M78 - the Headlight Nebula, a work in progress.

[Andy01 (Andy)]

Ok, bear with me on this - M78 from dark skies in Kilmore, Vic , Australia.

LRGB is not my comfort zone - for me it's hard and I'm not sure if my processing workflow is right. Anyway, this is a combination of two nights' data taken a year apart.

Approx 5 hrs Lum (10min subs binx1) 7 hrs Ha (20min subs bin x1) and roughly 40 mins ea of RG & B (5min subs bin x2).

Yes it's still a bit noisy, the colours arn't quite right and it needs yet more data, and the still undiagnosed reflection thing (which I've cropped out on the LHS) is a killer too. but despite all this, I'm convinced that with the proper processing, there's an image in here waiting to get out!

Happy to start again from scratch - but does anyone have a recommended LRGB workflow (or video link) using any combination of nebulosity, Startools & PS to share?

SI know it seems crazy but LRGB does my head in and to me NB is easier...

Hight res here -

C&C welcome

[Placidus (Mike and Trish)]

Hi, Andy,

Not talking about the tools but just the maths. The rough idea with LRGB is that you get three times as many photons per hour through L as through a colour filter, and humans are much more sensitive to variations in L than to variations in RGB, so it seems to make sense to shoot much more L than RGB, and to use the RGB to "colour" the more detailed L. To a first approximation, one parcels out the L to the diffrerent colour channels pro rata according to the RGB shots. In practice one takes into account that we perceive G as brighter than R, which is brighter than B, so we hand out more B and less G.

But there's a catch: In the darkest areas of the image, there may be almost no colour information in your short RGB stack. So you don't know how to hand out the L amongst the channels. Result: Hideous colour noise.

So I use four tricks:

(0) Take enough RGB. This might include 2x2 binning to reduce noise.
(1) Wavelet filter the RGB to remove colour noise in the RGB, before even considering using it with the L.
(2) Hand out more B than R, and more R than G, to make up for the human visual system.
(3) To the fuzzy extent that you have lots of RGB information at a particular pixel, hand the L out pro-rata according to the RGB. To the fuzzy extent that you have negligible RGB data at a particular pixel (and really don't know what the colour is), just apportion the L to produce a neutral grey.

No idea how PixInsight does it, but hope that the concepts help.

Best,
M

[gregbradley]

A good start Andy but perhaps you have your RGB files mislabelled. At a guess you have blue and red swapped. I did not see any yellow in the reflection nebs unless that is the Ha?

I sometimes swap my red and blue in CCDsoft as it labels the files red or blue but in reality I swap them to help with blue extinction (I want blue to be exposed before red usually whilst the scope is higher in the sky). My first LRGB looked like yours until I realised I had blue and red swapped.

Or is it simply the Ha component?

Greg.

[Andy01 (Andy)]

Thanks Mike, that really helps a lot- I'll process that bear it all in mind for the next one.

Cheers Greg, you may be right- I already got them mixed up once! I think I'll begin processing this one again from scratch.

Stay tuned...

[RickS (Rick)]

Originally Posted by Placidus

No idea how PixInsight does it

There are many ways to skin a cat with the PI swiss army knife, Mike

A typical workflow for PI would be a RGB Combine, DBE gradient removal (if needed), background neutralization using a preview, color calibration with a white reference (the stars in the image, a galaxy, the whole image, etc.)

Sometimes a linear fit of the R, G and B produces a nice result or an unlinked Histogram stretch.

I used to set my R, G and B values using eXcalibrator but it didn't really do any better than the conventional colour calibration options, IMO.

Like you, I do a heavy noise reduction on the RGB before combining with the luminance.

Cheers,
Rick.