Trifid Nebula Natural color Narrowband Experiment

[MarsG76]

Hello All,

It's been a while since I posted any images because I was upgrading & replacing faulty cables and cleaning up my observatory as well as Hypertuning my CGEM, doing this between family commitments so it has taken me about 2 months to complete it all.

The first object that I imaged using the Hypertuned mount was the Trifid nebula. Trifid nebula, aka M20 or NGC 6514 is a popular and bright nebula about 4300 light years away in the constellation Sagittarius.

As well as being a test image to determine whether the "Hypertune" has given me any improvement, this image was an experiment I was planning to do for a while, but never got around to doing until now. (I'll write about my adventures and Hypertuning results in another post for anyone interested.)

I always wondered if narrowband filters could be used to create a natural color looking image by using H-alpha as red, OIII as green and Hydrogen Beta as blue... logically these wavelengths would closely correspond to the visible Red, Green and Blue, so it made sense that it would be so.
I looked up this idea on a few forums and, even though it was mentioned, imagers were advising against it claiming that H-beta is the same as H-alpha except at half brightness... and I have not found any images with this kind of color channel combination.
The statement that Ha and Hb are essentially the same data didn't make sense to me simply because the Hb signal is in the blue part of the spectrum as opposed to Ha being in the deep Red, so I thought that surely this will translate to different signal in each of the channels.
I did a test exposure and Hbeta was indeed lower brightness but to me there was substantial amount of difference between the two filters, even when I matched the image brightness levels... enough difference for me to try imaging with both narrowband filters as seperate red and blue channels.

 

 

So seeing this difference between channels, I decided to do a full imaging session spending time to image through all 4 filters, SII, Ha, OIII and Hb, and use all but SII for this particular image below with the aim to use 7nm narrowband filters to create a natural color looking image, combining the narrowband channels in the order Ha, OIII & Hb as RGB.

 

I like the result, as I think that it's convincing of true color and I think that the benefit of using narrowband to create natural/true color images in this way is that light pollution will not limit sub lengths any more and fainter matter can be picked up from more light polluted skies. What do you think?

 

Imaged through an 8" SCT at f6.3 using a QHY268M camera, for a total exposure time of 9 hours and 5 minutes.

CS,
MG

[glafnazur]

That's  very nice, love the colours 👍 

[Adam J]

Very nice. In terms of the Hb I am left wondering if a fair amount of what you are seeing may be reflection nebula breaking through the Hb band pass as opposed to such a difference in the pure Hb in comparison to Ha. It's the first time I have ever seen a Hb shot produce anything that is more than just a weaker copy of the Ha data. 

[AndysAstroPix]

Very cool, I've been working on Natural colour narrowband images recently. I've just posted another one here on the forum.

Your idea of Hb is very interesting, must get myself on of those filters! 😍

[MarsG76]

14 hours ago, Adam J said:

Very nice. In terms of the Hb I am left wondering if a fair amount of what you are seeing may be reflection nebula breaking through the Hb band pass as opposed to such a difference in the pure Hb in comparison to Ha. It's the first time I have ever seen a Hb shot produce anything that is more than just a weaker copy of the Ha data. 

I'd think that a 7nm Baader H-beta CCD filter would be quite good at filtering out all but the desired spectra, Baader seems to have a good reputation.. but maybe you're right and that is a case of reflection nebulosity breaking through as a part of it will be at the same frequency... that said... I don't know if it matters  if this leak is the case because the end result is a difference in signal from pure Halpha and the desired effect is reached for the purpose of astro imaging... it would be more critical if this data was to be used for scientific analysis....

I'll image a few more objects using this method in the next few months, object which I imaged in natural color and se what results I get.... I'm confident that this might be a way to go deeper and capture fainter nebulosity from light polluted skies which would otherwise be limited or not possible to capture.