Flaming star and IC405. How to render the colour?

[timh]

For some objects it is relatively easy to make choices about the most appropriate coloration. Star clusters and galaxies - RGB plus optionally NB enhanced HA features - or Wolf Rayet nebula for example where just OIII and HA narrowband will suffice to capture most all of the 'interesting' visible light with which to form an image.

However image processing and colour choices that provide a consistent, scientifically meaningful and preferably 'naturalistic' representation seems to get more complicated where (ignoring the stars ) the object of interest combines both monochrome NB and broadband sources of light. So for example objects like M1 that combines narrow band with broadband synchrotron blue light or whereever reflection nebula get mixed up with HII regions.

The flaming star and surrounding HII region IC405 is one such example and I have been back and forth trying different recipes unable to decide.

As a bit of background this comprises a reflection nebula (implying the presence of dust) illuminated by a very bright young type O star, AE Auriga, surrounded by a 5 ly type II region also illuminated and ionized by AE Auriga. One simplifying factor is that there appears to be negligible OIII.

To image this object https://www.astrobin.com/inxrqq/F/ I collected 40 minutes of monocamera BLUE filter light, 70 minutes of mono-camera HA light , 70 min of OSC camera UHC-filtered light and ~ 180 min of unfiltered RGB OSC camera light mainly using an F4 VX12 Newtonian (set up as previously described).

Having collected these images I was then faced with the problem of working out the most logical way to combine them? First (having used RC Exterminator to remove the stars) I split the two RGB images into individual R, G and B channels and then - after liinear calibration - used Pixmath to combine the two red channels with the HA into one master red channel, the two green channels into one master green and the two RGB blue channels with the mono blue to make a master blue channel.

I was then left with a spectrum of choices about how to best stretch and weight the three channels to combine them in a way best matching 'reality'?

At one extreme I more or less suppressed green (except to neutralise the sky background) on the assumption that the green response was largely due to the fact that the green pixels also exhibit an overlap response with red and blue and that 'in reality' the HII region would be shining in monochrome red admixed with some blue (due to H beta) wherever there was less dust. This approach created thin trails of blue reflection nebulosity against a background of red through to purple type II nebulosity (which I note seems the predominant interpretation for this object on astrobin).

In another interpretation I admixed more of the green back in on the assumption that a) the reflection nebulosity of the thin trails would overlap into green light as well as blue and b) that there would likely be sufficient dust in the wider Type II region that, beyond the clear foreground 'trails' of dust, the parts of the HII region closest to AE Auriga would also be acting as a reflection nebula as well as emitting HA light. This approach leads to much of the Type II region appearing much more orange.

I don't know which physical interpretation is the more nearly correct and so ended up with three versions .. Personally I think that the less strongly coloured slightly more orange version is probably closer to reality (i.e the last of the three images which most closely matched the colour of the pure unfiltered RGB image)-- but I don't really know - or indeed if anyone else does so on Astrobin I just followed the herd as my brighter red 'final' choice.

Tim

[admin]

Hi Tim,

as always, a most interesting post

On a purely aesthetic level, my favourite of the above 3 is the first, orange version, and my least favourite is the middle one (where I feel the contrast of the red and purple/blue is excessive).

Mind you, I'd be happy if I personally had captured any of them!

cheers,

Robin

[timh]

Many thanks Robin,

Looking at the images more carefully you choice is right I think and not only aesthetically. Overly suppressing green to get closer to a pre- conceived notion that the nebula should look a deeper red distorted the overall colour balance and reduced the SNR. The bright blue does look a bit unlikely somehow. So the more orange image at the top is probably the best - and for all I know a truer representation - and so is restored to pride of place as my 'final image'.

Tim