain what duo-narrowband filters are, and what’s their use, and you’ll see images of the STC Astro Duo-Narrowband filter => https://beatriceheinze.com/portfolio/m42-the-orion-nebula-2/
Creating an SHO image, which stands for Sulfur (SII), Hydrogen-alpha (Ha), and Oxygen (OIII), using the Hubble Palette from data captured with an OSC (One Shot Colour) camera equipped with a Duo-Narrowband filter is a bit of a creative challenge but doable.
The reason why is that the Duo-Narrowband filter is designed to let through only the specific wavelengths of light emitted by hydrogen-alpha (Ha) and oxygen (OIII) while blocking most of the other wavelengths. This means there is a limitation because sulfur (SII) emissions aren’t directly captured.
Working with an OSC camera means that all the data is mixed in one image, an RGB image. You’ll need to use astrophotography software (Astro Pixel Processor, PixInsight, etc) to split your image into its RGB components (extracting the channels). The Duo-Narrowband filter’s characteristics mean that your Ha data will mostly be in the red channel, and your OIII data will be in the green and blue channels.
Since there is no actual SII data, you have to get creative and create a synthetic SII channel. One approach is to use the Ha data as a proxy for SII, considering they often illuminate similar structures within nebulae, though this isn’t scientifically accurate. Alternatively, you can blend your Ha and OIII data to create a synthetic SII channel that might more closely mimic the structures that SII would reveal. Experiment to see what gives you the best results for your particular image.
The software I like the most to create a narrowband image (SHO, HOO, and HSO are the only ones I’m familiar with at the time of writing), and I have the most experience with, are Astro Pixel Processor and PixInsight.
There are more methods to create an SHO image (an image in the Hubble Palette), but writing them all down would be too complicated.
On Feb. 14, Feb. 21, Feb. 28, March 6, and March 13, 2024, I was unable to post a new “Wisdom Wednesday” article due to certain circumstances therefore the delay.
*If you notice any errors in this article, please let me know via the contact form on my website. I will be grateful to you for it.
