The data for this article were captured by AstroDMx Capture through a William Optics Super Zenithstar 81mm ED Doublet APO refractor at f/5.5 with x 0.8 reducer/flattener, using an SVBONY SV605MC 14 bit, cooled, monochrome CMOS camera and Altair narrowband filters.
The Hubble Palette is one of six palettes made by assigning monochrome images taken through SII, H-alpha or OIII narrowband filters to the Red, Green and Blue channels of a resulting false colour image.
The Hubble Palette has S mapped to Red; H mapped to Green and O mapped to Blue in the false colour image.
The Hubble palette is highlighted in the above table of filter mappings to RGB channels.
The false colour image is generated according to the additive properties of the primary colours of light. So for example, where green light and red light are combined the resulting colour is yellow; where blue light and red light are combined the resulting colour is magenta; where blue light and green light are combined the resulting colour is cyan and where red, green and blue light are combined the result is white.
The additive properties of light
How the elements’ emission lines in the Hubble Palette combine to produce different colours in the false colour image.
Out of the six possible palettes that are available, most astro-imagers choose the Hubble palette probably because it has an iconic ring to it; being named for the Hubble Space telescope.
The other named palette; HOS, is the Canada, France, Hawaii telescope palette. This palette name just doesn’t have the same ring to it!
All of the palettes are equally valid and any one could be used, and for interpretation purposes, the Venn diagram of overlapping colours would have to be re-labelled for that particular palette's element combinations.
Montage of the six available unprocessed palettes made from SII, H-alpha and OIII filtered monochrome images.
The problem with the Hubble palette is that it is so green. This is because H-alpha is usually the dominant element in a nebula and in the Hubble palette it is assigned to the green channel. Therefore green usually dominates a Hubble palette image. The same problem exists for each of the possible palettes; one colour or other usually dominates the raw image.
Astro-imagers usually proceed to reprocess the raw Hubble Palette image to replace much of the green with yellow and gold hues partly for aesthetic reasons.
Of course, sensu stricto, once the hues of a Hubble palette image are changed, it is no longer a Hubble palette image, whatever we might call it; although it is derived from a Hubble palette.
Below is a basic Hubble palette image of the Horsehead-flame nebula region. It also contains luminance data acquired by capturing monochrome data through an Altair Quadband narrowband filter which is a narrowband filter with two bandpass zones:
1st band, Centred on 495nm FWHM 35nm, range 477.5nm - 512.5nm.
2nd band, Centred on 660nm FWHM 35nm, range 642.5nm - 677.5nm.
These two bands are wide enough to include the emission lines of H-beta and OIII in the 1st band, with H-alpha and SII in the second band.
The luminance layer simply provides structural luminance information but does not change the hue of the image.
Raw Hubble palette image
One way to look closely at the information in the image is to increase, globally and selectively, the saturation of the colours in the image. The hues remain unchanged by the increasing saturation, so this is still a Hubble palette image.
Hubble palette image with enhanced saturation
In this image it is possible to see green regions dominated by H-alpha; yellow regions where there are both SII and H-alpha; magenta regions where there are both OII and SII; blue regions where there is a lot of OIII, red regions where there is a lot of SII and cyan regions where there is both OIII and H-alpha.
However, in this example, whilst enhancing the saturation of the colours in the image does reveal more of the composition of the nebula, it neither produces an image that adequately distinguishes the regions of different composition, nor does it produce an image that is any more aesthetically pleasing than the original Hubble palette image.
This is why astro-imagers post-process Hubble palette images as mentioned previously.
Another method is where the individual monochrome images are colourised with the appropriate colours of red, green or blue and they are then combined together as layers with different % opacities. I shall call this the method of fractional channel blending.
This Hubble palette image was produced by layering 25% H-alpha, 50% SII and 100% OIII opacity; followed by global and specific colour saturation enhancements. The Quadband luminance layer was also incorporated for consistency with the previous images.
Hubble palette image by fractional channel blending
Strictly this is still a Hubble palette image and shows clear distribution of the composition of the nebula as well as being aesthetically pleasing.
Processing that selectively changes the hues of various colour components of the image can yield the required rendering of the image.
Hubble Palette image post-processed differently, including selective hue changes to reveal more differentiation in the compositional structure of the nebula.
Whilst this is strictly no longer a true Hubble palette image, it is derived from that palette and does show clear differentiation between the colours (derived from the composition) of the nebula. Moreover, it is an aesthetically pleasing rendering of the Hubble palette image.
It could be argued that one should allow the individual element channels to speak for themselves as monochrome images of the Hubble palette.
SII image
H-alpha image
OIII image
These three monochrome images tell the compositional distribution story perfectly, but not quantitatively. The reason for this is that the images have had to be stretched individually until they are ‘similar’ in their intensity, for composing into an RGB Hubble palette image. This also tells us that the ‘true’ Hubble palette image is not a quantitative image, but only a compositional structure image.
This is the luminance monochrome image captured through an Altair Quadband filter that was used to add luminance information to the original image
Various images could be used for luminance if it is to be incorporated at all into the image. Here we have used an image from a filter that passes all of the wavelengths of interest. However, H-alpha or H-alpha + SII could also be used and will influence the luminance in different parts of the image.
I believe that it is desirable to explore the other five palettes that result from mapping H-alpha, SII and OIII to the red, green and blue channels of false colour images.
By using the Hubble palette produced by fractional channel blending , all six possible palettes can be constructed by reassigning the colour channels:
