If you are an astroimaging amateur and capturing narrowband frames, you probably already needed to make a decision – how to compose all these channels into one colourful image. There is large number of options to choose among – starting from classic HST palette, to more natural look, and there is also lot of possibilities for imagers that does not have whole set of narrowband filters. In this entry I want to focus on mapping narrowband frames colours to pretty much natural look. We may want to achieve this, when we for example merge narrowband channels with RGB image of stars.
We capture narrowband signal usually for emission nebulae. We may find different emission lines in the signal that originates from emission nebula – depending on nebulae composition. Most common wavelengths that amateurs image narrowbands are hydrogen alpha, sulphur II and oxygen III, but there are also much more other signals, though they are usually much weaker. Example spectrum of emission nebulae may look like below (Eta Carinae nebula):
Let’s take for example hydrogen. When we capture images through H alpha filter, the light that passes the filter is of course red. But hydrogen radiated not only in H alpha band in the visible spectrum. At 468nm there is H beta line, that intensity is about 1/3 of the H alpha intensity. Still in the visible spectrum there is H gamma (431nm) and H delta lines (410nm). If we now would like to determine the nebula colour basing only on H alpha signal, we would need to mix the hue with all four hydrogen lines that exist in the visible spectrum part. First thing to do is to determine the colour that corresponds to given wavelength. That can be done with some hue calculator, like the one at https://academo.org/demos/wavelength-to-colour-relationship/ , or in MaxIm DL using Blend Narrowband Image function. Then next step will be to mix the colours with proper intensities. For H alpha and H beta lines their ratio will be 3:1.
The NGC1499 nebula colour presents as below (assuming there is only hydrogen emission lines present in the signal):
And here is the same nebula imaged with RGB filters and colour calibrated with G2V type star:
It can be noticed, that synthetic hydrogen colour is quite similar to the one achieved in the RGB palette image. HSL nebula hue is about 342 in both cases. Image below shows separate RGB channels of the California nebula. Most of the nebula signal is captured in the R channel – it is mostly H alpha signal. Also in B channel there is quite a lot of signal – this comes mostly from H beta emission line. Vast majority of light from California nebula lands in R and B channels, and that determines its natural colour, that is not pure red, but also with hint of blue-violet. The signal in B channel could also be oxygen III line, but that would be also visible in G channel (most of CCD RGB filter sets pass oxygen III signal in both G and B channels). But for this particular nebula the signal in G channel is extremely weak. The source of this light may be small amount of oxygen III, but it can also be reflected light. If there is some dust in California nebula, it may reflect nearby stars light and can be partially reflection nebula.
Is the H alpha to H beta ratio always 3:1? It is – at source. It may be later altered by interstellar matter selective extinction. Light at shorter wave lengths (blue) is scattered more, than longer wave lengths (red). After such process the combined light will be more red than at source. But for relatively close California nebula this effect is not significant.
(to be continued…)